CEC025 - Rail Track Code

Published on December 2016 | Categories: Documents | Downloads: 140 | Comments: 0 | Views: 834
of 411
Download PDF   Embed   Report

Rail Track Code

Comments

Content

RAILTRACK
COMPANY STANDARDS
INFORMATION NOTICE
For Printer Users

RT/CE/C/025 contains colour on the following electronic pages:
26, 30, 43, 66, 77-79, 83, 95, 97, 98, 128, 134, 135, 149, 163-165, 168, 169,
174, 175, 178, 196, 197, 217, 219, 222, 265, 268, 272, 293, 294, 337-340, 343,
349, 354, 360, 363, 368-370 & 384
Colour pages may be purchased separately from:
The Railtrack Document Centre
@ Rapidoc
Tel: 01344 404446
Fax: 01344 714440
Email: [email protected]

On to Document

RT/CE/C/025
Issue: 1
Date: February 2001

RAILTRACK LINE CODE OF
PRACTICE
The Structural Assessment of Underbridges

© Copyright 2001 Railtrack PLC
All rights reserved. No part of this publication may be reproduced, stored in a

retrieval system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying, recording or otherwise, without the prior written
permission of Railtrack PLC.

Endorsement and Authorisation
Endorsed by:
This publication, including the data
and information relating thereto, is

Kim Teager, Professional Head Of Structures Engineering

Accepted for Issue by:

not to be used, disseminated,
stored in a retrieval system,
reproduced, copied or adapted
either in whole or in part without
the express written permission of
RAILTRACK plc.

Graham Morris, Head Of Corporate Standards

Authorised by

Published & Issued by
Railtrack plc
Railtrack House
Euston Square
LONDON
NW1 2EE
© 2001 RAILTRACK PLC

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 11

SUMMARY
This Code of Practice provides recommendations for the parameters and methods to
be used for the assessment of underbridges owned by Railtrack.
ISSUE RECORD
This Code of Practice will be updated when necessary by distribution of a complete
replacement or revised sections. Amended or additional parts of revised pages will
be marked by a vertical black line in the margin. Due to the extensive number of
revisions compared with Issue 1 such changes have not be marked in this Issue.
ISSUE 1

DATE

February 2001 COMMENTS: New Code of Practice
to provide a limit state code for
assessment of underbridges in respect of
steel, wrought iron, and concrete and
composite bridges, and to codify
permissible assessment parameters and
methods for under bridges formed from
other materials of construction.

RESPONSIBILITIES AND DISTRIBUTION
This Code of Practice should be used by persons undertaking the assessment of
underbridges and by those responsible for managing the process of bridge assessment
carried out by others.
IMPLEMENTATION
This Code of Practice should be complied with from April 2001.
DISCLAIMER
Railtrack PLC has used its best endeavours to ensure that the content, layout and
text of this document are accurate, complete and suitable for its stated purpose. It
makes no warranties, express or implied, that compliance with the contents of this
document shall be sufficient to ensure safe systems of work or operation. Railtrack
PLC will not be liable to pay compensation in respect of the content or subsequent
use of this document for any purpose other than its stated purpose or for any
purpose other than that for which it was prepared except where it can be shown to
have acted in bad faith or there has been wilful default.
SUPPLY

Paper copies of this document will be available to Railtrack staff on request to the Document
Controller. Copies of this document will be available to other organisations from Technical
Indexes (01334 404409).

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 11

CONTENTS
SECTION 1

INTRODUCTION

SECTION 2

ASSESSMENT PHILOSOPHY

SECTION 3

INSPECTION FOR ASSESSMENT

SECTION 4

LOADING FOR ASSESSMENT

SECTION 5

STEEL AND WROUGHT IRON STRUCTURES

SECTION 6

MASONRY ARCHES

SECTION 7

CONCRETE STRUCTURES

SECTION 8

COMPOSITE STRUCTURES

SECTION 9

CAST IRON STRUCTURES

SECTION 10

TIMBER STRUCTURES

SECTION 11

SUBSTRUCTURES

SECTION 12

BEARINGS

APPENDIX A

ASSESSMENT OF STEEL AND WROUGHT
IRON

APPENDIX B

ASSESSMENT OF CONCRETE STRUCTURES

APPENDIX C

ASSESSMENT OF COMPOSITE STRUCTURES

APPENDIX D

FATIGUE ASSESSMENT OF STEEL AND
WROUGHT IRON

APPENDIX E

MODEL BRIDGE ASSESSMENT REPORT

APPENDIX F

INFORMATIVE ANNEX

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 11

CONTENTS
1. INTRODUCTION...................................................................................................................4
1.1 Purpose................................................................................................................................4
1.2 Scope....................................................................................................................................4
1.3 Units .....................................................................................................................................5
1.4 Definitions and Abbreviations.........................................................................................6
1.5 Competency .......................................................................................................................6
1.6 Procedures for Quantitative Assessment.....................................................................7
1.7 Qualitative Assessment Procedures............................................................................10
1.8 Railtrack’s Technical Approval Procedures ...............................................................10
1.9 Reporting...........................................................................................................................10
1.10 Informative Annex.........................................................................................................11
1. INTRODUCTION
1.1 Purpose
The purpose of this Code of Practice is to recommend applicable standards and
analytical methods which may be used to determine the load carrying capacity of
existing Railtrack underbridges, in terms of British Standard Units of Type RA1
loading. The load carrying capacity is determined in the context of the performance
requirements of an underbridge. The requirements are that the bridge meets safety
and serviceability criteria whilst regularly carrying rail traffic up to a level of traffic
load and speed in accordance with operational system requirements.
1.2 Scope
This Code of Practice may be used for the assessment of all Railtrack owned
underbridges and is applicable for permissible speeds up to a maximum 125 mph.
This Code of Practice provides recommendations for the assessment of underbridges
constructed from steel, wrought iron, cast iron, concrete, timber, or composite
steel/concrete construction. Recommendations for masonry arches, substructures
and bearings are also included. Limit state principles are used for underbridges of
steel, wrought iron, concrete and steel/concrete composite construction.
Permissible stresses or allowable loads are used for other materials and forms of
construction.
Where appropriate, guidance on the use of simple and more rigorous methods of
analysis is given. Unusual forms of construction such as cable stayed, moveable or
combined road/rail bridges are not specifically covered, but the principles outlined
may be applied in checking the elements of such structures.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 11

Requirements for the assessment of superstructures and supports under accidental
loading conditions are excluded from this document.
1.3 Units
The S.I. system of units is adopted throughout this Code of Practice unless otherwise
stated.
In the course of assessment frequent reference may have to be made to existing
records which may be presented in Imperial Units. Great care should be exercised in
the conversion between the two systems of units. The following table gives
conversion factors for some of the most commonly occurring units.

PROPERTY

IMPERIAL UNIT

METRIC
equivalent of
IMPERIAL UNIT

METRIC
UNIT

Length

inch
foot
yard
mile
chain

2.5400
0.3048
0.9144
1.6093
20.1168

cm
m
m
km
m

Area

inch²
inch²
foot²
yard²

645.1600
6.4516
0.0929
0.8361

mm²
cm²



Volume

inch³
foot³
yard³

16.387
0.0283
0.7646

cm³



Mass

lb
ton
ton

0.4536
1016
1.0160

kg
kg
tonnes

Modulus

inch³
inch³

16387
16.387

mm³
cm³

Inertia

inch4
inch4

416200
41.62

mm4
cm4

Speed

mph

1.6093

kph

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 11

Table 1.1
Conversion Factors
1.4 Definitions and Abbreviations
For the purpose of this Code of Practice the following definitions apply:
Bridge means a structure of one or more spans whose prime purpose is to carry
traffic or services over an obstruction or gap.
PSR means Permanent Speed Restriction.
Provisionally Sub-standard Bridge means a Bridge that has been assessed at the
Level 1 assessment stage of the Bridge Assessment process to have a safe load
capacity less than the RA Capacity of the route. The Bridge remains Provisionally
Sub-standard until it is confirmed on completion of the Bridge Assessment that the
safe load capacity is not less than the RA Capacity of the route or the Bridge is
classified as a Sub-standard Bridge.
Serviceability Limit State (SLS) means the condition at which the behaviour of a
Bridge becomes unsatisfactory to the extent that it can no longer satisfactorily
perform its function under service loads.
Sub-standard Bridge means a Bridge where, following completion of a Bridge
Assessment, action(s) is (are) required to protect the safety of the Bridge. A Bridge
remains classified as Sub-standard until actions are taken to remove the applied
controls, or the RA Capacity of the route is amended to not more than the safe load
capacity of the Bridge.
TSR means Temporary Speed Restriction.
Ultimate Limit State (ULS) means the condition at which the Bridge, or one of
its constituent parts, would fail due to loss of equilibrium, fatigue induced
deterioration, or exceedance of its collapse strength.
Railtrack Director’s Nominee means the Structures Engineer with formally
delegated responsibility for the assessment of underbridges within the Railtrack Zone.
1.5 Competency
The skills, expertise and training of those persons responsible for, and carrying out,
the assessment should be appropriate to the nature and complexity of the structure
under consideration.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 11

1.6 Procedures for Quantitative Assessment
The assessment should commence with a definition of the assessment objective. A
clear statement of the required load carrying capacity should be made. In particular,
it should state specifically the existing RA capacity of the structure, the existing RA
capacity of the route and whether an increased structure capacity greater than that of
the route is required. The initial assessment (Level 1) should generally comprise
three distinct phases as follows:
1.

Desk Study
All available information relevant to the structure, including record drawings,
inspection and maintenance records, details of past performance and previous
assessments, and any available ground investigation data should be collated
and examined. The documents should be verified for correctness and in
particular, whether they were updated after previous works on the structure.

2.

Inspection for Assessment
A detailed examination of the structure is required to verify the form of
construction, its dimensions and the nature and condition of the structural
parts.

3.

Analysis
Based on the information obtained from the first two phases of the
assessment process, structural analysis to determine the distribution of forces
within the structure and the load capacity of the structural parts is required in
most cases.

In order to determine the adequacy of a particular structure with the minimum
degree of effort, the assessment should be carried out in levels of increasing
refinement and complexity, with the initial level (Level 1) being based on the most
conservative distributions of loads and analytical assumptions. If the structure is
shown to be inadequate in relation to the required load carrying capacity at this level,
assessment work should continue, with subsequent levels seeking to remove
conservatism in the assessment where this can be justified. Subsequent more detailed
levels may use:


more refined structural analysis;



more precise estimates of loading based on real vehicles;



material properties based on testing of materials samples;



supplementary load testing.

As illustrated in Figure 1.1 the process is cyclical in nature, each cycle being at an
increasingly refined level until a decision on the adequacy of the bridge is reached.
Conceptually it is useful to envisage levels of assessment as follows:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 8 of 11

Level 1

Simplest level using assumptions known to be conservative.

Level 2

Use of more refined analysis and better structural idealisation. This
level may also include use of data on materials strengths based on mill
test certificates or recent material tests on another similar structure.

Level 3

Use of a bridge specific live loading based on the known traffic and/or
the use of tests on materials samples or the use of worst credible
strengths or the use of load tests.

Where, by inspection, it is considered that greater benefit may be gained by the
adoption if live loading based on real trains than from a more refined analysis, the
assessment may progress from Level 1 directly to level 3.
The conclusion from the assessment should be subjected to a plausibility check. In
particular, discrepancies between the results of structural analysis, indicating
inadequacy say, and the real structural condition, for example no sign of distress or
failure, should be explained.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 1 - Introduction

Page 9 of 11

Review past performance and inspection data.
Decide assessment objective.

No

Analytical
assessment?
Yes

Qualitative assessment
•Review structure

Pass

Fail

Level 1 assessment
•Analysis

Pass

Fail
Provisionally
sub-standard structure

Yes

Urgent safety
measures?

Urgent safety
measures

Yes

Implement
measures

No

No
Further investigation
and review

Pass

Level 2-3 assessment

Pass

Fail
Fail
Review safety measures
Review assessment
objective
Implement
measures

Review assessment
objective
Assessment report

Sufficient
capacity/
adequate condition?
No
Operational restrictions
/repair/upgrade?

Figure 1.1
Assessment Process Flow Diagram

Yes

Bridge
management
programme
•Periodic
inspection
•Maintenance
•Performance
review

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 10 of 11

1.7 Qualitative Assessment Procedures
For some types of structure where no established method of quantitative theoretical
assessment exists and where increased capacity is not required, assessment may be
made qualitatively on the basis of satisfactory past performance. Structures for which
this procedure may be considered are spandrel and dry stone walls, retaining walls,
jack arches, substructures and foundations. The requirements for assessment on this
basis are:


the structure has demonstrated satisfactory performance over a long period
of time (over 5 years) since any significant repairs or alteration;



careful inspection does not reveal significant damage, distress or
deterioration;



review of the structure confirms its force transfer system;



predicted future deterioration will not jeopardise safety;



no significant changes in the loads and actions on the Bridge are anticipated.

Where the assessing engineer proposed a qualitative method of assessment, this shall
be justified and recorded in accordance with Railtrack’s Technical Approval
Procedures.
1.8 Railtrack’s Technical Approval Procedures
All assessments shall be subject to Railtrack’s Technical Approval Procedures for
assessment.
Irrespective of whether the assessment is to be carried out on a quantitative or
qualitative basis, the chosen method should be recorded and justified within the
Form AA. Where a qualitative method is proposed for the assessment of one of the
structure types identified in Clause 1.7, reference to this document may be deemed
to be sufficient justification for adoption of the method.
For the assessment of Bridges or structural elements which are outwith the scope of
this document, the method of assessment should be agreed within the Technical
Approval Procedure by Railtrack’s Professional Head of Structures Engineering.
1.9 Reporting
When the assessment has been completed, a report should be prepared detailing the
various stages of the process, together with the results. A suitable format for the
assessment report is given in Appendix E. Summary tables for reporting the
assessment results have been included in Appendix E for metallic structures, masonry
arches and concrete structures. These summary tables should be completed and
incorporated in the final report where applicable.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 1 - Introduction

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 11

1.10 Appendices
Additional notes and further information relating to the assessment of underbridges
are contained in Appendices A to F. Clauses within each appendix are numbered
sequentially from 1.0 and are followed by a letter denoting the appendix to which
they belong. For example Clause 4.1.1B indicates Clause 4.1.1 of Appendix B.
1.11 Informative Annex
Background information on the derivation of certain clauses of this code of practice
and guidance on its usage is contained in Appendix F. It should be noted that this
Appendix is not intended to give comprehensive guidance, and should not be assumed
to indicate all aspects of a structure that should be checked in the course of an
assessment.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 8

CONTENTS
2. QUANTITATIVE ASSESSMENT PHILOSOPHY...............................................................1
2.1 Applicability.........................................................................................................................1
2.2 Basis for Quantitative Assessment.................................................................................1
2.3 Assessment Situations ......................................................................................................1
2.4 Limit States..........................................................................................................................3
2.5 Assessment Load Values ..................................................................................................4
2.6 Load Factors .......................................................................................................................4
2.7 Assessment Load Effects ..................................................................................................6
2.8 Assessment Resistance.....................................................................................................7
2.9 Verification of Structural Adequacy...............................................................................8
2. QUANTITATIVE ASSESSMENT PHILOSOPHY
2.1 Applicability
The analytical procedures for quantitative assessment given in this Section are
applicable to most structural forms. They are not applicable to structures where
analysis is impractical and where the original design was based on good construction
practice of the time and no codes existed. In these cases assessment can be based on
qualitative judgement of satisfactory past performance and the information obtained
from assessment inspections. In all cases the purpose of assessment is to determine
whether the bridge meets relevant safety and serviceability criteria, see Clause 1.1.
2.2 Basis for Quantitative Assessment
Assessment of steel, wrought iron, concrete and steel/concrete composite Bridges
should be undertaken by the application of limit state principles. Bridges and
structural elements constructed from cast iron, timber or masonry should be
assessed on permissible stresses or loads.
Irrespective of the basis on which a Bridge is to be assessed, the bridge is required to
satisfy the Operational Safety Limit State requirements given in Clause 2.4(d).
2.3 Assessment Situations
The circumstances in which the Bridge is required to fulfil its function should be taken
into account by selecting relevant situations for assessment. The situations should
encompass all conditions that can reasonably be foreseen during use of the Bridge by
rail traffic. The situations should be determined by making a critical selection of
conditions arising due to dead and imposed load, live traffic loads and where relevant
temperature and wind effects. The situations chosen, characterised by a dominant
live load and one or more coexistent loads, should include the most adverse live
loads as follows:

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy

Page 2 of 8

Situation (1)

Maximum vertical live load with coexistent transverse and longitudinal
live loads;

Situation (2)

Maximum longitudinal live loads with coexistent minimum vertical and
transverse live loads;

Situation (3)

Maximum transverse loads with coexistent minimum vertical and
longitudinal live loads.

In the above situations other live loads where required by Section 4 such as those due
to wind and temperature should also be included where a more onerous loading may
result.
The values of maximum and minimum live loads for each situation are determined by
multiplying the nominal live loads given in Section 4 by the applicable factors given in
Table 2.1. The coexistent loads should be taken as zero if this results in a more
onerous loading of the Bridge.
SITUATION (1)
Railway Live Loading Maximum Vertical +
Component
coexistent
Longitudinal and
Transverse

SITUATION (2)
Maximum
Longitudinal +
coexistent
minimum Vertical
and Transverse

SITUATION (3)
Maximum
Transverse +
coexistent
minimum Vertical
and Longitudinal

1.0

0.5

0.5

1.0 (0)

1.0

0.5 (0)

Nosing

1.0 (0)

0.5 (0)

1.0

Centrifugal

1.0 (0)

0.5 (0)

1.0

Vertical:
Type RA Loading
Longitudinal:
Traction & Braking
Transverse:

Table 2.1
Factors for Combinations of Components of Railway Live Loading
Partial factors for use in commonly occurring situations are given in Table 2.2. In
special cases, other situations may arise and govern the assessment.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 8

2.4 Limit States
Where Bridges are to be assessed under the selected situations using limit state
principles, the following should be considered:
(a)

Ultimate Limit State (ULS)
The ULS is generally the governing condition for the assessment of
underbridge capacity.
This condition relates to the collapse strength of individual elements of the
Bridge, and to the stability of a part or the whole of the Bridge when
considered as a rigid body. To verify that an ultimate limit state is not
reached, it is necessary to demonstrate that the criteria in the relevant
Section of this Code of Practice are not exceeded under the application of
ULS assessment loads.

(b)

Serviceability Limit State (SLS)
Serviceability limit states are those situations where excessive deformations
or a deterioration in structural condition may lead to a loss in utility of the
Bridge such that remedial action may be required. Circumstances in which it
may be necessary to carry out checks against SLS criteria are defined in
Clauses 4.2.2A, 4.1.1B and 4.3.2C.

(c)

Fatigue Limit State
The limit state for fatigue may be either an ULS or SLS. Where an assessment
situation exists requiring fatigue evaluation (see Clause 4.3.2A) it should be
checked taking the load factors γ fL and γ f 3 equal to 1.0. For cast iron
Bridges, see Section 9.

(d)

Operational Safety Limit State (OSLS)
These conditions are attained when specified limits which govern the safe
operation of the railway are reached. These limits will generally be related to
the changes in structural deformation that occur under the passage of a train
and which, if exceeded may lead directly to derailment, or to degradation of
the track which may, in time, have the same effect. They are limits of
serviceability beyond which a Bridge is operationally unserviceable. Further
information regarding OSLS requirements is given in Appendix F.

For the bridge structure as a whole, an Operational Safety check should be made
relating to track twist in accordance with Section 4. For some structures,
Serviceability Limit States, such as bridge deflections and rotations, may also need to
be checked. Appropriate criteria should be agreed in accordance with Railtrack’s
Technical Approval Procedures.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 8

2.5 Assessment Load Values
The assessment loads, Q *A , are determined from the nominal loads, QK according to
the equation:
Q *A =

γ fL ⋅Q K

Equation 2.1

where:
γ fL

is a partial factor for each type of loading as given in Table 2.2.

Nominal dead and superimposed dead loads may be determined using the
information given in Section 4. Details of the nominal live loading and its application
are also given in Section 4.
2.6 Load Factors
Dead and superimposed dead loads should be taken together with live loads using the
factors given in Table 2.2 and in accordance with Section 4. Where it is necessary to
consider loads, such as those due to wind or temperature, which are not defined in
Section 4 of this Code of Practice, reference should be made to BD37/88: Loads for
Highway Bridges in accordance with Clause 4.4.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy
LOAD

Dead:
Steel, wrought iron
Cast iron
Concrete, masonry, timber
Superimposed dead:
Ballast *1, *2
Track

*3

Fill
Services
Live:
The multiple components of Live
Loading should be considered to
act in accordance with Clause 2.3
Wind:

Temperature:
Restraint to movement or due to
frictional bearing restraint

Page 5 of 8

Limit
State

γfL to be considered in
Combination
1
2
3

ULS
SLS
ULS
SLS
ULS
SLS

1.05
1.0
1.1
1.0
1.15
1.0

ULS
SLS
ULS
SLS
ULS
SLS
ULS
SLS

1.75
1.2
1.2
1.0
1.2
1.0
1.25
1.0

ULS
SLS

ULS
SLS

1.4 *4
1.1

1.2 *4
1.0

1.2 *4
1.0

1.1
1.0

ULS
SLS

1.3
1.0

Table 2.2
Values of Partial Factors (γγfL) for Loads in Combinations
*1

A value of γ fL of 1.35 at ULS and 1.1 at SLS may be adopted provided the depth of
ballast is controlled or dictated by the form of construction. Control measures may
include datum plates or a Plimsoll line.

*2

Ballast more than 300 mm below underside of sleepers may be considered as fill.

*3

Track includes rails, fixings and sleepers, but excludes ballast between sleepers.

*4

Subject to the approval of the Railtrack Director’s Nominee a reduced value of 1.25
for combination 1 and 1.1 for combinations 2 and 3 may be adopted where the
loading is of a controlled nature as follows:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 8

(a)

There is reliable control over the trains that can enter the route in question,
and

(b)

For vehicles which comprise any of the following:
Locomotives;
Locomotive hauled passenger and/ or mail trains;
Other passenger and/ or mail trains;
Cranes and track plant not able to carry loads whilst in travelling mode;
Freight wagons where loading is physically controlled, for example fluid fuel
tank wagons, closed grain or closed cement wagons;
Standard coal hopper or similar wagons where the load is weighed before
dispatch.

Reduced values of γ fL can only be assumed for other vehicles where every vehicle
after loading is weighed or is otherwise subject to proper assessment of weight,
before details are submitted and accepted for such vehicles to cross the Bridge.
These vehicles include freightliner container wagons, open top wagons for
aggregates, spoil or waste and wagons for track infrastructure maintenance or
renewal.

2.7 Assessment Load Effects
The assessment load effects, S *A , are obtained from the assessment loads by the
relation:
S *A =
*
A

S =

(

γ f 3 ⋅ effects of Q *A

)

γ f 3 ⋅ (effects of γ fL ⋅ QK )

Equation 2.2A
Equation 2.2B

Note: For steel and wrought iron only (Section 5), γ f 3 is applied within the
resistance R * (see Clause 2.8) such that:
S *A =
*
A

S =

effects of Q *A

Equation 2.3A

effects of γ fL ⋅ QK

Equation 2.3B

where:
γf 3

is a factor that takes account of inaccurate assessment of the effects of
loading, such as unforeseen stress distribution in the structure, inherent
inaccuracies in the calculation model, and variations in the dimensional
accuracy from measured values. The effects of the assessment loads should

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 8

be obtained by the use of analytical procedures applicable to the form of
construction.
The factor γ f 3 should normally be taken as 1.1 for ULS and 1.0 for SLS. γ f 3
may be taken as 1.0 for the ultimate limit state for members where the
following conditions (a), (b) and (c) are all met:
(a)
members are either:
(i)

Rail bearers or cross girders of steel, wrought iron or
composite construction that are assumed to be simply
supported, or;

(ii)

Main girders of steel, wrought iron or steel/concrete
composite bridges with skew not greater than 25° (If main
girders are continuous, any splices should be welded or made
with HSFG bolts or rivets, and have cover plates to both
flanges) or;

(iii)

Main beams of reinforced or prestressed concrete bridges with
skew not greater than 25° that are assumed to be simply
supported.

(b)

load effects are based upon static distribution within the structure;

(c)

geometric dimensions of the members are verified during inspection.

2.8 Assessment Resistance
The assessment resistance, R *A , of any structural element is the calculated resistance,
R * , of that element, making appropriate allowance for any deterioration identified.
The calculated resistance, R * , determined from material strengths and measured
section properties should be calculated from the following equation:
R* =

function (f k γ m )

Equation 2.4

Except for steel and wrought iron structures only where (Section 5):
R* =
where:

function (f k (γ m ⋅ γ f 3 ))

Equation 2.5

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 2 - Assessment Philosophy
fk
γm

RT/CE/C/025
Issue: 1
Date: February 2001
Page 8 of 8

is the characteristic (or nominal) strength of the material;
is a partial factor for material strength.

Values of f k and γm are given in Sections 5, 7 and 8 according to the material of
construction.
For those materials where the calculated resistance is determined on a permissible
stress basis, the following may be applied:
R* =

function (f p ) where f p is the material permissible stress.

2.9 Verification of Structural Adequacy
Structures should be deemed to be capable of carrying a specified level of assessment
loading when the following relationship is satisfied:
R *A > S *A

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 18

CONTENTS
3. INSPECTION FOR ASSESSMENT .......................................................................................1
3.1 General ................................................................................................................................1
3.2 Requirements prior to Inspection..................................................................................2
3.3 Inspection for Loading ......................................................................................................2
3.4 Inspection for Resistance .................................................................................................3
3.4.1 General ........................................................................................................................3
3.4.2 Metal Bridges ..............................................................................................................4
3.4.3 Masonry Arch Bridges ..............................................................................................7
3.4.4 Reinforced and Prestressed Concrete Bridges .................................................13
3.4.5 Composite Bridges ..................................................................................................15
3.4.6 Timber Bridges .........................................................................................................15
3.4.7 Substructures............................................................................................................16
3.4.8 Bearings......................................................................................................................17
3.5 Report on Inspection ......................................................................................................18
3. INSPECTION FOR ASSESSMENT
3.1 General
This Section gives recommendations for the inspection of underbridges, following the
desk study of existing information. The purpose of the inspection is to obtain
information required for the structural assessment and determination of safe load
carrying capacities. The principles outlined below may be applied to all types of
underbridge, and all materials of construction referred to in this Code of Practice.
Inspection for assessment is necessary to verify the form of construction, the
dimensions of the structure and the nature and condition of the structural
components. Inspection should cover not only the condition of individual
components but also the condition of the structure as an entity, noting especially any
signs of distress and possible causes.
Should the inspection reveal a defect which is believed to seriously compromise the
structure’s ability to carry load safely, the Railtrack Director’s Nominee is required to
be advised urgently in order that consideration may be given to the appropriate
emergency action to be instructed. Examples of defects that may require urgent
action to maintain the safety of the Bridge would include cracks in metallic structures,
or in the case of a masonry arch bridge if part of the arch is sagging.
Where practicable, advantage should be taken of the presence of scaffolding for
repairs/painting, the removal of ballast, longitudinal timbers, walkway boarding,

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 18

periods of low water etc. which may improve access for inspection of concealed and
otherwise inaccessible parts.
Where reasonably practicable Bridges should be observed under rail traffic and any
signs of abnormal movement such as excessive deflection, working of connections,
vibration or movement should be noted and considered as part of the assessment.
Where possible, these observations should be made under the passage of the heaviest
rail traffic using the Bridge.
When inspection is to be carried out in the hours of darkness the Bridge should first
be observed in daylight.
The skill, expertise and training of the person carrying out the inspection should be
appropriate to the complexity of the structure being assessed. This person should be
involved in the subsequent assessment process.
Where the taking of samples is considered necessary to confirm material parameters
or condition, the number, position and size of samples to be taken and any
consequential making good is required to be agreed by the Railtrack Director’s
Nominee. With regard to metallic structures, material testing should generally only
be used to confirm the material types, allowing the adoption of typical material
properties form Table A2 for assessment. Only in circumstances where this process
shows the material to be untypical should additional testing be undertaken to confirm
the yield stress and other appropriate material properties. Guidance on material
identification, sampling and testing is included in Appendix F.
3.2 Requirements prior to Inspection
Prior to undertaking an inspection of a Bridge all existing information pertaining to
the Bridge should be examined, including as-built drawings, soils data, past assessment
and examination reports and details of mineral extraction, as appropriate. This
examination may be useful in determining what further information should be
obtained from the inspection and which items require special attention. Special
attention should be paid to checking whether previously identified defects have
worsened.
Emergency reporting arrangements should be established and inspection personnel
advised of these in advance of all site activities.
3.3 Inspection for Loading
The inspection should enable the material type and all dimensions necessary to
calculate an accurate estimate of the dead and superimposed dead loads to be
determined.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 18

The position of tracks, rail joints (e.g. fish plated, welded and expansion) or switches
and crossings (within 18 metres of the bridge bearings) relative to the Bridge and
whether timber or concrete sleepers are installed should be recorded.
Track cant, radii, permissible speeds and any PSR or TSR should also be recorded
where appropriate.
The presence of longitudinal timbers, methods of fastening and positions of joints and
notches in timbers should be recorded.
Where the Bridge carries ballasted tracks, the overall ballast depth and depth to
underside of sleepers should be determined. The extent and height of any ballast
heaped on the bridge should also be noted, and the level relative to any control
marks recorded.
The location, number, size and type of services and service troughs should be
recorded.
3.4 Inspection for Resistance
3.4.1 General
The Bridge should be inspected to record all the parameters needed to determine:


the strength of elements and joints, including any observed defects, such as
cracks, loss of section due to corrosion, settlement, defective materials,
damage etc.;



the form of the structure to enable, in particular, assessment of dynamic
effects (see Section 4).

This inspection should be carried out within touching distance.
The inspection should supplement and provide confirmation of any information
obtained from existing records, particularly:


dimensions of internal sections that may not be related to external features;



strengthening and repairs that may not appear on record drawings, as these
elements may limit the load carrying capacity of the Bridge.

All constituent parts of the structure should be inspected in sufficient detail so that
their respective strengths can be determined. In some cases sampling of materials
may be required. Those parts not inspected should be recorded clearly and reasons
given.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 18

For buried members and those with hidden parts, excavation of trial holes etc.,
should be considered where there is doubt about the above parameters, especially
where such parameters could be critical. Care should be exercised to ensure that
there is no permanent damage caused to the structure by such excavations.
3.4.2 Metal Bridges
3.4.2.1 General
Prior to the inspection, a preliminary review of the structure, should be undertaken
to identify and assess potentially fatigue prone components and details.
The location, extent and remaining section of members where corrosion or other
forms of deterioration has occurred should be recorded accurately (preferably in
sketch form) to enable calculations to be made of section properties. The extent of
corrosion should also be established where metal sections are in contact with timber
decking or longitudinal timbers.
The location, nature and extent of distortion of structural elements resulting from
bridge strikes should be recorded.
Samples should be taken where required for testing to determine yield stress or
other material properties. Signs of poor quality and inferior metal should be noted
and further tests carried out if appropriate.
All cast iron members should be checked for the presence of cracks and blow holes
especially in tensile areas. The location and extent of such defects should be
recorded.
Where suspension bolts support a live load carrying member, particularly where their
failure could directly lead to collapse of the member, consideration should be given to
removal of a bolt or plate for inspection purposes. The stability of the structure must
be maintained after removal of these components.
Evidence of water seepage which may have contributed to corrosion of parts that are
not directly amenable to inspection should be noted. Exploration to establish the
extent of any corrosion should be considered.
Loose or missing bolts or rivets, rivets with severely corroded heads and any
“working” or rust staining of any connections should be recorded.
The dimensions and condition of free spanning longitudinal timbers should be
recorded.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 18

The verticality, and magnitude and direction of horizontal bow of top flanges of main
girders as required by Clause 9.8.2A plus details of the end restraints including those
for vehicle restraint should be recorded. Out of flatness of web panels should also be
recorded.
For half through type bridges with solid web or truss girders the presence of and
condition of features which may be contributory to compression flange stability
should be noted such as:
(i)

cross girder to main girder connections including the relative locations of
vertical stiffeners;

(ii)

signs of loose or ‘working’ elements such as rivets, bolts or packings;

(iii)

presence of concrete or other haunching or infilling to the main girders;

(iv)

other connections between floor and main girders such as troughing, plate or
timber floor, resting onto the bottom flange etc.;

(v)

trimmers or end cross girders and any infilling at or adjacent to the bearings;

(vi)

type of bearings and whether they or any infilling or haunching is providing
torsional restraint to the main girders. A note should be made of any wear,
cracking or spalling of bedstones;

(vii)

details and location of bearing stiffeners, end plates and other stiffening local
to the bearings;

(viii)

verticality of the main girders at the bearings. Magnitude, shape and direction
of horizontal bow of the main girder top flanges. A note should be made of
any additional movements of the main girders under live loading.

3.4.2.2 Fatigue
Members particularly susceptible to fatigue should be closely examined for visible
cracks so far as reasonably practicable. In particular close attention should be paid to
the details shown in Figure 3.1 which are known to be fatigue susceptible. In addition
to these, areas of severe and/or pitted corrosion around areas which have been
subjected to mechanical damage and distortions, such as may arise from vehicle
impact should be closely examined.
Where visible cracks are found, their extent should be measured and recorded.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

LIABLE TO
FATIGUE
CRACK

NOTCHED CROSS GIRDER END

Page 6 of 18

LIABLE TO
FATIGUE
CRACK
NOTCHED RAILBEARERS
LIABLE TO
WELD
UNDERCUT

WELDED DOUBLER ENDS

FATIGUE CRACK
CRACK
RIVET HOLES IN
TENSION AREAS

STRESS
RAISER
TE
N
SIO
N
CRACK
ENDS OF TRUSS MEMBERS

WELDED REPAIRS
OR
ATTACHMENTS
TO
RIVETED MEMBERS

COMPRESSION

WELDED ATTACHMENTS
AT FLANGE EDGES
STRESS
RAISER

ATTACHMENTS
WELDED

WELDED REPAIR
PATCH

Figure 3.1
Fatigue Susceptible details

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 18

3.4.3 Masonry Arch Bridges
3.4.3.1 General
The external fabric should be inspected. The arch barrel should be inspected to
ascertain all the information needed to determine the loading and resistance in
accordance with Section 6. This information should be recorded on the arch data
sheets on Figures 3.2 and 3.3. In particular the following information should be
determined:
(i)

thickness of the arch ring carrying rail traffic (this may not be the same as the
number of rings visible on the face) and its shape;

(ii)

nature and condition of the brickwork, stonework and mortar, including the
location and extent of any crushing, and the direction of bonding in the case of
skew bridges;

(iii)

thickness of the joints and the depth of any mortar loss;

(iv)

presence of cracks, their width, length, position and number;

(v)

location and extent of any loss of section due to spalling or damage by vehicles
from bridge strikes;

(vi)

location of any displaced voussoirs and displacement across cracks;

(vii)

deformation of the arch barrel from its original shape;

(viii)

the presence and effectiveness of any previous strengthening such as saddling,
stitching, grouting or strengthening rings;

(ix)

the presence and extent of any ring separation, which may be deemed to have
occurred if the engineer has any reasons to believe that the ring is not acting
integrally with the rest of the arch;

(x)

haunching over abutments and piers of multispan structures.

On site measurements should be made in imperial units and then converted to metric
prior to commencement of assessment analysis.
If part of the arch exhibits a significant change in profile from that described in
previous reports, the bridge should not be assessed but the condition of the bridge
reported to Railtrack immediately.
Where there is uncertainty about the above information a site investigation should be
considered, including trial holes where necessary. Probing into the construction
should be carried out where the strength of the bridge is in doubt or if internal scour
and leaching of the fill is suspected.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 8 of 18

The extent and location of water seepage should be recorded. The colour and
nature of any leachates should be closely examined for signs of brick or stone slurry
that may indicate internal movement.
Parapets and spandrel walls should be inspected for evidence of any defects and their
extent recorded on Figures 3.2 and 3.3, including, but not limited to:


tilting, bulging or sagging;



lateral movement of parapet or spandrel wall relative to the face of the arch
barrel;



lateral movement of parapet or spandrel wall accompanied by longitudinal
cracking of the arch barrel;



weathering and lack of pointing;



cracking, splitting and spalling;



loosening of any coping stones;



presence, location and details of ties, straps and patress plates.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

Page 9 of 18

LINE OR
BRANCH

MAP REFERENCE

NEAREST
STATION

BRIDGE No.

M

C

ARCH PROFILE
SPAN DIMENSION
(SQUARE)
SPAN DIMENSION
(SKEW)
NUMBER OF RINGS

LINE REFERENCE

SEMICIRCULAR
SEGMENTAL
ELLIPTICAL
PARABOLIC
POINTED

A

C ARCH

=

=

=
SPAN

A

ELEVATION LOOKING :

SECTION A-A
The following information should be recorded above:
A. SKETCH PROFILE OF SURFACE BALLAST AND TRACKS.
B. DIMENSION FROM TOP OF PARAPET TO SOFFIT OF
ARCH.
C. DIMENSIONS FROM TOP OF PARAPET TO RAIL LEVEL.
D. DIMENSIONS BETWEEN PARAPETS.
E. POSITIONS OF TRACK ON STRUCTURE.
F. TYPE OF SLEEPER AND TRACK

Figure 3.2
Arch Data Sheet 1

=

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

Page 10 of 18
MAP REFERENCE

LINE OR
BRANCH
BRIDGE No.

NEAREST
STATION
HARD STONE
MEDIUM STONE
ARCH
SOFT
STONE
RING
MATERIAL ENGINEERING BRICKS
BUILDING BRICKS
CONCRETE

M

C

LINE REFERENCE

OTHER (STATE) :

ARCH RING JOINTS
MORTAR

LIME
STONE

ARCH RING
MATERIAL

GOOD
SOUND OR FRIABLE
RANDOM
TYPE OF LAYING SQUARED
COURSED

CORRECT BONDING
REGULAR JOINTS

WIDTH OF JOINTS
UP TO 6mm
6mm TO
12mm
OVER 12mm

YES

DEPTH OF JOINTS
0mm (FLUSH TO FACE)
UP TO 12mm
12mm TO 0.1 OF RING
THICKNESS
OVER 0.1 OF RING THICKNESS

GENERAL FAULTS
YES
DIAGONAL CRACKS FROM
SPRINGING TO CENTRE
LONGITUDINAL CRACKS IN
SOFFIT
TRANSVERSE CRACKS IN
SOFFIT
ARCH
RADIAL DISPLACEMENT OF
RING
INDIVIDUAL STONE OR BRICKS
PERMANENT
DEFORMATION
CONSTANTLY WET
OR DAMP
ABUTMENTS DIFFERENTIAL SETTLEMENT
&/OR PIERS
SPREAD
SPANDREL
CRACKS AT QUARTER POINTS
WALLS
BULGING
WING
CRACKS
WALLS
MOVEMENTS
CONCRETE SLAB OR SADDLE
FILLING
GROUTED MATERIAL
WELL COMPACTED MATERIALS

NO

IF `YES' GIVE DETAILS

WEAK MATERIALS EVIDENCED BY
`TRACKING' OF SURFACE
NOT KNOWN

Figure 3.3
Arch Data Sheet 2

NO

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 18

3.4.3.2 Cracking in Masonry Arches
The inspection should investigate all cracks in an effort to establish their size and
depth, any associated displacement and their age. Old cracks, which probably
occurred soon after the bridge was built, and are no longer propagating, may usually
be ignored. Recent cracks, on the other hand, usually show clean faces, with possibly
small and loose fragments of masonry. Although appearing as shear of the bricks or
masonry, cracks normally follow an irregular line through the mortar. For this
reason, care should be taken in checking that the defects are cracks and not
deficiencies of the pointing material.
Cracks in abutments may generally be ignored unless they are new or growing. If
cracks in abutments are caused by subsidence they may have affected the arch ring.
The possible causes of cracks in the arch are noted below:


longitudinal cracks outside the centre third of the arch between the spandrels
and the arch ring may be caused by shear stresses generated by the spanwise
deformation of the arch relative to the spandrels under the passage of live
load (see Figure 3.4);



longitudinal cracks within the centre third of the bridge emanating from the
abutments may be due to varying amounts of subsidence in different places
along the length of the abutment, and are dangerous if large, because such
cracks tend to indicate secondary breaking up;



longitudinal cracks along the centre of a twin track bridge, spreading outwards
from the midspan area, may be caused by the stresses generated by the arrival
on the bridge of trains travelling in opposite directions;



transverse cracks, usually found near the quarter points, due to permanent
deformation of the arch, may be caused by partial collapse of the arch or
movement at the abutments;



Diagonal cracks normally start near the sides of the arch at the springing and
spread up towards the centre of the bridge at the crown may be due to a
subsidence at the sides of the abutment. Diagonal cracks indicate that the
bridge could be in a dangerous state. Where diagonal cracks meet or cross,
there is a possibility that a portion at the joint could be punched out, as shown
in Figure 3.5 below, and therefore, action should be taken as soon as possible
to prevent this happening;



cracks in the corners and abutments of skewed arch bridges may be due to
the differential resistance provided by the backfill.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

Page 12 of 18
Spandrel walls

Face of abutment

(a)

(c)

(b)

Figure 3.4
Plan on Arch showing Longitudinal Cracks
a) Between arch ring and spandrels out with middle third
b) From abutment within middle third
c) Along centreline

Could be punched out

Diagonal cracks from arch springing

Figure 3.5
Diagonal Cracking in Arch

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 13 of 18

3.4.3.3 Inspection of Jack Arches
The inspection of a jack arch deck should record the following information:
(i)

the geometric configuration of the jack arches and their supporting members;

(ii)

the presence of arch ties, details of their size, spacing, condition and position
within the height of the arch;

(iii)

rotations or horizontal displacement of a supporting member;

(iv)

transversely braced bottom flange of a supporting member;

(v)

inadequate support to springings, for example, corrosion of the bottom flange
of supporting beam over a horizontal length or loss of bedding mortar;

(vi)

cracking at the crown of the arch due to spreading of springings;

(vii)

distortion and any associated cracking of the jack arch barrel;

(viii)

arch cracking associated with substructure cracking or distress.

3.4.4 Reinforced and Prestressed Concrete Bridges
A covermeter survey should be undertaken to check the cover and the location of
reinforcing bars and prestressing tendons particularly in critical areas. If there are no
drawings, if the available drawings do not give sufficient detail for assessment, or if
there is evidence that the bridge is not as shown in the drawings, further investigation
will be required. Other evidence may arise from records, from the covermeter
survey, or from other findings of the inspection. Further investigation usually consists
of a more comprehensive covermeter survey supplemented by local exposures of
reinforcement to determine its size and confirm the position of critical bars. It will
not normally be practical or desirable to expose sufficient reinforcing or prestressing
steel to fully determine, its position, cross-sectional area and condition. When it is
considered necessary to locally expose reinforcement, the extent and depth to be
removed and method of making good is required to be agreed by the Railtrack
Director’s Nominee.
The worst credible strength of concrete should generally be derived from tests
carried out on cores in accordance with BS 6089. Cores are destructive and cannot
generally be taken at the critical locations of an element; hence interpretation or
extrapolation is necessary to arrive at worst credible strengths in these locations.
To assist in interpreting or extrapolating the results of core tests, an integrated
programme of testing which may include destructive, semi-destructive (e.g. near
surface) and non-destructive tests is necessary for each element. Care and
judgement is required in selecting the locations and numbers of samples for such
tests. The non-destructive tests can be used to give an indication of whether the area

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 14 of 18

of concrete from which the cores are taken is representative of the concrete in the
critical areas.
For reinforcement or prestressing tendons and bars, a worst credible strength should
be obtained by testing samples taken from the element being assessed. It should be
noted that bars of different sizes are likely to have significantly different yield
strengths. Removal of prestressing steel for sampling will alter the stress distribution
in the concrete section and the change should be allowed for in the assessment
calculations.
The extent and nature of spalling, corrosion of reinforcement, rust staining, crazing
or soft or friable concrete should be recorded. Where cracking is present the
following information should be obtained:


details of position, extent and widths of significant or unusual cracks;



details of any cracks showing evidence of rust staining;



all cracks over approximately 0.2 mm wide;



all flexural cracks in prestressed elements.

Consideration should be given to undertaking additional tests to determine
constituents and condition of the concrete. The tests may include tests for chloride,
half cell potential, sulphates, carbonation, alkali silica reaction or ettringite formation
and cement content. However, in general these tests are not required unless there is
other evidence of the associated forms of deterioration.
For post-tensioned concrete structures the fundamental design and construction
details should be established by a desk study, prior to the inspection for assessment,
as outlined in BA 50/93: Post-tensioned Concrete Bridges. Planning, Organisation and
Methods for Carrying Out Special Inspections. The inspection should follow the
procedures for Special Inspections as described in BA 50/93, if the bridge may be at
risk of sudden failure following tendon corrosion or if the integrity of transverse
prestressing is to be assumed in the subsequent analysis of the adequacy of the
structure.
Any evidence of distress should be recorded, especially evidence of rust staining,
spalling, cracking or water penetration at anchorage or tendon positions. In
particular unexpected cracking and unexpected or changing deflection should be
recorded. Further investigation is required whenever there is evidence that suggests
tendon corrosion. Corrosion of tendons in post-tensioned members may not be
visually manifest during inspection. For pretensioned concrete members, significant
tendon corrosion usually causes visible rust staining and cracking of the cover
concrete.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 15 of 18

3.4.5 Composite Bridges
The steel and concrete elements of composite bridges or composite members known
to have been designed for composite behaviour should be inspected in accordance
with Clauses 3.4.2 and 3.4.4 as appropriate. For other bridges or members where
concrete (or brick jack arches) is in contact or surrounds steel or wrought iron
members then inspection should be used to decide whether composite behaviour can
be assumed using Appendix A or Appendix C. It should be noted (see Clause 8.3.1C)
that composite behaviour of cross girders is not to be assumed in Type A or other
filler beam type decks less than 300 mm deep where there is no encasement above
the top flange or below the bottom flange.
For concrete slabs supported on steel or wrought iron beams the steel/ metal
interfaces should be examined especially near the supports for signs of:


corrosion;



fretting;



relative longitudinal slip;



vertical separation;



cracking or spalling of concrete.

Any relative movement should be recorded including any under live loading.
For cased beams the soffit (and other surfaces where practicable) should be examined
for signs of:


rust forcing or leakage;



separation of or hollowness of the casing concrete.

For filler beams or concrete or brick arch decks the soffit (and other surfaces where
practicable) should be examined for signs of:


corrosion;



relative longitudinal slip;



separation;



cracking or spalling of concrete.

Where the infill consists of unreinforced concrete or brickwork or is unknown then
probing should be undertaken to prove the presence of dense material in contact
with the beam before composite behaviour can be assumed in assessment.
For concrete infilled troughs, probing or other inspection should be undertaken to
determine the depth and condition of concrete above the crests before composite
behaviour can be assumed in assessment.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 16 of 18

3.4.6 Timber Bridges
The timber should be inspected, noting the presence of any protective coatings and
searching for the presence of rot or infestation (especially around metal fixings and
where there is standing water round piers in estuary waters). Heart rot may only be
detected by probing. The use of pressure sensitive, non-destructive drilling
techniques should be considered. Inspection for splitting and rot should be carried
out especially in areas of notching. Care should be taken not to transfer fungal spores
to sound timber by the use of contaminated tools. Any holes drilled should be made
good with sound timber dowels. Particular attention should be paid to timber in
contact with metal.
If site inspection indicates modifications to the structure, especially to primary
members, and weak timber is suspected, or if the species is unknown, samples should
be taken for identification of the timber.
3.4.7 Substructures
It is not normally possible to inspect the foundations, but where they are exposed, for
example in tidal waters, their condition should be checked. Any defects present and
their extent should be noted; defects may include cracking, erosion, disintegration or
corrosion of reinforcement.
Dimensional checks are required for preparing sketches for analysis or for
confirmation of record drawings. The dimensional checks may require excavation or
probing to determine depth and the extent of foundations. Care should be taken to
ensure that exploratory work does not impair stability or damage underground
services.
In river beds and banks the removal of material by scouring, from around the base of
piers or abutments may lead to undermining of the foundations, especially during
flooding. Whilst assessment of the susceptibility of a substructure to scour is outside
the scope of this standard, evidence of scour holes and approximate dimensions
where possible should be recorded. The presence and type of scour protection
should be recorded.
Foundation deficiencies usually appear as movements which may be sufficiently large
to cause tilting, cracking or excessive movements at joints or bearings. In arch bridge
foundations movement or arch spreading is generally apparent from cracks showing
distress in the arch rings and spandrels; diagonal cracking may be indicative of
differential settlement of the foundations.
All accessible parts of the substructures should be examined and any defects,
including extent, and possible causes recorded.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 17 of 18

Some typical substructure defects are:


tilting and rotation, in any direction;



rocking;



cracking, splitting and spalling;



erosion beneath water level;



weathering and other material deterioration, including lack of pointing for
masonry and brickwork;



vegetation intrusion;



lack of effective drainage;



internal scour and leaking of fill;



settlement of structure;



settlement of fill.

Movement of substructures is likely to be caused by foundation movements.
Differential foundation movements may be evident on abutment or pier walls in the
form of vertical or inclined cracks.
The effects that any observed substructure movement may have on the
superstructure or deck should be investigated. For example, differential settlement
will cause a twist in the deck; inspection may reveal dislocated bearings. Where
continuous decks are encountered, substructure movements may be evident from
signs of distortion or distress consistent with a ‘sag’ over the settling support or
‘hogging’ over intermediate adjacent supports.
Movement of substructure may be related to the support of spans of unequal length
or character.
In arch bridges, predominantly horizontal cracks in piers or abutments may be the
result of the arch ‘spreading’.
3.4.8 Bearings
Bearings if present, should be inspected so that the general condition and efficiency or
operation of the bearings can be established. The following should be noted:


general condition of bearings and their type and articulation;



any binding or jamming, looseness, or reaching limits of rotational or
translational movement, or vertical movement under live load;



condition of seating bedding and plinth;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 3 - Inspection for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 18 of 18



whether correct operation of the bearings is prevented or impaired, such as
by structural members built into abutment or pier;



for metal girder bridges, where applicable, whether the bearings are fulfilling
their function of providing end torsional restraint.

In bridges without bearings or where the bearings have failed to function correctly,
there may be local crushing or cracking, especially where supports are stone or
brickwork.
3.5 Report on Inspection
A report containing all the relevant information obtained from the inspection should
be produced. The report should include:


a description of the structure including details of any services carried;



a description of the condition of the structure including any repairs and a
discussion of the effect of any significant defects on the operational safety and
assessment of the structure;



sketches, drawings or photographs identifying the nature, location and extent
of any defects;



sketches giving ‘as measured’ dimensions;



other photographs, including general views and specific details.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 33

CONTENTS
4. LOADING FOR ASSESSMENT.............................................................................................1
4.1 Dead Loads .........................................................................................................................1
4.2 Superimposed Dead Loads ..............................................................................................2
4.2.1 Ballast ...........................................................................................................................2
4.2.2 Track ............................................................................................................................2
4.2.3 Services ........................................................................................................................2
4.2.4 Miscellaneous..............................................................................................................2
4.3 Railway Live Load ..............................................................................................................3
4.3.1 Vertical Static Loading ..............................................................................................3
4.3.2 Dynamic Effects..........................................................................................................7
4.3.3 Dispersal of Railway Live Loading onto the Structure.....................................25
4.3.4 Nosing ........................................................................................................................28
4.3.5 Centrifugal Load.......................................................................................................29
4.3.6 Longitudinal Loads ...................................................................................................30
4.3.7 Load Combinations .................................................................................................31
4.3.8 Elements Supporting More Than One Track.....................................................31
4.3.9 Structures Carrying Light Rail Systems ...............................................................32
4.4 Other Live Loads .............................................................................................................32
4.4.1 Wind Loads...............................................................................................................32
4.4.2 Temperature.............................................................................................................32
4.5 Operational Safety Requirements ................................................................................33
4.5.1 Track Twist ...............................................................................................................33
4.6 Accidental Loads from Vehicles....................................................................................33
4.6.1 Bridges over Highways ...........................................................................................33
4.6.2 Intersection Bridges ................................................................................................33
4.6.3 Train Derailments on Bridges ...............................................................................33
4. LOADING FOR ASSESSMENT
4.1 Dead Loads
The dead loads should, where possible, be based on dimensions verified during the
inspection. For assessment Level 1 analysis the applicable values of unit weight given
in Table 4.2 should be used. Where, however, the initial assessment shows
inadequacies, or there is doubt about the nature of a particular material, tests should
be carried out to determine actual densities.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 33

4.2 Superimposed Dead Loads
4.2.1 Ballast
The superimposed dead load due to ballast should be based on the measured depth
with unit weight 1800 kg/m³.
4.2.2 Track
Where applicable, the superimposed dead loads due to track components given in
Table 4.1 may be used. Where a different configuration of sleepers and rails has been
identified during the inspection, the self weight to be used should be determined by
measurement of dimensions of the configuration and by reference to data on weights
of components produced by the manufacturer.
Component
Single Bullhead Rail
Single Type 113A Rail
Single UIC 60 Rail
Conductor Rail
Concrete Sleeper (Type F40 for use with 113A Rail) *1
Concrete Sleeper (Type G44 for use with UIC 60 Rail) *1
Timber Sleeper
Chair for Bullhead Rail
*1

Mass
47.07 kg/m
56.22 kg/m
60.22 kg/m
75.2 kg/m
300 kg
315 kg
94 kg
21 kg

Includes shoulder, clips and rail pads.
Table 4.1
Permanent Way Component Weights

4.2.3 Services
The superimposed dead load resulting from service cables and ducting should be
determined, where possible, from examination and measurement during the
inspection or from information provided by the service owner. Where this is not
possible, any assumptions made regarding such equipment should be clearly stated in
the assessment calculations.
4.2.4 Miscellaneous
Miscellaneous items such as walkways which are not deemed to be part of the
structure should be considered as superimposed dead load. The nature and
dimensions of such items should be established during the inspection, and the partial
factor γ fL for dead load applicable to the material (given in Table 2.2) should be used.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment
Material1
Metal

Concrete

Masonry

Timber2

Fill

Aluminium
Cast Iron
Wrought Iron
Steel
Reinforced or Prestressed
Plain
Breeze Block
Engineering Brickwork
Other Brickwork
Granite
Sandstone
Softwood
Hardwoods generally
Jarrah
Greenheart
Sand (dry)
Sand (saturated)
Hardcore
Crushed Slag
Packed Stone Rubble
Earth (dry, compact)
Earth (moist, compact)
Puddled Clay
Asphalt
Macadam

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 33
Unit Weights kg/m³
2750
7200
7700
7850
2400
2300
1400
2400
2100
2600 to 2930
2200 to 2400
640 typical (480 to 720)
640 to 1200
840 to 960
1040 to 1200
1600
2000
1920
1440
2240
1600
1800
1920
2300
2560

1

Reference may also be made to BS 648 and BS 5268: Part 2: 1996.

2

Wide range of unit weights because of the variability of timber. For densities
of specific timber types refer to BS 5268: Part 2: 1996
Table 4.2
Density of Materials used in Bridge Construction

4.3 Railway Live Load
4.3.1 Vertical Static Loading
4.3.1.1 Route Availability (RA) Number
The assessment of a Bridge should be determined in terms of its Route Availability
(RA) number, that is its safe traffic load capacity. Route Availability numbers generally

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 33

range from the lowest capacity RA0 to the highest at RA15 represented by 25 British
Standard Units (BSUs) of Type RA1 loading respectively as shown by Table 4.3.
R.A. NUMBER
RANGE OF BSUs
RANGE OF SINGLE AXLE
IN GROUP
WEIGHTS IN GROUP
RA0
Up to 10.99 units
Under 13.96 tonnes
RA1
11.00 to 11.99 units
13.97 to 15.23 tonnes
RA2
12.00 to 12.99 units
15.24 to 16.50 tonnes
RA3
13.00 to 13.99 units
16.51 to 17.77 tonnes
RA4
14.00 to 14.99 units
17.78 to 19.04 tonnes
RA5
15.00 to 15.99 units
19.05 to 20.31 tonnes
RA6
16.00 to 16.99 units
20.32 to 21.58 tonnes
RA7
17.00 to 17.99 units
21.59 to 22.85 tonnes
RA8
18.00 to 18.99 units
22.86 to 24.12 tonnes
RA9
19.00 to 19.99 units
24.13 to 25.39 tonnes
RA10
20.00 to 20.99 units
25.40 to 26.66 tonnes
RA11
21.00 to 21.99 units
26.67 to 27.93 tonnes
RA12
22.00 to 22.99 units
27.94 to 29.20 tonnes
RA13
23.00 to 23.99 units
29.21 to 30.47 tonnes
RA14
24.00 to 24.99 units
30.48 to 31.74 tonnes
RA15
25.00 units and over
31.75 tonnes and over
Table 4.3
Route Availability Classification for Bridges
Type RA1 loading excludes dynamic effects which should be added in accordance
with Clause 4.3.2 and are dependent upon train speed. RA numbers should therefore
be determined according to a given train speed. In some cases it may be necessary to
determine more than one RA number for a given Bridge, for example RA6 at
100 mph representing passenger trains (normally the permissible speed) and RA10 at
60 mph for freight trains.
The number of units of Type RA1 loading that the Bridge can carry should be
determined by calculating the live load capacity factor, C , as defined below:
C =

Live Load Capacity
Effects of 20 units of Type RA1 loading

Capacity in terms of units of Type RA1 loading = 20 × C
The RA number of the Bridge should be obtained from Table 4.3.

Equation 4.1

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 5 of 33

Where the assessed RA number is below the RA of the line, the effects under static
EUDLs for the “real” (actual) permitted vehicles and combinations, together with
dynamic factors for their respective permitted speeds, may be considered acceptable.
It should be noted that the RA effect of vehicles on a specific “span” (loaded length) is
often less than the RA classification for the vehicle which has to allow for a full range
of Bridge spans.
4.3.1.2 RA1 Loading
The static loading used to determine the RA number is shown in Figure 4.1 for
20 units of Type RA1 loading. The Short Lengths configuration should be used when
it produces more onerous effects than the axle and uniformly distributed load model.
4x200kN

4x150kN

4x200kN

4x150kN
65kN/m

2.4

1.5 1.5 1.5

2.7

1.8 1.8 1.8

4.0

1.5 1.5 1.5

2.7

1.8 1.8 1.8 1.5

2x250kN
SHORT LENGTHS
1.8

Figure 4.1
20 Units of Type RA 1 Loading
Note 1:

20 units of Type RA1 loading is equivalent to Route Availability RA10
without allowance for dynamic effects.

4.3.1.3 Equivalent Uniformly Distributed Loading
For simply supported spans (with the exception of Masonry Arches, see Section 6),
Type RA1 loading may be represented by an Equivalent Uniformly Distributed Load
(EUDL). Table 4.4 gives EUDL and maximum end shear values for simply supported
spans for 20 units of Type RA1 loading. The EUDL values equate with the maximum
bending moment within the span that occurs under RA1 loading.
For continuous spans the values in Table 4.4 are not strictly applicable, and loading
should be as shown in Figure 4.1. This loading should be considered as a whole, but
any parts of the loading that reduce the effects on the part of the element being
considered should be omitted.
4.3.1.4 Application of Loads
Type RA1 loading should be applied to each track and such as to produce the
maximum effect in the part of the element being considered.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 6 of 33

(SPAN)
(m)

EUDL
(kN

END
SHEAR
(kN)

SPAN
(m)

EUDL
(kN)

END
SHEAR
(kN)

1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
8.6
8.8
9.0
9.2

500
500
500
500
500
500
500
500
500
500
513
532
554
574
594
618
643
667
689
709
728
745
764
784
810
834
858
879
899
919
937
954
971
986
1001
1015
1029
1042
1054
1070
1088

250
250
250
250
269
291
308
322
335
346
356
364
372
378
384
390
395
401
417
432
447
459
471
483
493
503
512
521
529
537
544
552
563
573
582
591
599
608
615
623
631

9.4
9.6
9.8
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
34.0
36.0
38.0
40.0
42.0
44.0
46.0
48.0
50.0
52.0
54.0
56.0
58.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
100.0

1105
1121
1137
1152
1219
1282
1351
1411
1475
1547
1620
1687
1760
1837
1983
2126
2265
2415
2547
2702
2871
3039
3201
3358
3505
3651
3787
3921
4053
4186
4312
4437
4559
4677
4974
5260
5554
5846
6136
6427
6717

640
650
659
668
707
752
792
835
873
907
947
983
1017
1055
1146
1233
1319
1405
1488
1569
1649
1726
1803
1878
1952
2026
2099
2171
2242
2313
2384
2454
2525
2594
2767
2939
3109
3279
3448
3616
3784

Table 4.4
EUDLs and End Shears for 20 Units of Type RA1 Loading
(Static Load per Track)

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 33

4.3.2 Dynamic Effects
4.3.2.1 General
The vertical static loading defined in Clause 4.3.1 should be multiplied by the
applicable dynamic factor to allow for impact, oscillation and other dynamic effects,
including those caused by track and wheel irregularities.
4.3.2.2 Dynamic Factor for Members other than Transverse Floor Members
The dynamic factor (1 + ϕ ) should be applied where the speed of trains is 5 mph or
greater, up to 125 mph maximum, using the dynamic increment ϕ which should be
taken as in Table 4.5.
Dynamic Increment ϕ
for Bending

Dynamic Increment ϕ
for Shear

(ϕ1 + ϕ11 )

Normal track
Permissible speed ≤ 100 mph
Track maintained for
Permissible speed 100 - 125 mph
Fatigue calculations only
Permissible speed ≤ 125 mph

ϕ
1.3 ϕ1 + 11 
2 


2
× ϕ for Bending
3

ϕ
0.5 ϕ1 + 11 
2 


Table 4.5 - Dynamic Increment ϕ
In Table 4.5:
ϕ1 =

k
1− k + k 4

(

)

representing interaction of the structure

Equation 4.2

where:
k=

v
v
but not greater than
4.47 L φ n 0
358
2

ϕ11 =

 Lφ 
 Lφ 

−  
Ln 0  − 20 

10 

α 56e
+ 50
−1e

 80 


2






Equation 4.3

Equation 4.4

The dynamic factor as given in this Clause excludes the effects of any rail joints, or
points and crossings. Equation 4.4 represents track irregularities where:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

α = 0.0002v but not greater than 0.01


RT/CE/C/025
Issue: 1
Date: February 2001
Page 8 of 33

Equation 4.5

is the determinant length in metres obtained from Table 4.6, but not
to be taken as less than 4 metres. Alternatively L φ may be defined as
the length of the influence line for deflection of the element under
consideration. In the case of floor members, 3 metres may be added
to the length of the influence line as an allowance for load distribution
through the track.

L

is the effective span in metres.

n0

is the fundamental natural frequency in Hertz of the structural
element, considered as a simply supported beam with span L loaded
with a 20 kN uniformly distributed load in addition to dead loads.
17.75
Note: n0 may be assumed as
where δ 0 is the maximum
δ0
deflection in millimetres of the structural element with a simply
supported span and loading as defined for n0 . This expression is not
appropriate for elements of non simply supported form (for example
continuous girders or braced spandrel arches). For such structures,
the natural frequency under the loading defined for n0 should be
determined by other means.

v

is the speed in mph, normally taken as the permissible speed (or line
speed) on the Bridge. For freight trains v may be taken as 75 mph or,
if less, the permissible speed for freight trains on the Bridge.

4.3.2.3 Values of ϕ
Figures 4.02 to 4.14 show values of ϕ for bending for different train speeds from 5 to
125 mph based on the formulae in Clause 4.3.2.2 for a range of natural frequencies
(Hz) referred to as low frequency (LF) and high frequency (HF) and defined as
follows:
High frequencies (HF)
n 0 = 94.76L−0.748
Low frequencies (LF)

Equation 4.6

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment
n0 =

RT/CE/C/025
Issue: 1
Date: February 2001

80
for 4 metres ≤ L ≤ 20 metres
L

Page 9 of 33
Equation 4.7

n 0 = 23.58L−0.592 for 20 metres < L ≤100 metres

Equation 4.8

Determinant Length Lφ
φ

Element
Steel and Wrought Iron
Deck Plate
Discontinuous spanning longitudinally

Twice cross girder spacing plus 3 metres

Discontinuous spanning two ways

Three times cross girder spacing

Continuous over ribs or stringers

As for 4 span continuous beams

Rail Bearers
Continuous

3 times cross girder spacing

Simply supported

Cross girder spacing + 3 metres

Concrete Slabs & Other Elements
As part of box girders or upper flange of steel
or concrete beams
Spanning transversely

3 times beam or web spacing

Spanning longitudinally

3 times beam or web spacing or determinant
length of main girders - whichever is the lesser

Edge cantilevers

For cantilever length ‘e’ from outer face or web of
beam to centre of cess rail
e ≤ 0.5 metres - 3 times beam or web spacing
e > 0.5 metres - 4 metres

Continuous over cross girders

Twice cross girder spacing

For Half Through Bridges
Spanning transverse to main beams

Twice main beam spacing

Spanning longitudinally

Twice cross girder spacing or determinant length
of main girders - whichever is the lesser

End zones of transverse spanning
elements, extending from supports for
1 of span of transverse element
4

4 metres

Inverted U Units

As for portal frame

Table 4.6
Determinant Length Lφ

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 10 of 33

Table 4.6 (continued)
Determinant Length Lφ
φ

Element
Main Beams
Simply supported beams

Centre to centre of supports

Continuous beams

Kx

Lm , but not less than the maximum span

where K is taken from the following table:
n
K
where

2
1.2

3
1.3

4
1.4

≥5
1.5

L m is the average span
n is the number of spans

Web members of trusses

The length of span of the main beam which is
loaded such as to produce the maximum force in
the web member considered

Portal frames

As continuous beam, assuming legs are spans

Arch, arch rib or stiffening girder of bowstring

Half the span

Multi-span arches

Twice the clear maximum span

Suspension bars to bowstring girders
To bowstring having stiffening girder

4 times spacing of bars

Without stiffening girder

As for cross girders suspended from bars

Supports
Columns, trestles, cross heads, bearings,
tension anchors.

The total length of superstructure that is
supported by the element concerned.

4.3.2.4 Dynamic Factor for Transverse Floor Members
The dynamic factor for cross girders and other discrete transverse floor members
should be taken as (1+ I 4 ) , where I 4 is determined from Figure 4.15. For fatigue
calculations only the value of I 4 should be taken as 50% of the value shown in
Figure 4.15.
4.3.2.5 Reduced Dynamic Effect
Where the depth of ballast or non-structural fill exceeds 1.0 metre, the dynamic
increment ϕ may be reduced as follows:
 h − 1
Reduced dynamic increment = ϕ − 

 10 

Equation 4.9

where h is the depth in metres below underside of sleeper or track to top of arch
crown or structural element.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 11 of 33

0.1

5 mph
(8 kph, 2.2 m/s)

ϕ
Fatigue HF

Normal Track HF

0.05
100mph+ Track LF

Normal Track LF

100mph+ Track HF

Fatigue LF

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20

10

8

7

6

5

4

3

Figure 4.2
Dynamic Increment ϕ for Bending at 5 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 12 of 33

0.2

10 mph

0.15

(16 kph, 4.4 m/s)
Normal Track HF

Normal Track LF

ϕ
0.1

100mph+ Track LF

Fatigue HF

100mph+ Track HF
0.05

Fatigue LF

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2

20
10

8

7

6

5

4

3

Figure 4.3
Dynamic Increment ϕ for Bending at 10 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 13 of 33

0.5

0.4

20 mph
(32 kph, 8.9 m/s)

0.3

ϕ
Normal Track HF

Normal Track LF
0.2

100mph+ Track HF

100mph+ Track LF
0.1
Fatigue LF

Fatigue HF

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.4
Dynamic Increment ϕ for Bending at 20 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 14 of 33

0.6

0.5

30 mph
(48 kph, 13.3 m/s)

Normal Track HF

0.4

Normal Track LF

ϕ
0.3

100mph+ Track LF
0.2
100mph+ Track HF

Fatigue LF

0.1

Fatigue HF

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.5
Dynamic Increment ϕ for Bending at 30 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 15 of 33

1.2

1.1

1

40 mph
(64 kph, 17.8 m/s)

0.9

0.8

0.7

ϕ

Normal Track HF

0.6

0.5
Normal Track LF
0.4
100mph+ Track LF
0.3
100mph+ Track HF

Fatigue LF

0.2

Fatigue HF
0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2

20
10

8

7

6

5

4

3

Figure 4.6
Dynamic Increment ϕ for Bending at 40 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 16 of 33

1.2

1.1

1

50 mph
(80 kph, 22.2 m/s)

0.9

0.8

Normal Track HF
0.7

ϕ
0.6
Normal Track LF

0.5

0.4
100mph+ Track LF

0.3

100mph+ Track HF

0.2
Fatigue HF
Fatigue LF
0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.7
Dynamic Increment ϕ for Bending at 50 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 17 of 33

1.2

1.1

1

60 mph
0.9

(97 kph, 26.9 m/s)

0.8

Normal Track HF
0.7

ϕ
0.6
Normal Track LF

0.5

100mph+ Track LF
0.4

100mph+ Track HF
0.3

0.2
Fatigue HF
Fatigue LF
0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.8
Dynamic Increment ϕ for Bending at 60 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 18 of 33

1.2

1.1

1

70 mph
(113 kph, 31.4 m/s)

0.9

0.8
Normal Track HF

0.7

ϕ
0.6

Normal Track LF

0.5
100mph+ Track LF
0.4

0.3
100mph+ Track HF

0.2

Fatigue HF
Fatigue LF

0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.9
Dynamic Increment ϕ for Bending at 70 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 19 of 33

1.2

1.1

1

80 mph
(129 kph, 35.8 m/s)

0.9

0.8
Normal Track HF
0.7

ϕ
Normal Track LF
0.6

0.5
100mph+ Track LF

0.4

100mph+ Track HF
0.3

Fatigue HF

0.2
Fatigue LF
0.1

0
0

10

20

30

40

50

LF
n0
HF

60

70

80

90

Span - metres

2
20
10

8

7

6

5

4

3

Figure 4.10
Dynamic Increment ϕ for Bending at 80 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 20 of 33

1.2

1.1

1

90 mph
(145 kph, 40.3 m/s)

0.9

0.8
Normal Track HF
0.7

ϕ

Normal Track LF

0.6

0.5
100mph+ Track LF

0.4
100mph+ Track HF
0.3
Fatigue HF
0.2
Fatigue LF

0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.11
Dynamic Increment ϕ for Bending at 90 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 21 of 33

1.2

1.1

1

100 mph

Normal Track HF

(161 kph, 44.7 m/s)

0.9

0.8

Normal Track LF

0.7

ϕ
0.6

0.5

100mph+ Track LF

0.4

100mph+ Track HF

0.3

Fatigue HF

Fatigue LF

0.2

0.1

0
0

10

20

30

40

50

LF
n0
HF

60

70

80

90

Span - metres

2
20
10

8

7

6

5

4

3

Figure 4.12
Dynamic Increment ϕ for Bending at 100 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 22 of 33

1.2

1.1

1
Normal Track HF

110 mph
(177 kph, 49.2 m/s)

0.9

0.8
Normal Track LF

0.7

ϕ
100mph+ Track LF

0.6

0.5
100mph+ Track HF
0.4
Fatigue HF
0.3
Fatigue LF
0.2

0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.13
Dynamic Increment ϕ for Bending at 110 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 23 of 33

1.5

1.4

1.3

Normal Track HF

1.2

125 mph
(200 kph, 55.8 m/s)

1.1
Normal Track LF
1

0.9

ϕ

100mph+ Track LF

0.8

0.7
100mph+ Track HF
0.6

0.5

0.4

Fatigue HF

0.3
Fatigue LF
0.2

0.1

0
0

10

20

30

40

50

60

70

80

90

Span - metres

LF
n0
HF

2
20
10

8

7

6

5

4

3

Figure 4.14
Dynamic Increment ϕ for Bending at 125 mph
( 2 ϕ for shear)
3

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 24 of 33

1.1

1

0.9

0.8

0.7

0.6
I4
0.5

0.4

0.3

0.2

0.1

0
0

10

20

30

40

50

60

70

80

90

100

110 120

Speed mph

Figure 4.15
Dynamic Factor I 4 for Transverse Floor Members

130 140

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 25 of 33

4.3.3 Dispersal of Railway Live Loading onto the Structure
Dispersal through the track onto Bridge floor elements may be applied as in Clauses
4.3.3.1 and 4.3.3.2 where:
FEUDL

is a factor by which the unit EUDL loading is multiplied;

FA

is a factor which is multiplied by the axle loading;

t

is the depth of ballast between the underside of sleeper and the top of the
member in mm;

L

is the effective span in metres of longitudinal members spanning between
centres of cross girders or twice the spacing of cross girders in the case of
continuous longitudinal members.

4.3.3.1 Longitudinal Members positioned Directly Under the Rails including Rail
Bearers, Troughing, Slabs, Plates, Timber Decks etc.
For Longitudinal dispersal:
To allow for longitudinal dispersal through track:
To rail bearers, longitudinal
troughing, plate or timber
floor etc.
L (m)
FEUDL
<0.5
0.5
L+ 2.0
0.5 to 3.0
>3.0

Longitudinal timbers only
over cross girders
L (m)

0
up to 2.4

5

1.0

>2.4

FEUDL
0.60

L + 3. 6
6

1.0

Table 4.7
Live Load Factor For Dispersal Through Track - FEUDL
For Transverse distribution:
(i)

The effective width of longitudinal troughing, slabs or similar, carrying one track
load should be taken as shown in Table 4.8, but not greater than the actual
widths.
t (mm)
Up to 150
>150

Effective width (m)
3.0
3.6

Table 4.8
Effective Widths

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment
(ii)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 26 of 33

For longitudinal timber decks, barlow rails, old rails or similar, an effective width
of (1.8 + 0.004 t ) metres with a maximum of 3.0 metres.

4.3.3.2 Transverse Members
For Longitudinal dispersal:
(i)

Cross girder with cross sleepers and ballast.
For cross girders spaced at 1.8 metres or more, FA = 1.0.
For cross girders spaced at less than 1.8 metres the axle load should be
reduced in the ratio:
FA =

(ii)

Cross girder centres in metres
1.8

Equation 4.10

Cross girders with longitudinal timbers.
For cross girders spaced 1.5 metres or more, FA = 1.0
For cross girders spaced at less than 1.5 metres with longitudinal timbers
equal to or greater than 225 mm deep
FA =

Cross girder centres in metres
1.8

Equation 4.11

For cross girders spaced at less than 1.5 metres, with longitudinal timbers less
than 225 mm deep
FA =

Cross girder centres in metres
1.5

Equation 4.12

(iii)

Transverse reinforced concrete slabs, effective width = 1.8 metres or actual
width if this is less.

(iv)

The effective width of transverse members should be taken from Table 4.9.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001

Member Type

Page 27 of 33
Effective Width
(m)

Transverse Troughing
Cross sleeper track, sleepers in troughs
Cross sleeper track up to 150 mm depth of ballast below
underside of sleeper to top of troughing
Cross sleeper track more than 150 mm depth of ballast below
underside of sleeper to top of troughing
Longitudinal timber up to 150 mm deep directly on transverse
troughing
Longitudinal timber more than 150 mm deep directly on
transverse troughing
Transverse RC slabs

1.5
1.8
2.4
1.5
1.8

1.8

Transverse Timber decks
Chairs directly on the deck
Cross sleeper track up to 150 mm depth of ballast below
underside of sleeper to top of decking
Cross sleeper track more than 150 mm depth of ballast below
underside of sleeper to top of decking
Longitudinal timber up to 150 mm deep directly on decking
Longitudinal timber more than 150 mm deep directly on
decking

0.6
1.2
1.8
1.5
1.8

Table 4.9
Effective Width of Transverse Members
4.3.3.3 Dispersal through Ballasted Track
For sleepered track 50% of a wheel load may be assumed to be transmitted to the
sleeper beneath and 25% distributed to each of the sleepers on each side assuming a
sleeper spacing of 800 mm maximum. The load acting on the sleeper may be assumed
to be distributed uniformly over the ballast at the underside of the sleeper and over a
distance of 800 mm symmetrically about the centre line of the rail (or to twice the
distance from the centre of rail to the nearer end of the sleeper if less). A sleeper
width of 250 mm may normally be assumed. Dispersal through ballast or similar
granular fill may be taken at 15° to the vertical.
Where a flexible bridge floor such as flat or buckle plates is stiffened by rigid
members such as rail bearers, the relative flexibility of the floor construction may be

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 28 of 33

considered in the distribution of loading. For rail bearers a pressure, under nominal
live loading (including dynamic factor) and dead load, of up to a maximum of 1000
kN/m² may be assumed to occur over a width of 200 mm (or the stiff bearing width
of the rail bearer if greater). This pressure is reduced beneath the remainder of the
loaded area, as shown in Figure 4.16.
800mm
=

250mm

=

100%
50%

15°

25%

15°

1000 KN/m²
Maximum
200

Figure 4.16
Dispersal through Ballasted Track onto a flexible floor with Rail Bearer
4.3.4 Nosing
An allowance should be made for lateral loads applied by trains to the track due to
nosing which should be taken as two nominal loads spaced at 4.5 metres apart along
the track. Each load N should be taken as:
For all locomotives, passenger trains, and for freight vehicles where v does not
exceed 40 mph:
N=

0.72v

Equation 4.13

For freight vehicles where v exceeds 40 mph:
N=

28.8 + 2.56(v − 40 ) but not greater than 80

where:
N
v

is the value of each nosing force in kN;
is defined in Clause 4.3.2.2.

Equation 4.14

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 29 of 33

Nosing should be considered as acting in either direction at right angles to the track
at rail level and at a location so as to produce the maximum effect in the element
under consideration. For elements supporting more than one track nosing should be
applied to one track only. Nosing may be assumed applied wholly to one rail
corresponding with side contact from the wheel flange. Transverse distribution
equally between the rails may be assumed beneath sleepered track. Other than on
sleepered track, transverse distribution between members or longitudinal timbers
may be considered provided these members are adequately connected. The vertical
effects of nosing on elements supporting one rail only should be considered.
It may be assumed that 25% of the nosing load will be transmitted longitudinally to
each of the sleepers or track fastenings on each side assuming a sleeper or fastening
spacing of 800 mm maximum. No addition for dynamic effects should be made to the
nosing loads.
4.3.5 Centrifugal Load
Where the track on a Bridge is curved in plan, allowance for centrifugal action should
be made in assessing the elements, all tracks on the structure being considered
occupied. The nominal centrifugal load Fc in kN, per track acting radially at a height of
1.8 metres above rail Level should be calculated from the following formula:
P (v + 6 )
×f
50r
2

Fc =

Equation 4.15

where:
P
r
v
f

is the static axle load or equivalent uniformly distributed load for bending
moment as applicable;
is the radius of curvature of the track (in metres);
is the speed in mph as defined in Clause 4.3.2.2;
is a factor where for L less than 2.88 metres or v less than or equal to
75 mph:
f=

1.0

and for L greater than 2.88 metres and v t over 120 km/h:
f=

 v − 75  510

2.88 
1−  t
+1.751−


L 

 625  v

Equation 4.16

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

Page 30 of 33

where L is the loaded length of the element being considered.
The vertical effect of centrifugal load on elements supporting one rail such as
railbearers should be considered. This vertical effect may be reduced taking account
of any track cant that is present.
Centrifugal load may be dispersed using factor FEUDL or FA in Clause 4.3.3 as
applicable. No addition for dynamic effects should be made to the centrifugal loads.
4.3.6 Longitudinal Loads
Allowance should be made for loads due to traction and braking as given in Table 4.10
which are equivalent to 20 BSUs. Loads for a different number of BSUs may be taken
pro rata to these loads, using Equation 4.17, but not less than that applicable to
10 BSUs. Longitudinal loads should be considered as acting at rail Level in a direction
parallel to the tracks. No addition for dynamic effects should be made to the
longitudinal loads.
Load Arising From

Loaded Length L (m)

Traction (30% of load on
driving wheels)

up to 3
from 3 to 5
from 5 to 7
from 7 to 25
over 25
up to 3
from 3 to 5
from 5 to 7
over 7

Braking (25% of load on
braked wheels)

Longitudinal Load
(kN)
150
225
300
24 (L-7) + 300
750
125
187
250
20 (L-7) + 250

Table 4.10
Nominal Longitudinal Loads
(applicable to 20 BSUs of loading or RA10)
Longitudinal Load for X BSUs =

X
× Value from Table 4.10
20

Equation 4.17

For Bridges supporting ballasted track, up to one third of the longitudinal loads may
be assumed to be transmitted by the track to resistances outside the bridge
structure, provided that no expansion switches or similar rail discontinuities are
located on, or within, 18 metres of either end of the bridge.
Bridges and elements carrying single tracks should be assessed for the greater of the
two loads produced by traction and braking in either direction.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 31 of 33

Where a Bridge or element carries two tracks, both tracks should be considered as
being occupied simultaneously. Where the tracks carry traffic travelling
predominantly in opposite directions, the load due to braking should be applied to
one track and the load due to traction to the other. Bridges and elements carrying
two tracks in the same direction should be subjected to braking or traction on both
tracks, whichever gives the greater effect. Consideration should be given to braking
and traction acting in opposite directions producing rotational effects.
Where elements carry more than two tracks, longitudinal loads should be considered
as applied simultaneously to two tracks only.
Longitudinal loads may be reduced in accordance with Clause 4.3.8.
4.3.7 Load Combinations
All loads from rail traffic, including vertical loading with dynamic effects, nosing,
centrifugal and longitudinal loads should be considered to act simultaneously. Railway
live loads should be combined in accordance with Table 2.1 to produce the most
onerous effect in the element under consideration, except that nosing need not be
assumed to act simultaneously with centrifugal load on the same track.
4.3.8 Elements Supporting More Than One Track
Where an element supports more than one track, all tracks should be considered to
be loaded simultaneously. The track producing the most severe effect at the point
under consideration should be considered to be fully loaded. The remaining tracks
may be assumed to be loaded to 75% of the maximum value where this is specifically
approved by the Railtrack Director’s Nominee.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 32 of 33

4.3.9 Structures Carrying Light Rail Systems
Where an element supporting tracks used by mainline rolling stock, also supports
tracks used exclusively by light rail traffic, the Route Availability of the element should
be determined assuming the latter tracks to be loaded with Type RL Loading as
defined in BD 37/88: Loads for Highway Bridges. The dynamic factors appropriate to
RL Loading given in BD 37/88 should be adopted. Values of γ fL appropriate to live
load as given in Table 2.2 should be applied to all rail traffic.
It should be noted that the reduction factor for simultaneous loading of adjacent
tracks defined in Clause 4.3.8 is not applicable to Type RL Loading.
4.4
Other Live Loads
Other live loads in addition to dead, superimposed and railway live load may need to
be considered as required by Clauses 4.4.1 and 4.4.2. Loads and load combinations
should be derived according to BD 37/88.
4.4.1 Wind Loads
Wind loads need not normally be considered in assessment except for the following:


Superstructures exceeding 40 metres span, excluding masonry or brick
arches;



Piers exceeding 5 metres height in structural steel, wrought iron, cast iron or
timber where pier height is taken as the distance from soffit of the
superstructure at the pier to the base of the steel, wrought iron, cast iron or
timber construction.

In using BD37/88 the following may be assumed:
(i)

in BD 37/88 clause 5.3.2.1 it may be assumed that vc = 35 m/s;

(ii)

in BD 37/88 clauses 5.3.3.1.2 and 5.3.3.1.4, (a) superstructure without live load
need not normally be considered;

(iii)

in BD 37/88 clause 5.3.3 it may be assumed that q = 0.75 kN/m².

4.4.2 Temperature
Temperature changes should be considered where they result in load effects within
the superstructure. Differences in temperature between the top surface and other
levels in the superstructure need not be considered in assessment except for
continuous or rigid framed spans where the track bearing floor is monolithic or
composite with the primary members.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 4 - Loading for Assessment

RT/CE/C/025
Issue: 1
Date: February 2001
Page 33 of 33

4.5 Operational Safety Requirements
4.5.1 Track Twist
The deformation of the structure is required to be such that the twist of the track
does not exceed 0.0025 radians within any 3 metre gauge length. This criterion
should be considered under the passage of the most onerous real train for the
assessed RA number of the structure. This check should be carried out using a value
of γ fL = 1.0.
4.6 Accidental Loads from Vehicles
4.6.1 Bridges over Highways
Where agreed or instructed, railway Bridge supports should be assessed in
accordance with Highways Agency Standard BD60/94: The Design of Highway Bridges
for Vehicle Collision Loads for impact from errant road vehicles.
4.6.2 Intersection Bridges
Where it is necessary to consider impact on supports from train derailments beneath
a Bridge, applicable criteria should be agreed with the Railtrack Director’s Nominee.
4.6.3 Train Derailments on Bridges
Assessment for the effects of train derailments is not required unless agreed with or
instructed by the Railtrack Director’s Nominee.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment Of Underbridges
Section 5 - Steel & Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 1

CONTENTS
5. STEEL & WROUGHT IRON.................................................................................................1
5.1 General ................................................................................................................................1
5.1.1 Applicability.................................................................................................................1
5.1.2 Use of this Section and Appendix A ......................................................................1
5.2 Global Analysis ...................................................................................................................2
5.3 Material Properties............................................................................................................2
5.4 Partial Factors.....................................................................................................................2
5.5 Serviceability Limit State ..................................................................................................2
5.6 Structural Form..................................................................................................................3
5.7 Assessment Process..........................................................................................................3
5.8 Measured Deviations ........................................................................................................3
5.9 Fatigue ..................................................................................................................................3
5. STEEL & WROUGHT IRON
5.1 General
5.1.1 Applicability
This Section provides recommendations for the assessment of structural steel and
wrought iron superstructures. Where applicable this Section should be used for
structural steel and wrought iron elements within superstructures of different
materials, including composite bridges as covered by Section 8. This Section may be
used for structural steel and wrought iron elements within substructures where
required.
5.1.2 Use of this Section and Appendix A
This Section should be used in conjunction with Appendix A of this Code of Practice
for the detailed theoretical assessment of steel and wrought iron superstructures.
Unless noted otherwise references to Appendices A, B, C, D, E or F shall be taken to
mean Appendices A, B, C, D, E or F of this Code.
Appendix A is a set of additions and amendments to BS 5400: Part 3 (1982)
incorporating amendments numbers 1 and 2 with which it should be read as a
supplement. Clause numbers in Appendix A relate directly to those in BS 5400: Part
3. If a Clause does not appear in Appendix A then the original Clause, where
applicable, should be used. Clauses that appear in Appendix A but not in BS 5400:
Part 3 are additional Clauses applicable to assessment only.
The amendments enable strength aspects to be assessed that are not properly
covered by BS 5400: Part 3 which was written as a design code. When assessing
steel or wrought iron underbridges built before about 1960 using outmoded forms,

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment Of Underbridges
Section 5 - Steel & Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 2

assessing engineers should also consult the guidance in Appendix F which refers to
this Section 5 and Appendix A by specific cross reference.
5.2 Global Analysis
Plastic or yield line analysis is permitted at ultimate limit state for beams and for flat
plates provided the components are compact. This is relevant for stiffened or
unstiffened floor plates supporting ballasted track. Wrought iron elements are not
considered suitable for plastic global analysis.
5.3 Material Properties
Properties of materials should be determined from specified values, tests of the
material, or from mill test certificates. In the absence of this information then worst
credible strengths should be assessed as given by Appendix A.
5.4 Partial Factors
Values of the partial load factor γ fL should be obtained from Section 2. Values of the
partial material factor γ m , should be obtained from Appendix A of this Code of
Practice.
It is important to note that in Appendix A the factor γ f 3 is used in the resistance side
of the safety format as in BS 5400: Part 3 thus:
γ fL ⋅ QK ≤

fK
γf 3 ⋅ γm

Equation 5.1

γ f 3 is normally to be taken as 1.1 for the ultimate limit state (and 1.0 for the
serviceability limit state). γ f 3 may be taken as 1.0 where the following conditions are
all met.
(a)

Members are either:
(i)

Rail bearers or cross girders that are assumed to be simply supported;

(ii)

Main girders of bridges with maximum skew of 25°, and if continuous
any splices are welded, or made with HSFG bolts or rivets with cover
plates to both flanges.

(b)

Global analysis is based upon static distribution within the structure.

(c)

Geometric dimensions and properties of members are verified during the
inspection for assessment.

5.5 Serviceability Limit State
The Serviceability Limit State (SLS) need not normally be checked, except where
relevant for:


stiffened floor construction that forms part of a girder flange;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment Of Underbridges
Section 5 - Steel & Wrought Iron


certain trusses;



HSFG joints in continuous structures.

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 3

Where particular serviceability problems have been experienced or have been
reported, the serviceability limit state should be checked. Examples of serviceability
problems could include excessive:


deflection or twist;



vibrations;



movement at joints.

5.6 Structural Form
General guidance on common structural forms of steel and wrought iron rail
underbridges, especially on outmoded forms built before about 1960 is given in
Appendix F. Information is given on the approach for features of construction that
are not covered specifically by Appendix A, such as buckle plates.
The guidance in Appendix F is applicable to most types of underbridges. However,
there will be instances where a bridge may incorporate features of construction or
behaviour which are not covered. In these cases judgement may be required as to
the assessment procedure. In general it would is expected that assessment engineers
would seek guidance, but that the assessment would use the basis of this Code of
Practice and, in particular, Section 5 and Appendix A for the individual elements of
steel and wrought iron.
5.7 Assessment Process
The Levels of assessment are defined in Section 1. For Levels 1 & 2 the
recommended steps to be followed when carrying out assessment of the different
member types such as floor plates, railbearers, cross girders and main girders in
forms including plate girders or box girders whether riveted or welded is included in
Appendix F. At Level 3 the process should be similar to Level 2 results of actual
material or load tests are used and real train loadings are assumed.
5.8 Measured Deviations
Appropriate measured deviations from intended geometry should be taken into
account in the assessment. In particular, the bow in main girders and verticality of
webs and supports should be taken into account.
5.9 Fatigue
Fatigue should be considered in accordance with Appendices A and D.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 25

CONTENTS
6. MASONRY ARCHES...............................................................................................................2
6.1 General ................................................................................................................................2
6.1.1 Applicability.................................................................................................................2
6.1.2 Levels of Analysis .......................................................................................................2
6.1.3 Assumptions................................................................................................................2
6.1.4 Loading and Load Distribution................................................................................2
6.1.5 Material Properties....................................................................................................5
6.1.6 Skewed Arches...........................................................................................................5
6.1.7 Permissible Capacity .................................................................................................6
6.2 Single Span Structures ......................................................................................................9
6.2.1 General ........................................................................................................................9
6.2.2 The MEXE Method of Assessment.......................................................................10
6.2.3 Other Methods of Analysis....................................................................................20
6.2.4 Advanced Analysis Methods ..................................................................................22
6.3 Multispan Structures .......................................................................................................22
6.3.1 Modes of Failure.......................................................................................................22
6.3.2 Analysis for Failure Mode (i)..................................................................................23
6.3.3 Analysis for Failure Mode (ii).................................................................................23
6.3.4 Assessment................................................................................................................24
6.3.5 Advanced Assessment Methods ...........................................................................25
6.4 Spandrel Walls..................................................................................................................25
6.5 Jack Arches........................................................................................................................25

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 25

6. MASONRY ARCHES
6.1 General
6.1.1 Applicability
This Section should be used for the assessment of single and multiple span brick
and/or stone arch superstructures. The methods described are also applicable to
mass concrete arches. Recommendations for the assessment of the arch barrel, the
piers of multi-span structures, and spandrel walls are included. Whilst it is recognised
that spandrel walls can enhance the capacity of the arch barrel, the techniques
presented other than three dimensional finite element analysis are not able to take
this enhancement into account. Although considered on a purely qualitative basis,
spandrel walls may affect the decision governing the permissible capacity/speed for
the Bridge.
Abutments, which are defined as those parts of the end supports below springing
level, should be assessed on a qualitative basis in accordance with Section 11.
6.1.2 Levels of Analysis
For both single and multispan structures, analysis methods applicable to the three
Levels of assessment identified in Section 1 are presented. Assessment should be
carried out at the lowest Level possible. Higher Levels should be adopted only
where, on assessment at a lower Level, the Bridge does not meet the required load
carrying criterion.
6.1.3 Assumptions
Various assumptions are implicit in the methods of analysis presented. In addition the
assessing engineer may need to make further assumptions regarding the nature of the
structure being considered. The validity of all assumptions should be properly
considered, and the assessing engineer should be aware of the sensitivity of the
assessed capacity to all assumed parameters (such as the extent of backing in the
haunch area at supports; the level of passive pressure exerted on the arch by the fill).
6.1.4 Loading and Load Distribution
The assessment loading should be a single axle or group of axles based on the
Type RA1 load train defined in Section 4, or representing specific trains as directed by
the Railtrack Director’s Nominee. Loads should be distributed from the base of the
sleeper through the fill onto the arch barrel at a slope of 1 horizontally to 2 vertically
in the longitudinal direction (see Figure 6.1), and 1 horizontally to 1 vertically in the
transverse direction (see Figure 6.2). Alternative distribution models may be adopted
subject to approval in accordance with Railtrack’s Technical Approval Procedures.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 3 of 25

In determining the width of the arch barrel that is effective in carrying the load, due
account should be taken of significant longitudinal cracks.
Due account should also be taken of the vertical effects of nosing and centrifugal
action (on curved track), determined in accordance with Section 4. The effects may
result in an uneven distribution of load on the arch barrel. In such instances, loads
should be distributed on to the arch barrel in accordance with Figure 6.3, and the
analysis carried out considering the portion of the barrel associated with the more
heavily loaded rail. Nosing may be assumed to be shared by the two adjacent
sleepers, as detailed in Clause 4.3.4.

P
Rail

Sleeper
1
2

P/4

P/2

P/4

Ballast/fill

Arch
barrel

Loads may be distributed from the
underside of each sleeper, through
the fill onto the arch barrel at a
slope of 1 horizontal to 2 vertical

Figure 6.1
Longitudinal Distribution of Live Load Through Fill

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

C

Rail

C

Rail

C

Rail

400 400

Page 4 of 25

C

Rail

400 400

Ballast/fill

Spandrel

1

Angle of
1 dispersal
Arch barrel

Effective width

a) No longitudinal cracks in arch barrel.

C

Rail

C

Rail

C

Rail

400 400

C

Rail

400 400

Ballast/fill

Spandrel

1

Angle of
1 dispersal
Arch barrel

Longitudinal cracks
b) Arch barrel cracked.

Effective width

Figure 6.2
Transverse Distribution of Live Load Through Fill

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

P2

Page 5 of 25

P1

Spandrel wall

1

Angle of
1 dispersal
Arch barrel

L2

L1

Effective widths

Note: P1 > P2 due to centrifugal action.
Assessment to be based on a strip of width
L 1 subject to a force P1.
Figure 6.3
Transverse Distribution of Live Load Through Fill
Considering Centrifugal Action
6.1.5 Material Properties
For methods other than the Military Engineering Experimental Establishment (MEXE)
method it may be necessary to make an assessment of the strength of the masonry
from which the arch barrel is constructed. Figures 6.4 and 6.5 give typical
compressive and characteristic strengths to be expected for various types of masonry
by brick or stone type and mortar. Strength tests, if considered necessary, should be
carried out on the individual components. BS 5628 and TRRL Contractor
Report 244: Masonry Properties for Assessing Arch Bridges give information on suitable
tests and strengths.
6.1.6 Skewed Arches
The assessment of skewed arches may be undertaken using a two dimensional
analysis for skews up to and including 35°. The span length should be taken as the
clear distance between abutment faces measured on the skew (see Figure 6.6). For
skew angles greater than 35° a three dimensional analysis should be undertaken.
Where, however, the applied live loads are located at a significant distance from the
edges of the Bridge (for example tunnels) the assessment may be based on a two
dimensional analysis of the square span.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 25

For skewed multispan structures, the analysis should be based on the skew spans and
pier widths. Such structures may exhibit defects resulting from out of balance lateral
thrusts at supporting piers (“racking” effects) and torsional effects. As it is difficult to
assess these effects quantitatively, skewed multispan arches should also be subject to
a qualitative assessment, which should consider the following:


whether the capacity of the structure is at risk owing to unknown implications
of the defects;



whether monitoring of the structure would assist assurance of the safety of
the structure under rail traffic.

The implications of both the quantitative and qualitative assessments should be
considered in determining the safe load capacity of the structure.
6.1.7 Permissible Capacity
When using the MEXE method, the safe load capacity should be determined by
comparing the assessed permissible axle capacity with the single axle weight in the
group indicated in Table 4.3. It should be noted that this can only be taken as a guide
to the RA number of any arch since the assumptions made in the development of the
capacity formulae apply to axle spacings of 2.0 metres.
When using methods other than MEXE to determine the permissible capacity from
the Provisional Axle (Ultimate) Capacity, the partial factors noted below should be
used. Assessments carried out by these methods should be based on the actual
thickness of the arch barrel (that is a Geometric Factor of 1).
Dead and Superimposed Dead Loads
Arch Fill
Live Loads

γ fL - Refer to Table 2.2.

γ fL = 1.2
γ fL = 1.4
γ f 3 = 1.35
γ m = 1.0

A dynamic factor of 1.8 should be applied to a single axle load or to the critical axle of
a train of loads. Where the depth of ballast and fill from the underside of sleeper to
the crown of the arch ring is greater than 600 mm, consideration may be given to
reducing the value of the dynamic factor. Any alternative value proposed should be
agreed in accordance with Railtrack’s Technical Approval Procedures.
In certain circumstances permanent loads on the arch barrel may generate relieving
effects. To ensure that the assessment does not produce an unconservative result,
the analysis should also be undertaken with γfL = 1.0 for all such permanent effects,
and the RA number determined from the lower calculated capacity.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 7 of 25

12

10

1:2:8 Mortar Designation (iv)

8

fK
(N/mm2)
6

1:3 Lime Mortar
4

2

0
0

10

20

30

40

50

60

70

80

2

Compressive Strength of Unit (N/mm )

(Material Designations as defined in BS 5628)

Figure 6.4
Characteristic Strength of Normal Brick Masonry fk

90

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 8 of 25

20

Ashlar

15

Squared Rubble with 1:2:8 Mortar

fk
(N/mm2)
10

Random Rubble with Lime Mortar

5

0
20

40

60

80

100

120

140

160

180

2

Compressive Strength of Unit (N/mm )

(Material Designations as defined in BS 5628)

Figure 6.5
Characteristic Strength of Normal Stone Masonry fk

200

220

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 9 of 25

In determining the load carrying capacity of the Bridge by any of the methods
presented, due account should be taken of known defects and any deterioration
identified during inspection.

C

s
cro

Spandrel
wall

sed
Skew angle of
Bridge <35°

Face of
abutment

C

Bridge

Span length to be adopted
for Assessment

Figure 6.6
Definition of Span Length for the Assessment of Skewed Arches with
Skew ≤ 35°°
6.2 Single Span Structures
6.2.1 General
Assessment of single span structures may be carried out using the MEXE method
described in Clause 6.2.2, or by one of the alternative methods given in Clause 6.2.3.
The MEXE method is approximate and may only be adopted where:
(i)

the clear span is less than 19.8 metres (see Figure 6.6 for skew arches);

(ii)

the arch barrel is not severely deformed;

(iii)

there is no evidence of significant ring separation for those rings under
consideration;

(iv)

the arch does not support internal spandrel walls with vaulted construction.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 10 of 25

6.2.2 The MEXE Method of Assessment
6.2.2.1 General
The MEXE method of assessment assumes that the loading on arches is represented
by that from the most common heavy freight wagons, such as 100 ton wagons with
2.0 metre bogie axle spacing, or two axle type wagons.
Figures 6.7 to 6.12 are enveloping curves giving the provisional axle capacities of
arched bridges. The envelope curves have been based on the application of single
axles, bogie axles and adjacent bogies of coupled vehicles to the spans, and should be
assumed to allow for dynamic effects. For this reason the values read from the
curves may in some cases vary from calculated values by ± 2%.
For each arch the permissible axle capacity in tonnes should be determined from the
provisional axle capacity by modifying it using the factors given in Clause 6.2.2.3.
6.2.2.2 Provisional Axle Capacity (QP)
Provisional axle capacity (QP) for arches of different spans, ring thickness and depths
of fill may be obtained from Figures 6.7 to 6.12, where h is the depth of fill beneath
the sleeper soffit and arch ring.
The provisional axle capacity obtained using this method is generally less than the
actual capacity. It should not be taken as more than an indication that the arch may
be suitable to carry rail traffic for the following reasons:


minimum longitudinal distribution of loading has been assumed;



only that part of the arch under the track has been assumed to carry load;



the adjustment factors for condition are subjective.

In marginal cases, justification of a higher capacity may be possible through further
consideration of the above factors, or by means of other, more refined methods of
assessment.
Where the characteristics of an arch are such that graph readings require
interpolation, it may be more convenient to calculate the provisional axle capacity
directly, using the formulae given in Appendix F.
For an arch carrying more than one track, the axle capacity for the portion under
each track should be assessed as if it were a separate arch.
6.2.2.3 Modifying Factors for Provisional Axle Capacity
The provisional axle capacity should be adjusted to give the permissible axle capacity
by taking into account various characteristics of the arch such as shape and condition,
using the factors defined below.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 25

The actual conditions of the bridge being assessed may not exactly align with
conditions defined below. The selected values of the factors should be those that
approximate nearest to the conditions of the arch being assessed.
The factors allocated to the various characteristics are as follows:


Profile Factor (KP)
This factor is obtained from Figure 6.13.



Shape Factor (KS)

For parabolic arches KS = 1.0. For all other arch shapes this factor is obtained from
Figure 6.14.


Material Factor (KM)
Soft brick and soft stone
Hard brick
Mass concrete
Masonry

1.0
1.2
1.2
1.5

It is permissible to interpolate for particular types of brick and stone.
In allocating material factors the possibility of previous relining should be considered.


Condition Factor (KV)
(a)

Brick

Good condition - No spalling
Fair condition, Slight spalling (between 0% and 25% of
arch surface) and no bricks missing
Poor condition, Significant spalling (over 25% of arch
surface) and/or bricks missing

1.0

0.9

0.8

If a large number of bricks are missing (over 10% of the arch surface) or the
joints are only partly filled with mortar, or the jointing material is in a very
poor and deteriorated condition, the arch should be treated as having one less
ring when obtaining the provisional capacity (QP).
(b)

Stone

Good condition - No spalling
Fair condition, Slight spalling (between 0% and 25% of arch
surface)
Poor condition, Significant spalling (over 25% of arch surface)
but no stones loose or missing
Some stones loose or missing, severe loss of jointing material in
undressed stone arches

1.0
0.9
0.8
0.75

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 12 of 25



Crack Factor (KC)

a)

No cracks

1.0

b)

Longitudinal cracks:
outside the centre third of the arch, less than one tenth
of the span in length

0.95

outside the centre third of the arch, longer than one tenth of the
span in length

0.90

within the centre third of the arch, less than one tenth of the
span in length

0.90

within the centre third of the arch longer than one tenth of the
span in length

0.85

c)

Lateral and diagonal cracks:
up to three small lateral or diagonal cracks less than
3 mm in width and less than one tenth of the arch width

numerous small cracks as above in the centre third of the arch


0.90
0.60

Deformation Factor (KD)
Deformation of the arch may be due to partial failure of the arch ring or
movement of the abutments. It should not be mistaken for irregularity due to
inaccurate falsework centres being used during construction.
If the deformation has resulted in a flat section, the final axle capacity of the
bridge should be reduced, using the Deformation Factor (KD), in the
proportion of rise at centre of flattened portion to original rise at this point.
KD = Rise at centre of flattened portion
Original rise at the same point

6.2.2.4 Permissible axle capacity (CF)
The permissible axle capacity should be obtained by multiplying QP by all the above
factors as applicable.
Permissible axle capacity = QP x KP x KS x KM x KV x KC x KD
Any number of axles up to the permissible capacity may be permitted.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 13 of 25

70

60

50

QP
(t)

40

h=900 mm
30
h=750 mm

h=600 mm

h=450 mm
20

h=300 mm

h=150 mm
10
2

3

4

5

6

7

8

9

10

Span L (m)

Figure 6.7
Arch Ring Thickness d at the Crown = 300 mm

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 14 of 25

70

60

50

QP
(t)

40

h=900 mm

h=750 mm
30
h=600 mm

h=450 mm

20
h=300 mm

h=150 mm

10
3

4

5

6

7

8

9

10

11

Span L (m)

Figure 6.8
Arch Ring Thickness d at the Crown = 400 mm

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 15 of 25

70

60

50

h=900 mm
QP
(t)

40

h=750 mm

h=600 mm

30

h=450 mm

h=300 mm
20

h=150 mm

10
4

5

6

7

8

9

10

11

12

13

Span L (m)

Figure 6.9
Arch Ring Thickness d at the Crown = 500 mm

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 16 of 25

70

60

50
h=900 mm

h=750 mm
QP
(t)

40
h=600 mm

h=450 mm

30
h=300 mm

h=150 mm
20

10
6

7

8

9

10

11

12

13

14

15

16

Span L (m)

Figure 6.10
Arch Ring Thickness d at the Crown = 650 mm

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 17 of 25

80

70

60

h=900 mm

h=750 mm
QP
(t)

50
h=600 mm

h=450 mm
40

h=300 mm

30

h=150 mm

20
7

8

9

10

11

12

13

14

15

16

17

18

Span L (m)

Figure 6.11
Arch Ring Thickness d at the Crown = 800 mm

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 18 of 25

80

70
h=900 mm

h=750 mm
60

h=600 mm

h=450 mm
QP
(t)

50

h=300 mm
40

h=150 mm
30

20
8

9

10

11

12

13

14

15

16

17

18

19

20

Span L (m)

Figure 6.12
Arch Ring Thickness d at the Crown = 1000 mm

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 19 of 25

1.0

0.9

KP 0.8

0.7

0.6
3

4

5

6
Span Rise Ratio L/RC

Figure 6.13
Profile Factor KP

7

8

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 6 - Masonry Arches

Page 20 of 25

1.0

KS 0.5

0.0
0.75

0.80

0.85

0.90

0.95

1.00

¼ Point / Centreline Rise Ratio Rq/Rc

Figure 6.14
Shape Factor
6.2.3 Other Methods of Analysis
6.2.3.1 General
The following methods of analysis may be adopted where, for the reasons given in
Clause 6.2.1, the MEXE method is considered inappropriate:
(i)

Mechanism methods which consider the failure of the arch by the formation of
hinges (generally four - see Figure 6.15) close to the extremities of the arch
ring. An iterative process is required to determine the critical position for
hinge formation and the associated minimum load, which is considered to be
the ultimate capacity of the arch.

(ii)

Elastic methods which, by the incremental application of loads, allow the area
of the arch ring to be modified as tension develops and masonry yields (see
Figure 6.16). The application of load is continued until, ultimately, sufficient
hinges form to cause collapse as a mechanism. This form of analysis may be
carried out using classic elastic theory or by a finite element approach.

Computer software for assessment by any method other than MEXE should be
approved by the Professional Head of Structures prior to its use.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 21 of 25

P

Denotes
hinge

Figure 6.15
Single Span Failure Involving the Formation of Four Hinges

Tension zone
Crushed zone

Load applied
incrementally

The arch ring is modified after
each iteration to take account of
crushed and tension zones.

Figure 6.16
Thinning of Arch Ring During Elastic Analysis

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 22 of 25

6.2.3.2 Limitations
Where the span/rise ratio of the arch is greater than 6 and the clear span exceeds
15 metres, the failure mode may be a ‘snap through’ involving the formation of a
three, rather than four, hinged mechanism. As this form of failure involves the gross
deformation of the arch in the vicinity of the load, mechanism methods, and elastic
methods which do not take account of geometric non-linearity should not be used.
Care should be exercised in modelling the soil/structure interaction of deep arches,
where it may be possible to mobilise sufficient passive pressure to eliminate the
horizontal thrust at the springing furthest from the point of loading.
6.2.4 Advanced Analysis Methods
The following methods of analysis may be applicable when a more detailed
investigation is required:
(i)

plane strain, (two dimensional finite element analysis);

(ii)

full three dimensional finite element analysis using curved shell or other
appropriate elements.

Method (i) allows the analysis of problems such as ring separation to be investigated
and may allow some advanced soil models to be adopted.
Method (ii) should only be used where specific structural problems warrant such
complex analytical techniques, and when approved by Professional Head of Structures
Engineering in accordance with Railtrack’s Technical Approval Procedures.
6.3 Multispan Structures
6.3.1 Modes of Failure
In considering the ultimate capacity of multispan arches two distinct modes of failure
should be examined:
(i)

the collapse of a single span which is not modified by the presence of an
adjacent span;

(ii)

the interaction of adjacent spans leading to partial or total collapse of the
structure.

Mode (i) predominates where intermediate piers are sufficiently stocky to ensure that
the structure can be considered as a series of independent spans, and may be
considered to apply when:
H
≤ 2
t

Equation 6.1

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 23 of 25

Mode (ii) may be considered to apply when:
H
>2
t

Equation 6.2

where h and t are as defined below and shown in Figure 6.17:
t
H

is the pier thickness;
is the height of the pier above its foundation level.

t
H

Figure 6.17
Definition of Parameters h and t
6.3.2 Analysis for Failure Mode (i)
Where the minimum actual pier thickness is greater than H 2 , each span of a
multispan structure may be analysed using an applicable single span method.
6.3.3 Analysis for Failure Mode (ii)
Interaction of adjacent spans should be considered where the pier thickness is less
than or equal to H 2 . Analysis may be carried out in these cases on the basis of:
(i)

An equilibrium approach, in which a zone of thrust is traced throughout the
structure which is in equilibrium with the applied forces. Redundancy may be
dealt with by specifying the location, direction and magnitude of the thrust at
the highest point of each arch;

(ii)

A mechanism analysis, which for a multispan structure, is generally the
formation of seven hinges within two adjacent spans and the intermediate pier
as shown in Figure 6.18. Where span lengths and pier heights vary, the
analysis should consider successive pairs of adjacent spans.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 24 of 25

Denotes hinge position

Figure 6.18
Twin Span Failure Involving the formation of Seven Hinges
Notwithstanding the limit of pier height to thickness in Clause 6.3.1, the capacity of an
individual span can still govern the overall capacity of the structure, and therefore,
single span analysis should also be undertaken.
The safe load capacity of the structure should be determined from the lowest
capacity calculated from single and multispan assessment.
6.3.4 Assessment
In assessing a multispan arch structure using either of the analysis methods outlined in
Clause 6.3.3 due account should be taken of the following features:
(a)

Haunching
The capacity of multispan arches can be greatly influenced by the extent of
haunching (or backing) above the intermediate piers. In the absence of
definite information the minimum level of haunching should be taken to be the
level where the extrados crosses the vertical through the intrados at the
springing point, unless there are features or other structural evidence to the
contrary. No minimum haunching level at abutments should be assumed
without some evidence of its presence;

(b)

Voided Elements of the Structure
Allowance should be made for reduced weight over or within the piers which
may arise due to the presence of voids such as internal spandrel walls with
vaulted construction shell piers, or piers with cutouts;

(c)

Piers with Cut Outs

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 6 - Masonry Arches

RT/CE/C/025
Issue: 1
Date: February 2001
Page 25 of 25

Consideration should be given to the increased width of thrust zone that may
arise due to the variation in cross sectional area throughout the height of the
pier;
(d)

Foundations
In the absence of definitive information, the top of each pier foundation may
normally be assumed to be 0.5 metres below ground level;

(e)

Lateral Loading on Piers
Lateral loading resulting from soil or surcharge pressure acting on the face of
the piers, such as may arise from a significant difference in ground levels on
opposite sides of a pier, should be considered in the analysis.

6.3.5 Advanced Assessment Methods
For multispan structures where the methods outlined in Clauses 6.3.2 and 6.3.3 are
not applicable, structural modelling using plane (two dimensional) strain finite element
analysis may be considered.
6.4 Spandrel Walls
Although spandrel walls affect the load carrying capacity of arch bridges, current
analytical techniques do not satisfactorily model their interaction with the arch barrel.
Consequently, these elements should be assessed qualitatively by considering their
condition and the significance of any defects.
6.5 Jack Arches
Jack arches, which span between supporting members to form the deck of metallic
beam bridges, should be assessed qualitatively. Details of an empirical approach
which may be appropriate are contained in Bridgeguard 3 Current Information
Sheet 22. Prior to the use of this method, the assessing engineer must satisfy himself
as to it’s appropriateness to the structure under consideration.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 7 - Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 5

CONTENTS
7. CONCRETE STRUCTURES..................................................................................................1
7.1 Introduction........................................................................................................................1
7.1.1 Scope ............................................................................................................................1
7.1.2 Background and Origins ...........................................................................................1
7.2 Assessment of Strength....................................................................................................2
7.2.1 General ........................................................................................................................2
7.2.2 Material Strengths......................................................................................................2
7.2.3 Damaged and Deteriorating Structures................................................................4
7.3 Serviceability and Other Considerations......................................................................5
7. CONCRETE STRUCTURES
7.1 Introduction
7.1.1 Scope
This Section should be read in conjunction with Appendix B for the assessment of
concrete rail underbridges and elements of rail underbridges. It covers elements of
plain, reinforced and prestressed concrete construction, including both pre- and posttensioned prestressed concrete construction with internal bonded tendons. It does
not cover in-fill joist type structures such as the decks of Z-type bridges that are
considered to be composite, for which Section 8 should be used. It does not cover
elements with external or unbonded tendons.
7.1.2 Background and Origins
Appendix B is based on BD 44/95: The Assessment of Concrete Highway Bridges and
Structures. Acknowledgement is made to the Highways Agency for allowing Standard
BD 44 to be used. This document was being drafted at the same time that a revision
to BD 44 was being prepared, there are inevitably some differences that are not
directly related to the differences between bridges carrying roads and railways.
The Highways Agency Advice Note BA 44: The Assessment of Concrete Highway
Bridges and Structures provides guidance on the use of BD 44/95 much of which is also
relevant in this Section. However, where it gives alternative approaches which will
frequently be used in the assessment of railway underbridges, the relevant clauses
have been included here.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 7 - Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 5

7.2 Assessment of Strength
7.2.1 General
The objective of Section 7 is to provide recommendations in order that a realistic
assessment of the strength of concrete elements may be obtained. This is in part
achieved by using measured values of parameters which at the design stage could have
only been predicted. Section 7 enables the following aspects to be considered:
(a)

actual strength of steel and concrete;

(b)

details and designs not complying with the recommendations of BS 5400:
Part 4;

(c)

less conservative assessments of some aspects compared to the interpretation
used in design codes, or where better information has become available since
these design standards were written.

In addition, equations which were originally written in true ‘design’ format, giving
required steel area as a function of required design strength, have been rearranged in
assessment format, giving strength as a function of steel area.
7.2.2 Material Strengths
7.2.2.1 Worst Credible Strength
The term worst credible strength has been used to allow a conservative estimate of
the actual material strength of structures and structural elements to be used for
assessment. Worst credible strength can be defined as the worst value of that
strength which the engineer, based on experience and knowledge of the material,
realistically believes could be present in the structure or element being considered.
This value may be greater or less than the characteristic strength of the material
assumed at the design stage. Since this value eliminates some of the uncertainties
associated with the use of characteristic strengths, reductions may be made in the
partial factor for material γ m .
The use of worst credible strengths should be considered in the following
circumstances:
(i)

when an initial assessment using characteristic, assumed or recorded values
has not shown the element to be capable of carrying the full assessment
loading to Section 4;

(ii)

if a structure has suffered damage or deterioration in such a way that the
actual strengths are, or are thought to be, less than assumed characteristic
values;

(iii)

if there is reason to believe the original materials were deficient and below
intended characteristic values;

(iv)

where no information exists on the values used in design or recorded in
construction.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 7 - Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 5

The worst credible value should generally be taken as the lower bound value of the
estimated in situ strength for the element under consideration. Advice on the
assessment of worst credible strength is given in BA 44.
7.2.2.2 Assumed, Specified and Recorded Strengths
It is usually desirable to undertake initial assessments at Level 1 without undertaking
the material tests required to determine worst credible strengths. Guidance on
material which may be used are given below at (a), (b) and (c). The strengths taken
should be treated as characteristic strengths and used with the applicable γ m as
defined in Clause 7.2.2.3.
Where assessment using these values does not give the required element strength,
consideration should be given to taking samples and using worst credible values
instead. However, before undertaking the tests, the sensitivity of the assessed
strength to assumed material strength should be investigated. In particular, the
strength of lightly reinforced members is often very insensitive to the strength of the
concrete.
(a)

Concrete. Pre-1939 concrete may be assumed to have a characteristic
strength not greater than 20 N/mm². The strength of more modern concrete
is often specified on the record drawings. Where a characteristic strength is
specified, this value may be used directly. Where concrete strength has been
defined in terms of a 28 day minimum cube strength, this value may be
assumed to be equivalent to characteristic cube strength. It may be noted,
however, that for statistical reasons this is theoretically a conservative
assumption. Less modern concretes also tended to gain strength with age
more than modern concrete. Scope for obtaining a higher strength in
accordance with Clause 7.2.2.1 may be considerable.
Nominal mixes prescribed by constituents and grades designated by standard
letters were frequently used. Information on some of these is given in
Appendix B1. Where concrete strengths from Appendix B1 are used, they
should be treated as characteristic strengths.
The mix design of prestressed elements, particularly pre-tensioned ones, is
often controlled by the requirement to achieve transfer at an early age. This
also often resulted in 28-day strengths well above specified values.
If construction records include 28-day cube results for concrete which is
representative of the critical areas, these values may be used. They should be
processed in the same way as core results are processed to obtain a
representative value in accordance with BA 44. Alternatively, if the number of
test results available exceeds 6, the actual standard deviation of the test
results may be calculated and the representative value taken as the mean
minus 1.65 times the standard deviation. Because of the difference between
British Standard wet cured cubes and in situ concrete, the reduction in
uncertainty is less than for in situ core tests so the representative value

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 7 - Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 5

obtained from records should therefore be treated as a characteristic, rather
than a worst credible value.
Judgement is needed in using the results of non-standard cube tests. Older
age tests and tests on cubes stored with the structure may be used.
However, because the early age strength gain of cements varies significantly,
extrapolation of early age cube results is not normally reliable.
(b)

Reinforcement and Prestressing Steel - In the absence of definite information,
a characteristic reinforcing steel strength of 230 N/mm² may be assumed for
steel produced before 1961. Strengths of more recent reinforcement and
prestressing steels can be obtained from contemporary standards.
If construction records include actual test results for representative steel, the
test results may be used. It should be noted that tests on one size of bar are
unlikely to be representative of the strength of bars of another size.
Where test results are used, they can be processed in the same way as tests
on samples taken from the structure to obtain a representative value in
accordance with BA 44. This value may be taken as the worst credible value.

7.2.2.3 Partial Factor for Materials, γ m
The values of γ m for concrete and reinforcement or prestressing steel should be as
given in Appendix B. The values for use with characteristic and worst credible
strengths are different. To enable the correct value of γ m to be used, all limiting
criteria are expressed in Appendix B as formulae with γ m stated explicitly, rather
than as tabulated values.
7.2.3 Damaged and Deteriorating Structures
The strength prediction methods in Appendix B assume that the structure is in the
‘as-built’ condition. It will often be necessary to assess damaged or deteriorated
structures. Some general principles for taking damage and/or deterioration into
account are outlined below.
It is not normally appropriate to apply overall ‘condition factors’ (such as are used
with the ‘MEXE’ method for masonry arches) to concrete structures. It is preferable
to attempt to quantify the damage/deterioration in particular areas and allow for it in
the assessment calculations. Where the deficiency takes the form of weak materials,
the use of worst credible strength based on tests on samples from the affected areas
automatically takes account of this and further allowance is not required.
The commonest form of damage to concrete structures is caused by corrosion of the
embedded steel. Where local corrosion is suspected, an estimate of the loss of steel
section is required. For reinforced concrete structures, the reduced area of steel can
be used directly in assessment. If the loss of area in a specific bar is taken to be

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 7 - Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 5

greater than 30%, consideration should be given to the resulting reduced ductility of
the bar as well as its reduced strength.
Bonded prestressing wires are likely to break due to prestress load when the loss of
section reaches approximately 40%. Hence, when it is estimated that the amount of
loss has reached 30% of the area, the relevant wires should be ignored in the
assessment calculations. For post-tensioned tendons, provided the grout is in good
condition, even whole tendons that have been broken are able to re-anchor in lengths
of the order of one metre. The position of individual breaks as well as their number
is therefore important. In assessing the strength of a structure allowing for damage to
tendons, it is very important to be aware that the assumed loss can only ever be very
approximate and to allow for upper and lower bounds to the actual loss. The bounds
may be very far apart.
For the more easily detected and common form of corrosion of reinforcement, the
loss of steel area is not normally the dominant factor. The corrosion products (rust)
occupy a greater volume than the parent steel causing cracking and eventually spalling
of the cover concrete. A loss of bond strength occurs when the cover concrete
cracks. The significance of the loss should be considered where reinforcement details
are sensitive to bond.
Where the cover concrete has seriously delaminated or spalled off over the whole
length of a particular reinforcing bar, that bar should be disregarded in assessment.
However, delaminating and spalling in the critical flexural area is less significant if the
bar is anchored at either end of the damaged area. Anchorage may be present due to
the local nature of the damage or because the ends of the bars are bent up or
hooked. In such cases, the bar may be considered to contribute to the element
strength with due allowance for loss of section or incomplete anchorage as
appropriate.
7.3 Serviceability and Other Considerations
Assessment is normally carried out for the ultimate limit state only. Appendix B does
not give criteria for serviceability and deflection. Where checks for serviceability are
required by the Railtrack Director’s Nominee, the criteria should be agreed with him.
The initial approach is usually to take criteria from BS 5400: Part 4. However,
calculated stresses and cracks widths that exceed these criteria frequently do not
require immediate action and the requirements should be agreed with the Railtrack
Director’s Nominee.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 8 - Composite (Steel or Wrought Iron with Concrete)

Page 1 of 1

CONTENTS
8. COMPOSITE (STEEL OR WROUGHT IRON WITH CONCRETE)...........................1
8.1 General ................................................................................................................................1
8.2 Use of Appendix C............................................................................................................1
8. COMPOSITE (STEEL OR WROUGHT IRON WITH CONCRETE)
8.1 General
This Section provides recommendations for the assessment of composite elements of
rail underbridges involving components of steel or wrought iron interconnected with
concrete. Assessment of the steelwork, wrought iron and concrete elements should
be carried out using Sections 5 and 7 of this Code augmented by this Section where
the materials act compositely.
8.2 Use of Appendix C
This Section should be used in conjunction with Section 5 and 7 of this Code of
Practice. Appendix C contains relevant clauses and appendices as a set of additions
and amendments to BS 5400: Part 5: 1979 incorporating amendment no 1 dated 31
May 1982, and should be read as though it is a supplement to BS 5400: Part 5.
References within Appendix C to “this standard” or “that part” should therefore be
taken as referring to Appendix C.
Where there is no addition to a clause in BS 5400: Part 5, the existing design clause
should be used. It should be noted that some clauses in BS 5400: Part 5, for example
those dealing with construction aspects or recommended forms are generally not
applicable to assessment and should be ignored.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 9 - Cast Iron

Page 1 of 6

CONTENTS
9. CAST IRON...............................................................................................................................1
9.1 General ................................................................................................................................1
9.2 Material Properties............................................................................................................1
9.3 Strength................................................................................................................................1
9.3.1 Permissible Stresses...................................................................................................1
9.3.2 Beams Continuously Restrained by Surrounding Fill..........................................2
9.3.3 Beams with Intermediate Lateral Restraints ........................................................2
9.3.4 Compression Members ............................................................................................2
9.3.5 Members subject to Bending and Axial Compression .......................................3
9.3.6 Restraints to Elements in Compression................................................................3
9.4 Fatigue ..................................................................................................................................4

9. CAST IRON
9.1 General
This Section provides recommendations for the assessment of Bridges that contain
cast iron elements such as beams, columns forming part of a support framework, and
arch ribs.
The strength of all elements should be assessed on a permissible stress basis using the
values given in Clause 9.3.
9.2 Material Properties
The following properties may be adopted for assessment purposes:
Unit weight

7200 kg/m³

Modulus of elasticity

114000 N/mm²

Coefficient of linear thermal expansion

10.2 x 10-6 oC-1

9.3 Strength
9.3.1 Permissible Stresses
The stresses in cast iron under the combined effects of permanent and live loads
should not exceed the following limits:
Compression
Tension
Shear

154 N/mm²
46 N/mm²
46 N/mm²

RAILTRACK LINE CODE OF PRACTICE

The Structural Assessment of Underbridges
Section 9 - Cast Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 6

9.3.2 Beams Continuously Restrained by Surrounding Fill
The section modulus of cast iron girders may be increased for live loading by the
factor D/d, where D is the overall depth of the deck less an appropriate allowance for
ballast and loose fill, and d is the depth of the bare girder at midspan, provided the
following conditions are present:
(i)

the girders are known to be firmly embedded in well consolidated filling
material, other than pure sand or pure clay;

(ii)

there are no services in the fill that would decrease the support which it
renders, such as stoneware pipes or large diameter water or gas mains.

The factor D/d should not be applied to longitudinal girders consisting of cast iron
troughs. The maximum value for D/d which may be applied to the section modulus of
cast iron sections for live load, should not exceed 2.0. If openings are made in the
deck after an assessment that used the D/d factor, the opening should be back filled
with concrete, or the assessment reconsidered.
9.3.3 Beams with Intermediate Lateral Restraints
Members subject to bending which have discrete, rather than continuous, restraint to
their compression flange may be susceptible to lateral torsional buckling. In these
cases a reduced value of permissible compressive stress due to bending, pbc , should
be adopted, which takes due account of this phenomenon. The procedure for
determining pbc should be agreed in accordance with Railtrack’s Technical Approval
Procedures.
9.3.4 Compression Members
Cast iron struts that are adequately braced in accordance with Clause 9.3.6 should be
assessed by the Gordon-Rankine equation as follows:

P =



f A
(2 x10 −4 ) cy 2
FaL
1+ 2s
Kr









Equation 9.1

where:
P
f cy

is the safe load (kN);
is the compressive yield stress which should be taken as 555 N/mm²;

A
Ls
Kr

is the cross section area (mm²);
is the length (mm);
is the least radius of gyration (mm);

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 9 - Cast Iron
F
a

Page 3 of 6

is the end fixity factor given in Table 9.1;
1
is the material factor,
.
1600
End Condition

F

Both ends pin jointed
One end fixed, one end pin jointed
Both ends rigidly fixed
One end fixed, one end entirely free

1
0.5
0.25
4

Table 9.1
Values of End Fixity Factor (F)
9.3.5 Members subject to Bending and Axial Compression
Members subject to both axial compression and bending, such as arch ribs, should
satisfy the following condition at all points:
f c f bc
+
≤ 1.0
pc pbc

Equation 9.2

where:
fc
f bc
pbc
pc

is the calculated average axial compressive stress;
is the compressive stress due to bending about the centroidal axis;
is the permissible compressive stress determined in accordance with
Clause 9.3.3;
is the allowable compressive stress in N/mm² determined in accordance with
the following expression:
pc =

0.2f cy

 FaL
1+ 2s
 Kr

2





Equation 9.3

where f cy ,F ,a ,L s and K r are as defined in Clause 9.3.4.
9.3.6 Restraints to Elements in Compression
The load effects to be considered in assessing the adequacy of restraining members
and their connections should be determined in accordance with Appendix A. The
adequacy of members and their connections should be considered in accordance with
the permissible stress requirements of this Section.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 9 - Cast Iron

Page 4 of 6

9.4 Fatigue
In addition to the limits defined in Clause 9.3.1, the live load stresses, f L and qL ,
should not exceed the values defined in Equations 9.4 to 9.7. Equations 9.4 and 9.5
are represented graphically in Figure 9.1 and 9.2. In determining the live load effects,
the dynamic factor applicable to fatigue as defined in Table 4.5 should be used.
For tensile values of f L , f L should not exceed the greater of the values given by:
fL =
fL =

24.6 − 0.44 f d N/mm², or
19.6 − 0 .76f d N/mm²

Equation 9.4

where f d is the permanent load stress and tensile stresses are positive.
For compressive values of f L , f L should not exceed the lesser of the values given by:
fL =
fL =

− 43.9 + 0.79f d N/mm², or
− 81.3 + 3.15f d N/mm²

Equation 9.5

Where the live load shear stress qL acts in the same sense as the dead load shear
stress qd :
qL ≤ 24.6 − 0.44q d N/mm²

Equation 9.6

Where the live load shear stress qL acts in an opposite sense to the dead load shear
stress qd :
qL ≤ 43.9 − 0.79 qd N/mm² when qL ≤ 2qd, or
qL ≤ 24.6 + 0.44qd N/mm² when qL > 2qd

Equation 9.7

In Equations 9.6 and 9.7, the signs of the shears have been taken into account and
only numerical values of qL and qd should be substituted.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 9 - Cast Iron

Page 5 of 6

150
fL=

100
Live Load
Stress
fL
N/mm²

fd=

19.6-0.76fd

-154

50

fL=

24.6-0.4fd
fL=

0

46-fd

fd=
fL=

46

-43.9+0.79fd

-50
fL=

-154-fd

fL=

-81.3+31.5fd

-100
-150
-200

-150

-100

-50

0

Stress due to Permanent Loads N/mm²
Note Tensile Stresses are Positive
Figure 9.1
Permissible Stresses in Cast Iron

50

100

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Section 9 - Cast Iron

Page 6 of 6

40
fL=

19.6-0.76fd

30
fL=

Live Load
Stress
fL
N/mm²

24.6-0.4fd

20
10
fL=

46-fd

0
fd=
fL=

46

-43.9+0.79fd

-10
-20
Note Tensile Stresses are Positive

-10

0

10

20

30

Stress due to Permanent Loads N/mm²
Figure 9.2
Permissible Stresses in Cast Iron

40

50

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 10 - Timber

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 2

CONTENTS
10. TIMBER.....................................................................................................................................1
10.1 General..............................................................................................................................1
10.2 Assessment .......................................................................................................................1
10.3 Loading ..............................................................................................................................1
10.4 Modifications to BS 5268: Part 2: 1996......................................................................1
10.4.1 Species .......................................................................................................................1
10.4.2 Dimensions and Geometrical Properties ...........................................................2
10.4.3 Grades........................................................................................................................2
10.4.4 Grade Stresses for Strength Classes and Individual Species ..........................2
10.4.5 Duration of Loading ................................................................................................2
10. TIMBER
10.1 General
This Section provides recommendations for the assessment of timber elements of
underbridges. It covers timber elements of both superstructures and substructures.
It also deals with timber decking and is applicable to longitudinal timbers.
10.2 Assessment
Timber Bridges and timber elements within Bridges should be assessed to provide a
safe load carrying capacity using permissible stress principles in accordance with
BS 5268: Part 2: 1996 unless modified by Clause 10.4 below.
10.3 Loading
For the purpose of assessment, the loading applied to the structure should be as
defined in Section 4: Loading, except that, for decking timbers, no impact factor
(1 + ϕ) should be applied.
10.4 Modifications to BS 5268: Part 2: 1996
10.4.1 Species
Replace BS 5268: Part 2 Clause 2.3 with the following:
Where available, the species and grading of timber should initially be taken as that
shown on record drawings or other record documentation.
In the absence of definite species identification, initial assessment calculations should
assume the timber as Douglas Fir (Canada/North American), SS grade. Obtained

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 10 - Timber

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 2

samples should be identified if calculations show an inadequate assessment capacity
relative to existing or future loading requirements.
10.4.2 Dimensions and Geometrical Properties
Replace BS 5268: Part 2 Clause 2.4 with the following:
Member dimensions should be verified on site. Assessment should be based on the
measured site dimensions. therefore taking production tolerances into account.
Any loss of section area due to timber decay should be taken into account in the
derivation of member capacity.
10.4.3 Grades
Replace BS 5268: Part 2 Clause 2.5 with the following:
If record drawings or other information do not show the grade of timber, on-site
visual stress grading by a specialist consultant should be carried out. The timber
consultant should be qualified by a suitable certification body.
10.4.4 Grade Stresses for Strength Classes and Individual Species
Replace BS 5268: Part 2 Clause 2.6 with the following:
Timber employed in railway underbridges should be considered as service class 3
(wet exposure). The grade stresses for service classes 1 and 2 given in BS 5268: Part
2, Tables 7 to 12 should be multiplied by the applicable modification factor given in
Table 13.
10.4.5 Duration of Loading
Replace BS 5268: Part 2 Clause 2.8 with the following:
The modification factor for duration of loading applicable to the assessment of
underbridges, excepting decking timbers, should be taken as 1.5 as applicable to short
term duration of loading.
For decking timbers the modification factor for duration of load should be taken
as 1.0.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 11 - Substructures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 2

CONTENTS
11. SUBSTRUCTURES.................................................................................................................1
11.1 General..............................................................................................................................1
11.2 Assessment .......................................................................................................................1
11. SUBSTRUCTURES
11.1 General
This Section provides recommendations for the assessment of substructures and
foundations for all types of bridges. Substructures and foundations are taken to
include all elements of the bridge beneath the soffit of the deck, excluding bearings,
but including piers, bank seats, abutments, wing walls and foundations, including piles.
For arch bridges the substructure and foundations include the springings and all
elements beneath the ground.
For substructures and foundations, any failure is likely to be progressive and there
will usually be some warning signs (such as movement, settlement, rotation and/or
cracking) well before final collapse is imminent. For this reason the history of the
observed defects should be determined if possible. For example, movements or
rotations may have occurred early in the life of the structure and subsequently a state
of equilibrium has been achieved.
Most substructure and foundation elements, especially structures of brick or stone,
are not amenable to assessment by calculation.
Where increased capacity is not required, adequacy may be determined by a
qualitative assessment of the structure, including the significance of any defects. The
capacity may be considered sufficient if the substructure has performed satisfactorily
over a long period of time and it is in adequate condition and shows no signs of
distress or undue settlement.
Where increased capacity is required, structural and/or geotechnical analysis should
be carried out.
11.2 Assessment
The assessment of substructures and foundations should be based on the results of
their detailed inspection, reference to record drawings and other available
information.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 11 - Substructures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 2

If a foundation or substructure shows no signs of significant distress, if there is no
evidence of scour either externally or internally, and if no significant increases in load
are envisaged, the foundation or substructure may be assumed to be adequate and no
further assessment is necessary. Where substructures however include elements
amenable to assessment by calculation such as, metal trestles or concrete columns,
the relevant Section of this Code should be used.
In assessing elements which fall outside the scope of this Code, reference may be
made to design standards. These documents are likely to contain conservative
requirements which if unmodified, especially for serviceability aspects, are likely to
produce unduly pessimistic estimates of load carrying capacity. Useful guidance can
be found in the BA 55/94: The Assessment of Bridge Substructures and Foundations,
Retaining Walls and Buried Structures. It suggests for example, that where wing walls
are integral with abutment stems, and in good condition, consideration may be given
to assessment of the substructure as a whole, rather than, on a unit width basis.
A substructure that shows signs of significant defects should be quantitatively assessed
taking the defects into account. For an analytical approach, realistic parameters (such
as earth pressure coefficients) should be used. Detailed soil surveys should be
carried out if such information is likely to improve the reliability of the assessment.
Where it is necessary to take account of railway traffic surcharge loading in the
assessment of abutments and other soil-retaining substructure elements, the values
given in Table 11.1 should be adopted. The tabulated values may be deemed to take
into account dynamic effects.
Substructures and foundations should be considered inadequate in relation to current
(not increased) capacity only if they exhibit signs of distress and do not meet the
acceptance criteria of the numerical assessment.
RA
NUMBER
RA0
RA1
RA2
RA3
RA4
RA5
RA6
RA7

SURCHARGE LOAD
(kN/m²)
22
24
26
28
30
32
34
36

RA
NUMBER
RA8
RA9
RA10
RA11
RA12
RA13
RA14
RA15

SURCHARGE LOAD
(kN/m²)
38
40
42
44
46
48
50
52

Table 11.1
Nominal Railway Traffic Surcharge Loading

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Section 12 - Bearings

RT/CE/C/025
Issue: 1
Date: February 2000
Page 1 of 1

CONTENTS
12. BEARINGS...............................................................................................................................1
12.1 General..............................................................................................................................1
12.2 Assessment .......................................................................................................................1
12. BEARINGS
12.1 General
This Section provides recommendations for the assessment of bearings, where
present, for all types of Bridges.
The condition of a bearing and its seating is an important indicator, not only of the
bearing itself, but sometimes of some defect in the structure. Bearings are located
where movement is intended to take place. If they do not function adequately, the
structure may suffer excessive stress.
12.2 Assessment
The assessment of bearings should be based on the results of their detailed inspection
and reference to record drawings and other available information. For proprietary
bearings, reference to manufacturer’s information should be made if possible.
If a bearing, or its seating, shows no signs of distress, if movements including rotations
are free to take place, and if no significant increases in load are envisaged, the bearing
may be assumed to be adequate and no further assessment is necessary.
Bearings that show signs of significant defects should be assessed using the design
principles of BS 5400: Part 9.1 and where applicable using the manufacturer’s design
recommendations. Any references within BS 5400: Part 9.1 to BS 5400: Part 3
should be read as Appendix A of this Code.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 66

CONTENTS
Modifications and additions to BS 5400: Part 3 Clauses:
3.2.2A Main Symbols ..........................................................................................................3
3.2.3A Subscripts ................................................................................................................3
4.2.2A Serviceability Limit State ......................................................................................4
4.2.3.A Fatigue .....................................................................................................................4
4.3.3A Values of Partial Factors.......................................................................................5
6.1A Performance ...................................................................................................................6
6.4A Ductility ...........................................................................................................................7
6.6A Properties of Steel and Wrought Iron......................................................................8
7A. GLOBAL ANALYSIS FOR LOAD EFFECTS ..................................................................8
7.1A General ............................................................................................................................8
8A. STRESS ANALYSIS...............................................................................................................8
8.5.1A Imperfections Allowed For..................................................................................8
8.8A Originally Unintended Composite Action ...............................................................9
8.8.1A General ....................................................................................................................9
8.8.2A Cased Beams or Filler Beams or Jack Arch Decks .........................................9
8.8.3A Concrete Slab and Steel/Wrought Iron Beam Decks..................................10
9A. ASSESSMENT OF BEAMS ................................................................................................10
9.3A Shape Limitations.........................................................................................................11
9.3.1A General ..................................................................................................................11
9.3.5A Flanges Curved in Elevation...............................................................................13
9.3.6A Circular Hollow Sections...................................................................................14
9.3.7A Compact Sections................................................................................................14
9.3.7.5A Flat Plates ...........................................................................................................15
9.4.2A Effective Section...................................................................................................15
9.6A Effective Length for Lateral Torsional Buckling ....................................................15
9.6.1A General ..................................................................................................................15
9.6.2A Beams with intermediate lateral restraints ....................................................16
9.6.3A Beams (other than cantilevers) without Intermediate Lateral Restraints16
9.6.5A Beams with Intermediate U-Frame Restraints ..............................................19
9.7A Slenderness ...................................................................................................................21
9.7.1A General ..................................................................................................................21
9.7.2A Uniform I, Channel, Tee or Angle Sections ...................................................23
9.7.3A Other Uniform Sections ....................................................................................25
9.7.4A Varying Sections...................................................................................................26
9.7.5A Other Cases and Alternative Methods ...........................................................26
9.8A Limiting Moment of Resistance.................................................................................27
9.8.1A General ..................................................................................................................27
9.8.2A Allowance for flange straightness imperfection ............................................28
9.9A Beams Without Longitudinal Stiffeners ..................................................................31

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 66

9.9.1A Bending Resistance ..............................................................................................31
9.9.2A Shear Resistance ..................................................................................................31
9.10A Flanges in Beams with Longitudinal Stiffeners in the Cross Section...............34
9.12A Restraints to Compression Flanges.......................................................................34
9.12.1A General................................................................................................................34
9.12.2A Intermediate U-Frame Restraints ..................................................................35
9.12.3A Continuous Restraint Provided by Deck .....................................................36
9.12.4A Restraint at Supports........................................................................................37
9.14.4A Strength of Bearing Stiffeners .........................................................................42
10A. DESIGN OF COMPRESSION MEMBERS...................................................................42
10.3.1A Unstiffened Outstand .......................................................................................42
10.3.3A Circular Hollow Section ..................................................................................42
10.3.4A Assessment of Sections not Complying with Shape Limitations .............43
10.7.2A Evaluation of Stresses .......................................................................................43
11A. TENSION MEMBERS ......................................................................................................43
11.1A General........................................................................................................................44
11.3.2A Effective Area .....................................................................................................44
11.3.5A Pin Connected Members .................................................................................44
11.4A Thickness at Pin Holes .............................................................................................44
12A. TRUSSES ............................................................................................................................45
12.1A General........................................................................................................................45
12.4.1A General................................................................................................................45
12.4.2A Lateral Restraint by Deck to Compression Chord....................................45
12.5.1A Effective Length..................................................................................................45
12.5.2A Restraints to Compression Chords ..............................................................50
12.6.1A General................................................................................................................50
12.6.2A Forces on Bracing..............................................................................................50
12.6.3A Lateral Bracing not providing Adequate Restraint .....................................50
12.7A Curved Members ......................................................................................................51
14A. CONNECTIONS.............................................................................................................51
14.3.5A Connection of Restraints to Parts in Compression...................................51
14.4A Splices ..........................................................................................................................52
14.4.5A Obsolete Splicing Methods..............................................................................52
14.5A Connections made with Bolts, Rivets or Pins .....................................................53
14.5.2A Edge and End Distance .....................................................................................54
14.6A Welded Connections ...............................................................................................55
14.6.1A General................................................................................................................55
BS 5400: PART 3 APPENDIX B..............................................................................................60
BS 5400: PART 3 APPENDIX D.............................................................................................64
BS 5400: PART 3 APPENDIX E ..............................................................................................65
BS 5400: PART 3 APPENDIX G.............................................................................................66

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 66

AMENDMENTS TO BS 5400: PART 3 (1982)
3.2.2A Main Symbols
Add to the list given in BS 5400: Part 3 Clause 3.2.2 the following symbols:
ΨR
FR
FS
lw
lR
δe
δi
δt
δr
θL

Restricted shear lag factor;
Lateral restraint force on compression flange;
Lateral force at support;
Half wave length of buckling;
U-frame spacing;
Unit force related displacement at end support;
Unit force related displacement at internal supports;
Lateral displacement at end torsional restraint for unit force to each flange;
Lateral displacement at intermediate restraints (other than internal supports);
Slope (radians).

Replace the existing definitions of k and β given in BS 5400: Part 3 Clause 3.2.2 with
the following:
k
β

Buckling co-efficient; ratio of principal stresses;
Slope of web to vertical; factor.

3.2.3A Subscripts
Add the following subscripts to the list given in BS 5400: Part 3 Clause 3.2.3:
c
e
f
i
L
min
max
s
T
ult

compressive flange;
end;
flange;
internal;
longitudinal;
minimum;
maximum;
at support;
transverse;
ultimate condition.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 66

4.2.2A Serviceability Limit State
Delete the existing BS 5400: Part 3 Clause 4.2.2 and substitute the following:
For underbridges serviceability need not be checked except where relevant for
BS 5400: Part 3 Clauses:
9.10.3.3

In stiffened steel plate decking construction that forms part of a girder
flange.

12.2.3

In trusses as defined in BS 5400: Part 3 Clause 12.2.3.

14.5.4.1.2

In HSFG connections where slip would cause redistribution in
continuous structures.

4.2.3.A Fatigue
Delete the existing BS 5400: Part 3 Clause 4.2.3 and substitute the following:
4.2.3.1A General
The fatigue endurance of steelwork and wrought iron underbridges should be
checked in accordance with BS 5400: Part 10 as amended by Appendix D of this
Code.
4.2.3.2A Bridges requiring Assessment
Assessment of fatigue life of steelwork and wrought iron underbridges is required
when one or more of the following conditions apply:
(i)

there are visible cracks in components;

(ii)

the design is unable to be confirmed as having been carried out to
BS 5400: Part 10;

(iii)

the construction is unable to be confirmed as having been carried out in
accordance with BS 5400: Part 6;

(iv)

visual inspection has revealed, or records show, that the bridge has been
subject to structural modification during or since construction. Note that this
may include remaining temporary works features, new fixtures or repair or
damage on the structural members, such as welded attachments, flame cut
holes, strengthening etc.;

(v)

the bridge contains details not specifically covered by the scope of BS 5400:
Part 10, in particular orthotropic steel decks. Standard Box girder decks in
accordance with Railtrack’s standard bridge design may be considered as
covered by BS 5400: Part 10;

(vi)

there has been evidence of traffic resonance in any of the structural members
resulting in cracking of components or loosening of connections or supports.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 66

4.3.3A Values of Partial Factors
Delete the existing BS 5400: Part 3 Clause 4.3.3 and substitute the following:
The values of partial factors are as follows:
γ fL

the values of γ fL are given in Section 2 for each type and combination of
loading;

γf 3

the factor γ f 3 in BS 5400: Part 3 should be taken as 1.1 for the ultimate limit
state and 1.0 for the serviceability limit state.
γ f 3 may be taken as 1.0 for the ultimate limit state where the following
conditions are all met:
(a)

members are either:
(i)

rail bearers or cross girders that are assumed to be simply
supported;

(ii)

main girders of bridges with maximum skew of 25°. If
continuous any splices are welded, or made with HSFG bolts
or rivets with cover plates to both flanges.

(b)

global analysis is based upon static distribution within the structure;

(c)

geometric dimensions of the members are verified during inspection.

For the sake of simplicity, the expressions for strength in BS 5400: Part 3 contain a
single factor γ m = γ m1 ⋅ γ m 2 . Values of the factor to be used where γ m is explicitly
shown in the design strength equations in BS 5400: Part 3 are given in Table 2.
Where explicitly expressed for Ultimate Limit State in a strength requirement in
BS 5400: Part 3, γ m should be taken as 1.05, except where the applicable value of γ m
is tabulated in Table A1 for particular Clauses.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
Structural Component and
Behaviour

Clauses

Buckling resistance of stiffeners

9.10.2.3(a) and (b),
9.11.5.2, 9.13.5.3A,
9.13.6, 9.14.4.3,
9.17.6.7, 9.17.7.3.2,
9.17.8
14.5.3.2, 14.5.3.3A,
14.5.3.5
14.5.3.4

Fasteners in tension
Fasteners in shear
Friction capacity of HSFG bolts
Welds

Compression members

14.5.4.2
14.6.3.11.1A,
14.6.3.11.2A,
14.6.3.11.3A
10.6.1.1A,
10.6.3

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 66
γm

1.20

1.20
1.10 excl. rivets
1.33 rivets
1.30
1.20

0.95 +

1.8
≤1.05
 L +5
 
r

Table A1
Partial Factors γ m for ULS
Where explicitly expressed for Serviceability Limit State in a strength requirement in
BS 5400: Part 3, γ m should be taken as 1.00, except when considering friction
capacity of HSFG bolts in accordance with Clause 14.5.4.2A, in which case γ m should
be taken as 1.20.
Note:
Any other Clause making cross-reference to any of the above Clauses contained in
Table A1 should incorporate the applicable γm value given in Table A1 above.
6.1A Performance
Delete the existing BS 5400: Part 3 Clause 6.1 and substitute the following:
The mechanical properties of materials should be determined from specified values,
tests of the material or from available mill test certificates. In the absence of this
information, minimum ultimate tensile strengths may be taken for materials produced
before BS 4360:1962 as given in Table A2.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
σy

Material
Steel

Pre 1905
BS 15:1906
BS 15:1948

Wrought Iron
Steel Rivets * Pre 1905
After 1905
Wrought Iron Rivets *

N/mm²
205
230
245
190
335
300
275

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 66

Minimum
Ultimate
Tensile N/mm²
370
430
430
285
450
385
300

Table A2
Mechanical Properties
*

The yield and ultimate tensile stress of rivets is taken as the mean strength
from test results.

6.4A Ductility
Delete the existing BS 5400: Part 3 Clause 6.4 and substitute the following:
Steel or wrought iron should have a ductility not less than that corresponding to an
elongation of 15%. Where plastic global analysis is used for steel under Clause 7.1A
the ductility should not be less than that corresponding to an elongation of 19%.
Elongation should be based on the standard proportional gauge length of 5.65 So
where So is the cross sectional area of the test piece.
If a different gauge length is used the percentage elongation value should be
converted to the value for the standard gauge length in accordance with BS 3894.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

Page 8 of 66

6.6A Properties of Steel and Wrought Iron
Delete the existing BS 5400: Part 3 Clause 6.6 and substitute the following:
The properties given in Table A3 should be assumed:

Modulus of elasticity E (N/mm²)
Shear Modulus G
Poisson’s ratio, ν
Coefficient of thermal expansion/oC

Steel
205000
0.4E
0.3
12x106

Wrought Iron
190000
0.4E
0.3
12x106

Table A3
Properties of Steel and Wrought Iron
7A. GLOBAL ANALYSIS FOR LOAD EFFECTS
7.1A General
Add to end of existing BS 5400: Part 3 Clause 7.1:
Plastic (or yield line) global analysis at ULS is permitted for beams and flat plates
provided the components are compact sections under Clause 9.3.7A. Plastic global
or yield line analysis is not permitted for wrought iron.
The plastic modulus may be used for stress analysis of wrought iron see BS 5400:
Part 3 Clause 9.4.2.
8A. STRESS ANALYSIS
8.5.1A Imperfections Allowed For
Add to end of existing BS 5400: Part 3 Clause 8.5.1:
For bridges that are not known to have been constructed to the specification
requirements of BS 5400: Parts 6 and 9, bearing misalignment, errors in level, bearing
inclination, and imperfections in flatness and straightness should, where relevant, be
taken into account in assessments. The assessment of half-through girders should
take account of the measured bows of girders and verticality at supports. The
strength of web panels should take account of measured out-of-flatness where
significant distortion is evident from inspection.
For elements of bridges known to have measured imperfections outside the
tolerances required by BS 5400: Parts 6 and 9, the magnitude of these imperfections

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 9 of 66

should be taken into account in strength assessment. Imperfections less than the
tolerances may be taken into account when this is beneficial.
The imperfections should be assumed to be 1.2 times the measured imperfections to
allow for inaccuracies. A factor of 1.2 is already contained in the relevant expressions
of BS 5400: Part 3 and modifications by this Appendix are so that actual measured
values may be used.
8.5.2.2A Column
Add to end of existing BS 5400: Part 3 Clause 8.5.2.2:
Eccentricities exceeding 10 mm of bearings at top or bottom to the axes of columns
should be taken into account.
8.8A Originally Unintended Composite Action
Add the following additional Clause 8.8A to BS 5400: Part 3.
8.8.1A General
Stiffness and strengths calculated for sections not originally intended as acting
compositely can be enhanced by consideration of composite action with adjacent or
surrounding structure using Appendix C of this Code of Practice where conditions
are as given in Clauses 8.8.2A or 8.8.3A.
8.8.2A Cased Beams or Filler Beams or Jack Arch Decks
For cased beams and concrete filler beams, the stress analysis should be based on
composite properties to Section 8 where there is no evidence of excessive corrosion,
fretting action or cracking sufficient to adversely affect the composite action.
Sections can be assumed to be compact where the compression flange and webs are
cased on both sides. Where the requirements for resistance to longitudinal shear are
not met, the beams should be assessed on the basis of the properties of the steel
section only, which may be assumed to be compact carrying the entire load.
Alternatively where attachments to the beams are sufficient to prevent relative
longitudinal slip (such as rivet or bolt heads or other transverse elements) as
demonstrated by push-out tests or by relevant evidence, these attachments may be
assumed to transmit the longitudinal shear forces. For dense brickwork filler beam
or jack arch decks, global and stress analysis should be based on composite
properties provided that the bending resistance of the composite section is not taken
as greater than 30% in excess of the calculated resistance of the steel section alone.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 10 of 66

8.8.3A Concrete Slab and Steel/Wrought Iron Beam Decks
For concrete slab and steel beam decks, global and stress analysis using composite
properties can be used provided both (a) and (b) or (c) below apply:
(a)

there is no significant evidence of corrosion, fretting, relative longitudinal slip
separation or cracking at the steel/concrete interfaces sufficient to adversely
affect the required composite action;

(b)

attachments to the beams at the interface (such as rivet or bolt heads or
other transverse elements) are sufficient to prevent relative slip;

(c)

site testing is carried out to demonstrate that the live load: stiffness
relationship is supportive of the composite action achieved, where the amount
of composite action is required to increase the assessment strength by more
than 30%. Normally test loading approximately equivalent to the nominal
calculated live load capacity of the steel section only should be applied.

9A. ASSESSMENT OF BEAMS
9.2.1.2A Effects to be Considered
Delete the existing BS 5400: Part 3 Clause 9.2.1.2 and substitute the following:
The effects at the ultimate limit state should be obtained for the relevant
combinations of:
(a)

flexure, shear, torsion (and, for box girders, distortion) due to any loads
transverse to the longitudinal axis of the member;

(b)

the effects of axial load;

(c)

creep, shrinkage and differential temperature (see Section 8 for composite
structures);

(d)

settlement of supports.

9.2.1.3A Effects that may be Neglected
Delete the existing BS 5400: Part 3 Clause 9.2.1.3 and substitute the following:
Effects that may be neglected for the ultimate limit states are:
(i)

shear lag;

(ii)

restraint of torsional warping;

(iii)

items (c) and (d) of Clause 9.2.1.2A provided that:
(a)

the section is compact through the span being considered in
accordance with the provisions of Clause 9.3.7A, and

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
(b)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 66

the member is not prone to lateral instability which may be deemed to
be satisfied when the slenderness parameter, λ LT is less than:
30

355 Mpe
σ y Mult

Expression A1

where:
λ LT
σy

is defined in Clause 9.7.1A;
is defined in Clause 9.7.1A;

Mpe

is defined in Clause 9.7.1A;

Mult

is defined in Clause 9.8.1A.

9.3A Shape Limitations
9.3.1A General
Delete the existing BS 5400: Part 3 Clause 9.3.1 and substitute the following:
BS 5400: Part 3 Figure 1 and Figure A1 in this Appendix set out the geometric
notation used in this Appendix A.
Where the proportions of flanges, stiffeners or hollow sections comply with the
requirements of Clause 9.3.2.1A, BS 5400: Part 3 Clauses 9.3.4, or Clause 9.3.5A and
9.3.6A, taking σys or σy as the yield stress of the material as defined in Clause 6.1A and
BS 5400: Part 3 Clause 6.2, the strengths of sections should be determined as
specified in the applicable Clauses of this Appendix where:
σ ys

relates to the stiffener;

σy

relates to the flange, the web or the circular hollow sections, as applicable.

Where the proportions do not thus comply, a lower value or σ ys or σ y should be
determined such that compliance with Clause 9.3.2.1A, BS 5400: Part 3 Clause 9.3.4,
or Clauses 9.3.5A or 9.3.6A as applicable is achieved. This lower value of σ ys or σ y
should be used in all subsequent assessments of strength.
For riveted construction the outstand dimension bfo or hs may be taken from the
edge of the head of rivets where the heads are consistently intact. A rivet head
diameter may be considered to be 1.6 x nominal rivet diameter.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

Page 12 of 66

b fo
0.8 x nominal
rivet diameter

For individual flanges
t fo
t fo

b fo
0.8 x nominal
rivet diameter C

dwe

rivet

b fo

0.8 x nominal
rivet diameter
Bf
b fo

0.8 x nominal
rivet diameter
tw
=

b fo

0.8 x nominal
t fo rivet diameter

dwe
=

Bf
0.8 x nominal
rivet diameter

tw

t fo

=

t fo
tw

tw
=

b fo

dwe

t fo

ts
ts

=

=

32 t w

0.8 x nominal
rivet diameter

0.8 x nominal
rivet diameter

hs

tw

b fo

hs

tw

=

tw

=

32 t w

Total 32 t w
b fo

tf

t fo
tf

tf

dw = d we

Figure A1
Geometric Notation for Sections that may be encountered in Assessment

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

Page 13 of 66

9.3.2.1A Outstands in Compression
In the existing BS 5400: Part 3 Clause 9.3.2.1, in the third and fourth lines, delete “or
16 whichever is the lesser”.
Add to the end of the existing BS 5400: Part 3 Clause 9.3.2.1:
Refer to Clause 9.3.1A for assessment of non-complying outstands in compression.
9.3.2.2A Outstands in Tension
For assessment BS 5400: Part 3 Clause 9.3.2.2 may be ignored.
9.3.4.1.1A General
Add to end of existing BS 5400: Part 3 Clause 9.3.4.1.1:
Refer to Clause 9.3.1A for the assessment of non-complying stiffener configurations.
Other shapes of stiffeners other than those specified should be assessed on the basis
of the nearest standard shape. Reference may also be made to Appendix S of Advice
Note BA 56/96: The Assessment of Steel Highway Bridges and Structures.
9.3.4.1.3A Bulb Flat Stiffeners
For assessment BS 5400: Part 3 Clause 9.3.4.1.3 may be ignored.
9.3.4.1.4A Angle Stiffeners
For assessment BS 5400: Part 3 Clause 9.3.4.1.4 may be ignored.
9.3.4.1.5 Tee Stiffeners
In BS 5400: Part 3 Clause 9.3.4.1.5 item (c) (2), delete

σ ys
355

and substitute

σy
355

.

9.3.5A Flanges Curved in Elevation
In the existing BS 5400: Part 3 Clause 9.3.5 delete the expressions in (a) and (b) and
substitute:
Flanges curved in elevation should be such that:
a)

b)

bfo
t fo

σ ys

RF
355 6bfo


b σ ys RF

t f 355 2b

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 14 of 66

where:
RF
b
tf

is the radius of curvature of the flange;
is the distance between adjacent longitudinal stiffeners and/or webs;
is the thickness of the flange in the panel such longitudinal stiffeners and/or

webs;
bfo , t fo are defined in BS 5400: Part 3 Clause 9.3.2.
9.3.6A Circular Hollow Sections
In BS 5400: Part 3 Clause 9.36 delete the expression and substitute 60

355
.
σy

Add to end of existing BS 5400: Part 3 Clause 9.3.6:
Refer to Clause 9.3.1A and BA 56: Appendix S for the assessment of non-complying
sections.
9.3.7A Compact Sections
9.3.7.2.3A Alternative Method
Add additional Clause 9.3.7.2.3 to BS 5400: Part 3 Clauses 9.3.7.2.1 and 9.3.7.2.2 the
depth of the web should not exceed:
34t w
m

355
when m is less than 0.5
σ yw

or
374t w 355
when m is greater than 0.5
13m − 1 σ yw

Expression A2

Expression A3

where:
m

tw
σ yw

is the ratio of the depth of the web plate that is on the compressive side of the
plastic neutral axis of the beam to the depth of the web plate. The depth of
the web referred to in this Clause should be measured in its plane and taken
clear of root fillets for rolled sections and welds or flange angles for fabricated
sections;
is the thickness of the web plate;
is the nominal yield stress of the web material or any other lower stress
assumed in the assessment.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 15 of 66

9.3.7.3.3A Composite Compression Flanges
In BS 5400: Part 3 Clause 9.3.7.3.3 delete the existing expressions and substitute the
following expressions respectively:
30t f

355
;
σ yf

15t f

355
;
σ yf

22t f

355
.
σ yf

9.3.7.4A Circular Hollow Sections
In BS 5400: Part 3 Clause 9.3.7.4 delete the existing expression and substitute
355
46
.
σy
9.3.7.5A Flat Plates
Add additional Clause 9.3.7.5 to BS 5400: Part 3.
Flat steel or wrought iron plates having a width at least equal to their thickness may
be assumed as compact for bending applied about an axis parallel to their width.
9.4.2A Effective Section
9.4.2.2A Deduction for Holes
In BS 5400: Part 3 Clause 9.4.2.2 add ‘ 11.3.2 and’ before ‘ 11.3.3’.
9.6A Effective Length for Lateral Torsional Buckling
9.6.1A General
Delete the existing BS 5400: Part 3 Clause 9.6.1 and substitute the following:
For all beams there should be restraint against rotation about the longitudinal axis in
accordance with Clause 9.12.4A at or adjacent to the supports. A restraint or
restraints within the following distance from a support may be considered as a
support restraint:
Half Through

le 3

but not greater than L 5

Beam and Decking Type

le 7.5

but not greater than L 5

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 16 of 66

where L is the span of the beam.
In all cases the effective length for lateral torsional buckling le should be determined
in accordance with Clauses 9.6.2A, 9.6.3A, BS 5400: Part 3 Clauses 9.6.4 and
Clause 9.6.5A as applicable. However, if the second moment of area of a cross
section about the axis of bending is smaller than that about an axis perpendicular to it,
the cross-section as a whole is stable against overall lateral torsional buckling, and the
effective length le may be taken as zero.
Where the resistance of the restraining systems is less than required to resist the
force FS in accordance with Clause 9.12.4.1A, the slenderness parameter λ LT
applicable to the length le at the support under consideration, should be modified as
follows:
λ LT
λ LT ' =
Equation A4
1  5FRD 
+ 3

8  FS

where:
λ LT '

is a modified value of λ LT as defined in Clause 9.7.1A;

FS
FRD

is as defined in Clause 9.12.4A;
is the available resistance which is less than FS excluding the effects of wind,
frictional and other applied forces.

9.6.2A Beams with intermediate lateral restraints
Amend the reference to Clause 9.12.1 to 9.12.1A.
9.6.3A Beams (other than cantilevers) without Intermediate Lateral Restraints
Delete the existing BS 5400: Part 3 Clause 9.6.3 and substitute the following:
This Clause applies to half-through bridges which do not have effective intermediate
U-frames. When there is no intermediate lateral restraint to a compression flange le
should be taken as the greater of the values calculated in accordance with (a) and (b)
below as applicable:
(a) for single-span or continuous beams:
le =
where:

k1k2 ke L

Equation A5

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
L
k1

RT/CE/C/025
Issue: 1
Date: February 2001
Page 17 of 66

is the span of the beam between lateral restraints at supports;
may conservatively be taken as:
i)

1.0 where the compression flange is free to rotate in plan at the points
of support; or

ii)

0.85 where the compression flange is partially restrained against
rotation in plan at the points of support or if it is fully restrained
against rotation in plan at one support, and free to rotate in plan at the
other; or

iii)

0.7 where the compression flange is fully restrained against rotation in
plan at the points of support.

A more accurate value of k1 allowing for the degree of restraint in plan, may
be obtained from Figure A7(a).
k2
ke =

should be taken as 1.0 unless the load is applied to the top flange and both the
flange and the load are free to move laterally in which case 1.2 should be used.
1
Equation A6




60Et f max βδ t 

1− 
3



L
   ν4 

  ry 
where:
t f max is the maximum thickness of the compression flange in the span;
δt

β

υ

is the relative lateral deflection of the centroid of one flange of the
beam with respect to the centroid of the other flange which would
occur when equal and opposite unit forces act laterally on the end
torsional restraint only at the same levels as shown in Figure A8;
should be taken as 1.0 for a simply supported beam, or for an internal
support to a continuous beam with restraint in plan to the
compression flange at the support, or as
should be taken as 2.0 for an internal support to a continuous beam
with restraint in plan to that support.
is the value of υ calculated in accordance with Clause 9.7.2A which
may derived using ke = 1.0 in calculating λF.

ke should be taken as the greater of the values obtained for either support.
The restraint should be such that the denominator has a positive sign.
(b) for continuous beams only:

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

le =

k1k 2 ∑ L

Page 18 of 66

1

Equation A7



 2(∑ L )3

1+ 

 π 4 EI c  δ i + δ e  

2  


where:
∑L
Ic

is the sum of the adjacent spans;
is the second moment of area of the compression flange about its Y-Y
axis, as defined in BS 5400: Part 3 Figure 1, at the point of maximum
bending moment;
is the value of δ t for an internal support;
is the value of δ t for the end or internal supports at the opposite ends
of the adjacent spans.

δi
δe

1.0

1.0

0.9

0.9

0.8

0.8

0.7

0.7

1=1.0

0.9
0.8

1

3

0.6

0.6

0.5

0.5

0.4

5

15

10

20

25

o

0.4

0.7
0.6
0.5

0.2

0.4

µo

0.6

0.8

1.0

c

(a) Effect of rotational end restraint
Note 1: ko is the smaller value, at either end, of the
rotational stiffness to lateral bending of the
compression flange, chord or strut.
I c is as defined in Clause 9.6.5A or 12.5.1A or

BS 5400: Part 3 Clause 10.4.1 as applicable.

(b) Effect of bending restraint
L is the span of the beam or truss or
length between the ends of a
compression member effectively held in
position.
lo is the value of le obtained from
Clause 9.6.3A or 12.5.1A but calculated
with k1 = 1.0
NOTE 2: For basis of curves, see
BS 5400: Part 3 Clause G6.

Figure A7
Influence on Effective Length of Compression Flange Restraint

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 19 of 66

Structural connection

9.6.5.2
t

(9.6.6.)

t
unit
force

Figure A8
Restraint of Compression Flange by
U-Frames on Deck End Torsional Restraint
(Note: Clause number in brackets refers to BS 5400: Part 3)
9.6.5A Beams with Intermediate U-Frame Restraints
Delete the existing BS 5400: Part 3 Clause 9.6.5 and substitute the following:
Where restraint to a compression flange is provided by U-frames in accordance with
Clause 9.12.2A, l e should be taken as:
le =

k2 k3k5 l1 but not less than k3l R and not greater than the value given by
Clause 9.6.3A.
Equation A8

where:
k2
k3

is as defined in Clause 9.6.3A;
may be taken as 1.0 but, where the compression flange is restrained against
rotation in plan at supports, a lower value of k 3 may be obtained from
Figure A7(b);

k5 =

2.22 +
where:

0.69
X + 0.5

Equation A9

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 20 of 66

3

X=
l1 =

l1
2EI c δ t

(EI c l R δ R )0.25

Equation A10

Equation A11

where:
Ic
lR
δR

is as defined in Clause 9.6.3A;
is equal to the U-frame spacing;
is equal to the lateral deflection which would occur in any intermediate
U-frame at the level of the centroid of the flange being considered,
when a unit force acts laterally to the U-frame only at this point and
simultaneously at each corresponding point on the other flange or
flanges connecting to the same U-frame. The direction of each unit
force should be such as to produce the maximum aggregate value of
δ R . The U-frame should be taken as fixed in position at each point of
intersection between the cross member and a vertical, and as free and
unconnected at all other points;
is as defined in Clause 9.6.3A.

δt

Provided that δ R + δ t 2 for U-frame restraints adjacent to an end support, and that
2δ R for U-frame restraints not adjacent to an end support, are both not greater than
3

lR 20EIc , the restraints may be taken as fully effective and k5 l1 may be taken as lR .
Where the lateral stiffness of the decking system does not comply with Clause
9.12.2.2A then the intermediate U-frames should be ignored in deriving the effective
length.
In cases of symmetrical U-frames, where cross members and verticals are each of
constant moment of inertia throughout their own length, as shown in Figure 8, it may
be assumed that:
3

δR =

2

d1 uBd 2
2
+
+ fd 2
3EI1 EI 2

Equation A12

where:
d1

is the distance from the centroid of the compression flange to the nearer face
of the cross member of the U-frame, as shown in Figure 8;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
d2
I1

I2

u
B
f

RT/CE/C/025
Issue: 1
Date: February 2001
Page 21 of 66

is the distance from the centroid of the compression flange to the centroidal
axis of the cross member of the U-frame, as shown in Figure 8;
is the second moment of area of the effective section of the vertical about its
axis of bending perpendicular to the plane of the U-frames. A width of web
plate of up to 16 times the web thickness may be included on each side of the
centreline of its connection when determining the effective section of the
vertical;
is the second moment of area of the cross member of the U-frame about an
axis perpendicular to the plane of the U-frame. A width of deck on either side
of the U-frame equal to B 8 or lR 2 whichever is less, may be taken as the
effective cross member when no other discrete member is present, or may be
taken together with a cross member if structurally connected to it. In
calculating the transformed area of a concrete deck, the gross area of
concrete within this effective width may be considered;
is 0.5 for an outer beam, and 0.33 for an inner beam if there are three or
more beams interconnected by U-frames;
is the distance between centres of parallel beams, or where the beams are not
parallel the maximum distance within the mid-third of the span;
is the flexibility of the joint between the cross member and the verticals of the
U-frame, expressed in radians per unit moment. Values of f may be:


taken from BS 5400: Part 3 Figure A42;



determined from test results (which should cover the required
ultimate capacity of the joint);



determined from theoretical methods as described in BS ENV 1993:
Part 1 Annex J.

9.7A Slenderness
9.7.1A General
Delete the existing BS 5400: Part 3 Clause 9.7.1 and substitute the following:
The slenderness parameter λ LT required for the calculation of the limiting moment of
resistance (see Clause 9.8A) should be determined for all beams in accordance with
Clauses 9.7.2A to 9.7.5A applicable to the type of beam, using the effective length for
lateral torsional buckling obtained from Clause 9.6A.
The half wavelength of buckling of the compression flange should, if required be taken
as:
(a)

the distance between supports for beams without intermediate restraints;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
(b)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 22 of 66

the distance between effective discrete intermediate restraints
(Clause 9.6.2A).

The derivation of the moduli of the cross sections should be based on the following
requirements:
Z pe

is the plastic modulus of the effective section derived in accordance with
BS 5400: Part 3 Clause 9.4.2, and is defined as Mpe σ ye ,
where:
Mpe

is the plastic moment of resistance of the effective cross section
derived in accordance with BS 5400: Part 3 Clause 9.4.2 and based on
rectangular stress blocks of intensity equal to the strength of the
elements. For elements in structural steel, the strength should be
taken as the nominal yield stress of the elements, as defined in
Clause 6.1A and Clause 6.2A. For concrete flanges in compression,
the area of reinforcement should be ignored and the strength should
be taken as 0.4 f cu γ m . For concrete flanges in tension the area of
concrete should be ignored and the strength of the reinforcement
taken as 0.87 f y γ m ;
where:

σ ye
Z xc

Z xt

Z xw

f cu
fy

is the concrete cube strength in accordance with Section 7;
is the characteristic strength of the reinforcement in

γm

accordance with Section 7;
is taken from Clause 4.3.3A.

is the nominal yield stress value, as defined in Clause 9.3.1A for the
compression flange;
is the elastic section modulus with respect to the extreme compression fibre,
based on the effective section derived in accordance with BS 5400: Part 3
Clause 9.4.2;
is the elastic section modulus with respect to the extreme tension fibre, based
on the effective section derived in accordance with BS 5400: Part 3
Clause 9.4.2;
is the minimum elastic modulus of the section with respect to the web, based
on the effective section derived in accordance with BS 5400: Part 3
Clause 9.4.2.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 23 of 66

For composite sections Z xc , Z xt and Z xw should be based on the transformed
section. The transformed area of the concrete compression flange should be
obtained using the short term modular ratio of the concrete. Concrete in tension
should be ignored but the area of reinforcement in concrete subject to tension
should be included.
Where the web is discontinuous at the location considered, its area should be
ignored in the calculation of Mpe, Zxc Zxt, but with the neutral axis location calculated
assuming that the area of web is present.
9.7.2A Uniform I, Channel, Tee or Angle Sections
Delete the existing BS 5400: Part 3 Clause 9.7.2 and substitute the following:
The value of λ LT for overall lateral buckling of a beam of I, channel, tee or angle
section, uniform within the half-wavelength of buckling of the compression flange, and
bending about its X-X axis, as defined in BS 5400: Part 3 Figure 1, should be taken as:
λ LT =

le
k 4 ηυ
ry

Equation A13

where:
le
ry

is the effective length determined in accordance with Clause 9.6A;
is the radius of gyration of the gross cross-section of beam about its Y-Y axis

k4

(see BS 5400: Part 3 Figure 1);
should be taken as 0.9 for rolled I or channel section beams in accordance
with BS 4: Part 1 or any I section symmetrical about both axes with t f not

η

υ

greater than twice the web thickness, or 1.0 for all other beams;
should be taken as 1.0, but where the bending moment varies substantially
within the half-wavelength of buckling of the compression flange, advantage
may be obtained by using a value of η, from BS 5400: Part 3 Figure 9(a), if the
loading is substantially concentrated within the middle-fifth of the halfwavelength or from BS 5400: Part 3 Figure 9(b), for other loading patterns;
is dependent on the shape of the beam, and may be obtained from BS 5400:
Part 3 Table 9, using the parameters:
λF =
where:

le
ry

 tf 
  and i =
D

Ic
Ic + I t

Equations A14(a) and (b)

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
tf

D
Ic

It

RT/CE/C/025
Issue: 1
Date: February 2001
Page 24 of 66

is the mean thickness of the two flanges of an I or channel section
including any horizontal legs of connections angles, or the mean
thickness of the table of a tee or leg of an angle section;
is the depth of the cross section (see BS 5400: Part 3 Figure 1);
is the second moment of area of the compression flange, about its Y-Y
axis, as defined in BS 5400: Part 3 Figure 1, at the section being
checked;
is the second moment of area of the tension flange, about its Y-Y axis,
as defined in BS 5400: Part 3 Figure 1, at the section being checked.

For beams with Ic ≥ It or with λ F ≥ 8 , λ LT may conservatively be taken as l e ry .
When in accordance with BS 5400: Part 3 Clauses 9.6.4.1.3 or 9.6.4.2, le is greater
than l R , λ LT should be taken as l e ryc where ryc is the radius of gyration about the
Y-Y axis of the gross cross-section of the compression flange plus one third of the
height of the web.
Where a flange is common to two or more (n number) beams (for example in a
girder bridge with a composite deck) the properties ry , I y , Ic ,or I t may be calculated
by attributing a fraction n −1 of the lateral second moment of area and of the area of
the common flange to the section of each beam.
For beams restrained by U-frames the lateral stiffness of the decking system may be
taken into account whether or not its lateral stiffness complies with the minimum
value given by Clause 9.12.2.2A. Where the decking is assumed to act compositely as
part of the beams it may be treated as a flange common to the beams as above but
with any concrete in tension ignored. Where the decking is not assumed to act
compositely then its lateral inertia may be proportioned between the beams and
added algebraically to the value of I t or otherwise combined to take account of the
plan stiffness through the cross members. Concrete decking may be assumed as
uncracked provided it is continuously reinforced in the longitudinal direction.
In calculating t f , Ic and I t for composite beams, the equivalent thickness of the
composite flange in compression should be based on the short term elastic modulus
for concrete. Concrete in tension should be ignored and the equivalent thickness of
tension reinforcement should be taken as the area of reinforcement divided by the
flange width over which it is placed.
The calculation of ry should include the web even when the web is discontinuous at
the location considered.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 25 of 66

9.7.3A Other Uniform Sections
Delete the existing BS 5400: Part 3 Clause 9.7.3 and substitute the following
Clauses 9.7.3.1A and 9.7.3.2A:
9.7.3.1A Uniform Rectangular or Trapezoidal Box Sections
The value of λ LT for overall lateral buckling of a beam of rectangular or trapezoidal
box section, uniform within the half-wavelength of buckling of the compression flange,
should be taken as:
λ LT = 2.25ηξ

Z pe le

Equation A15

ry AJ

where:
η, ry

are as defined in Clause 9.7.2A;

Z pe

is as defined in Clause 9.7.1A. For non-compact sections, Z pe need not be
calculated explicitly since it may be replaced by Mpe σ yc and in the
subsequent application of Clause 9.8A, Mpe will cancel;

le

is as determined in accordance with Clause 9.6A;

A
J

is the area of the gross cross section;
2
is the torsional constant 4 A0 ∑ (B t ) ;
where:
A0
B, t

ξ=

is the area enclosed by the median line of the perimeter material of the
section;
are the width and thickness, respectively, of each wall of the section
forming the closed perimeter. In the case of a wall made from
concrete it should be taken as the actual thickness multiplied by the
ratio of the short term modulus of elasticity of the concrete to the
E of the section.

 (I x − I y )(I x − 0.385 J ) 
2


Ix



0.25

Equation A16

where I x and I y are the second moments of area of the gross cross section
about axes through the centroid normal to the plane of bending, and in the
plane of bending respectively.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 26 of 66

9.7.3.2A Uniform Solid Rectangular Sections
The value of λ LT for overall lateral buckling of a beam of homogeneous solid
rectangular section, which is uniform within the half-wavelength of buckling of the
compression flange, should be taken as:
λ LT =

2.8η

l eD
B

Equation A17

where:
η
le

is as defined in Clause 9.7.2A;
is determined in accordance with Clause 9.6A;

D
B

is the depth of the section in the place of bending;
is the width of the section.

9.7.4A Varying Sections
Delete the existing BS 5400: Part 3 Clause 9.7.4 and substitute the following:
The value of λ LT for overall lateral buckling of a beam of varying section within the
half-wavelength of buckling should be taken as (1.5 − 0.5ρf ) times the value obtained
from Clause 9.7.2A or 9.7.3A using the values of ry and υ applicable to the section
where the limiting compressive stress is to be derived.
where:
ρf =

minimum total area of two flanges at any section in lw
total area of two flanges at section being considered

lw

is the half wavelength of buckling as defined in Clause 9.12.2A.

This Clause is not applicable to beams with U-frame restraints.
9.7.5A Other Cases and Alternative Methods
Delete the existing BS 5400: Part 3 Clause 9.7.5 and substitute the following:
For cases not covered by Clauses 9.7.2A, 9.7.3A or 9.7.4A or as an alternative, λ LT
for overall lateral buckling may be taken as:
λ LT =

π 2 EZ pe
Mcr

Equation A18

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 27 of 66

where:
Z pe

is as defined in Clause 9.7.1A. For non-compact sections, Z pe need not be
calculated explicitly since it may be replaced by Mpe σ yc and in the
subsequent application of Clause 9.8A, Mpe will cancel;

Mcr

is the bending moment at which, under the given pattern of loading, the beam
reaches its theoretical elastic buckling condition as determined by an elastic
analysis.

9.8A Limiting Moment of Resistance
Delete the existing BS 5400: Part 3 Clause 9.8 and substitute the following:
9.8.1A General
The limiting moment of resistance, MR , should be obtained from Figure A10 or A11
as applicable, according to the value of:

β =

 σ  M 
λ LT  yc  ult 
 355  Mpe 

Equation A19

where:
λ LT
Mult

is obtained from Clause 9.7A;
is the moment of resistance of the cross section if lateral-torsional buckling is
prevented, defined as:
Mult = Mpe for compact sections; or
Mult = Z xc σ yc for non-compact sections.

MR is determined as:
M 
a) for compact sections  R  Mpe
 Mult 
M 
b) for non-compact sections the least of  R  Mult ,or Z xt σ yt
 Mult 
M 
where  R  is taken from Figure A10 or A11 as applicable;
 Mult 
Mpe , Z xc , Z xt are as defined in Clause 9.7.1A;
σ yt

is the nominal yield stress of the tension flange material;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

σ yc

RT/CE/C/025
Issue: 1
Date: February 2001
Page 28 of 66

is the nominal yield stress of the compressive flange material.

For Level 1 assessment or in the absence of measured values of flange straightness the
curve for k = 0 should be used where k is defined in 9.8.2A, corresponding to the
tolerances given by BS 5400: Part 6. Where measured values of flange straightness
are obtained such values should be used to derive the applicable value of k when
using Figure A10 or A11.
Figure A10 should be used for unwelded sections; it should be used for rolled and
riveted sections and for sections having welding limited to vertical stiffeners and local
gussets etc. Figure A11 should be used for sections fabricated by welding. For the
basis of the curves to Figures A10 and A11 see to BS 5400: Part 3 Appendix G7 as
modified by this Appendix.
9.8.2A Allowance for flange straightness imperfection
Where allowance is to be made for measured flange straightness imperfections, ∆ F
should be measured in accordance with BS 5400: Part 6 Table 5. MR Mult should be
obtained from Figures A10 and A11, corresponding to the value of the imperfection
factor k given by:
y
k=
( ∆ F − 0.001g ) 2
Equation A20
ry
where:
∆F

g
y
ry

is the greater of the values measured in accordance with 4(a) and 4(b)
respectively of BS 5400: Part 6 Table 6 but not less than 3 mm over a gauge
length normally equal to the span of the beams between points of support;
Gauge length used to measure ∆ F ;
is the distance in the x-direction from the Y-Y axis to the extreme fibre of the
compression flange (see BS 5400: Part 3 Figure 1);
is the radius of gyration of the gross cross section about its Y-Y axis.

The curve k = 0 represents ∆ F = 0.001g as specified by BS 5400: Part 6. Positive
values of k = 0.5, 1, 2, 3 and 4 represent bows greater than 0.001g. The curves for
negative values of k (-0.0029β , for Figure A10 and -0.0067 β for Figure A11)
represent bows of less than 0.001g and allow for enhancement of MR Mult up to the
maximum possible, equivalent to η=0 where η is defined in Clause G.7A.
Measured bow ∆ F and gauge length g should be applicable to the shape of bow which
exists. For example a double curvature bow may require additional measurements.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 29 of 66

1

0.9

0.8

0.7

0.6

MR/Mult
0.5

0.4

k
-0.0029β

0.3
0.5
2
4

0

1

3

0.2

0.1

0
0

50

100

150

200

250

β

Figure A10
Limiting Moment of Resistance MR
for Unwelded Sections

300

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 30 of 66

1

0.9

0.8

0.7

0.6

MR/Mult
0.5

0.4

k
-0.0067β

0.3
0.5
2
4

1

0

3

0.2

0.1

0
0

50

100

150

200

250

β

Figure A11
Limiting Moment of Resistance MR
for Welded Sections

300

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 31 of 66

9.9A Beams Without Longitudinal Stiffeners
9.9.1A Bending Resistance
Delete the existing BS 5400: Part 3 Clause 9.9.1 and substitute the following:
9.9.1.1A General
Beams should be checked for bending in accordance with Clause 9.9.1.2A.
Beams that are known to have been constructed in several stages during which the
loading and section properties changed should be assessed in accordance with
BS 5400: Part 3 Clause 9.9.5. Effects due to differential temperature where required
by Section 4 and concrete shrinkage where required by Appendix C should be taken
into account in accordance with BS 5400: Part 3 Clause 9.9.7.
9.9.1.2A All Sections
The bending resistance MD of all sections should be taken as:
MD =

MR
γ mγ f 3

Equation A21

where MR is the limiting moment of resistance derived in Clause 9.8A.
9.9.1.3A Non-compact sections
For Assessment BS 5400: Part 3 Clause 9.91.3 may be ignored.
9.9.2A Shear Resistance
9.9.2.1A General
Delete the existing BS 5400: Part 3 Clause 9.9.2.1 and substitute the following:
The shear resistance of a web of a beam with transverse stiffeners at supports and
with or without intermediate transverse stiffeners should be determined in
accordance with BS 5400: Part 3 Clause 9.9.2.2. provided that:
(a)

there are no longitudinal stiffeners on the web which are assumed effective in
resisting bending or shear resistance of the beam;

(b)

the web panel considered has no openings other than those within the limits
set out in BS 5400: Part 3 Clause 9.3.3.2 (a) (b) and (c);

(c)

the provisions of BS 5400: Part 3 Clauses 9.9.4 and 10.6 are met if the beam is
subjected to axial load;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
(d)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 32 of 66

flange or flanges are straight and parallel in elevation where the beam is other
than simply supported.

Web panels which do not meet these conditions should be checked in accordance
with BS 5400: Part 3 Clause 9.11.
For simply supported hog back beams with intermediate transverse web stiffeners or
beams with sloped bottom flange, the shear resistance should be checked to
Clause 9.9.2.3A.
9.9.2.3A Shear Resistance of Simply Supported Hog Back Beams or Sloped Bottom
Flange
Add additional Clause 9.9.2.3 to BS 5400: Part 3.
For simply supported hog-back beams (top flange sloped at the supports so that the
beam depth increases towards the span centre) having intermediate transverse web
stiffeners, a contribution to the vertical shear may be assumed to be carried by the
sloping top flange. Each web panel may be considered as part of a truss mechanism
with the web acting as a tie connecting opposite corners of the panel as shown in
Figure A17. The connection between the sloping flange and the bearing stiffeners
including any portion of web extending beyond the bearing stiffeners should be
checked for resistance to horizontal shear. In this Clause d w should be taken at the
shallowest end and d we at the mid length of each web panel. The shear resistance of
the web should be taken as in BS 5400: Part 3 Clause 9.9.2.2.
For simply supported beams with sloped bottom flange, that is with flange that is
sloped at the supports, so that the beam increases in depth along the span, a
contribution to the vertical shear may be assumed to be carried by the sloping
bottom flange provided λ does not exceed 50. The web may be assumed as divided
into panels between vertical stiffeners (if any), each panel not exceeding 1.2d we in
length. Each web panel may be considered as part of a truss mechanism with the web
acting as both struts and ties as shown in Figure A17.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 33 of 66

dwe
>30
Vertical
stiffeners
Slope of
top flange

Reaction
Contribution of
bottom flange

Contribution of
top flange

Reaction

HOG-BACK

SLOPED BOTTOM FLANGE

Figure A17
Shear on a Hog-Back Beam and a Beam with Sloped Bottom Flange
9.9.3.1A Webs with Intermediate Transverse Stiffeners
In BS 5400: Part 3 Clause 9.9.3.1, replace:
“ MR ” with “ MF ” (7 times).
Definitions of σ f with:
σf

is σ yt (for the tension flange) the nominal yield stress, as defined in Clause
6.1A, or BS 5400: Part 3 Clause 6.2, or (for the compression flange) is the
lesser of σ yc the nominal yield stress value, as defined in Clause 9.3.1A and

MR
Z xc

MR Z xc
is defined in Clause 9.8.1A;
is defined in Clause 9.7.1A.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

Page 34 of 66

9.10A Flanges in Beams with Longitudinal Stiffeners in the Cross Section
9.10.1.1A Flanges Straight in Elevation
In BS 5400: Part 3 Clause 9.10.1.1 in expression (a) replace “

σ lc
MR
” with “

γm γf 3
Z xc γ m γ f 3

Delete the definition for σ lc and add the following definitions:
MR
is as defined in Clause 9.8.1A;
Z xc
is as defined in Clause 9.7.1A.
9.12A Restraints to Compression Flanges
9.12.1A General
Delete the existing BS 5400: Part 3 Clause 9.12.1 and substitute the following:
The load effects on restraints to a compression flange may be determined by nonlinear analysis of representative structural models of beams and the restraint system
under ultimate limit state factored loads with imperfections in geometry of the beams
and their supports corresponding to 1.2 times the relevant tolerances given in
BS 5400: Part 6.
Alternatively, where the compression flange is provided with discrete elastic
restraints connected to the end supports by a system of plan bracing, the system may
be considered to be effective provided that it complies with the following
requirements.
The arrangement and proportions of bracing members should be able to resist, at all
transverse sections of the beam, a restraining lateral shear force F having a value
equal to:
F=
or
F=

where

∑P

f

plus the direct shear arising from wind and other laterally applied
80
forces,
Equation A22

∑P

f

when the effects of wind and other laterally applied forces are not
40
included.
Equation A23

∑P

f

is the sum of the greatest forces in two of the compression flanges of

the beams connected by the bracing at the section under consideration.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 35 of 66

The bracing system should also be checked such that the lateral shear force F as
calculated and applied at the position of maximum flange forces can be transmitted to
the restraints at supports.
9.12.2A Intermediate U-Frame Restraints
9.12.2.2A Strength
Delete the existing BS 5400: Part 3 Clauses 9.12.2.1 and 9.12.2.2 and substitute the
following:
The decking system in combination with the tension flange of the beams should have a
lateral stiffness such that the deflection which would occur when a unit force acts
l
laterally is no greater than e
where le and I c are defined in Clause 9.6.5A.
40EI c
Where the effective length is determined in accordance with Clause 9.6.5.A, each
intermediate U-frame and its connections should be checked for, in addition to the
effects of wind and other applied forces, the effect of horizontal forces FR acting
normal to the compression flange at the level of its centroid given by:
FR =

 σ fc

 σ ci − σ fc

 lw

but not greater than
667
δ
R


 σ fc

 σ ci − σ fc

 EI c

2
16.7 lR

Equation A24

where:
is the half wavelength of buckling, and is determined by taking L lw as the
next integer below L l e but not less than unity;
le , δ R ,Ic ,lR are defined in Clause 9.6.5A;
σ fc
is the maximum compressive stress in the flange;
lw

σ ci =

π 2 ES
2
λ LT

Equation A25

where:
S

= Z pe Z xc ;

Z pe

is the plastic modulus of the section defined in Clause 9.7.1A;

Z xc

is the elastic modulus of the section with respect to the extreme
compression fibre defined in Clause 9.7.1A;
is as derived in Clause 9.7A.

λ LT

Equation A26

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 36 of 66

When there are several interconnected beams, two such forces FR should be applied,
in the same or opposite directions, in such a way as to produce the most severe
effect in the part being considered.
9.12.2.3A U-Frames with Cross Members subjected to Vertical Loading
Delete the existing BS 5400: Part 3 Clause 9.12.2.3 and substitute the following:
The following additional effects should be included for U-frames with cross members
subjected to vertical loading:
(a)

Additional force FC applied to the U-frame, in the same manner as FR in
Clause 9.12.2.2A, and resulting from the interaction between the bending of
the cross members and verticals may be taken as:
θd 2
FC =
Equation A27
3
lR
1.5δ R +
12EI c
where:
lR , d 2 , δ R and I c are as defined in Clause 9.6.5A;
θ
is the difference in rotation under the coincident loading between the
cross member of the U-frame under consideration, and the mean of
the rotations of the cross members of the adjacent U-frames on either
side. The rotations are calculated in radians under the loading, at the
junction of the relevant cross member with the main beam under
consideration, assuming that the cross member is simply supported.

(b)

The lateral flexure of a compression flange due to loading on a cross member
should be taken into account. A conservative method of determining the
resulting transverse moment, and of combining it with other effects, may be
obtained from BS 5400: Part 3 Appendix E as amended by this Appendix.

Alternatively, the effects in (a) and (b) may be evaluated using an analysis that takes
account of the lateral flexure of the compression flange such as an upstand grillage.
Alternatively for (b), the value of MR given in Clause 9.8A may be reduced by 5% and
the effect of lateral flexure disregarded.
9.12.3A Continuous Restraint Provided by Deck
9.12.3.2A Deck not at Compression Flange Level
In BS 5400: Part 3 Clause 9.12.3.2(b) replace expression for f c by:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001

θd 2
1.5δ
where δ and d 2 are as defined in BS 5400: Part 3 Clause 9.6.6.2.
fc =

Page 37 of 66
Equation A28

9.12.4A Restraint at Supports
9.12.4.1A Restraining Forces
Delete the existing BS 5400: Part 3 Clause 9.12.4.1 and substitute the following:
All beams, including cantilever beams assessed in accordance with Clause 9.6A,
should be restrained against rotation about their own axes at or adjacent to each
support in accordance with this Clause and Clause 9.12.4.2A as applicable. The
restraint may be assumed to be shared between support restraints in proportion to
their relative stiffnesses where more than one restraint has been assumed as part of
the support restraint under Clause 9.6.1A.
The restraining system should be capable of resisting, in addition to the co-existent
effects of wind, frictional and other applied forces, two equal and opposite forces Fs
applied normal to the beam and in the planes of its two flanges. Where several beams
are restrained by a common lateral member, two pairs of such forces should be
taken, in the same or opposite directions, such as to produce the most severe effect
in the part under consideration.
Where the resistance is less than required to carry the force Fs the slenderness
parameter λ LT should be modified as under Clause 9.6.1A.
The value of each force Fs in a direction normal to the longitudinal axis of the beam
should be taken as follows:
Fs =

Fs1 + Fs 2 + Fs 3 + Fs 4

Equation A32

where:
Fs1 =

0.004

M
1000∆ F 

2 
  σ fc    g 
D1−   
  σ ci  

Equation A33

which represents the force on a support due to the end torque on the beam
resulting from the initial bow of the compression flange. If ∆ F is not
measured, ∆ F should be taken as g 1000 ;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
Fs 2 =

1.2β ∆σ fc
(σ ci − σ fc )δ t

RT/CE/C/025
Issue: 1
Date: February 2001
Page 38 of 66
Equation A34

which represents the force on the support resulting from the compressive
force in the flange magnified due to the initial departure from verticality of
the supports;
Fs 3 =

Rd 1.2 ∆
+ θL tanα 

D D


Equation A35

which represents the force at a support due to the eccentricity of the lateral
location of the centre of the applied loading relative to the centre of bearing
reaction resulting from departure from verticality of the support;
Fs 4 =

2βθ L tanα
 δ t DL 
 + 
 D GJ 

Equation A36

which represents the force at a support due to any twisting of the beam
caused by interconnection with adjacent beams at a skewed support. Force
Fs 4 may alternatively be derived from a two dimensional grillage analysis of
the structure.
where:
∆F ,g


L

d

G
J
Ic

are defined in Clause 9.8.2A;
is the departure from verticality of the web at the support as measured but
not less than 3 mm. For Level 1 assessment or in the absence of
measurement a value of D 300 should be assumed;
is the span of a simply supported beam or is the greater of the spans on each
side of an internal support when deriving Fs 2 , or the mean of the 2 spans on
either side of a support in a continuous beam when deriving Fs 4 ;
is the vertical distance between the levels of the bearing support and the
applied load respectively. For composite beams the levels of the applied loads
should be taken as the top of the steel beam. For cambered beams d should
be taken as the sum of the individual applied loads multiplied by their relevant
values of d , divided by the total applied loads;
is the shear modulus;
is the St. Venant torsion constant for the beam;
is defined in Clause 9.6.3A;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
M
σfc

σci
D
R
β
θL
α
δt , k2

RT/CE/C/025
Issue: 1
Date: February 2001
Page 39 of 66

is the largest bending moment occurring either at the support or within length
lw of it, whether sagging or hogging;
is the maximum compressive stress in the flange averaged over the whole
flange width, either at the support under consideration or in the span either
side of it;
2EI c δ t
πk 2
for simply
is as defined in Clause 9.12.2.2A taking le =
L
supported beams or as Clause 9.6.3A(b) for continuous beams;
is the overall depth of the beam at the support;
is the bearing reaction;
is 1.0 for an end support, or 2.0 for an internal support in a continuous beam;
is the change due to live loads in the longitudinal slope of the beam adjacent to
the support;
is the angle of skew defined as the angle between the axis normal to the
longitudinal axis of the beam and the axis of the support in plan for a skewed
bridge;
are defined in Clause 9.6.3A.

9.12.4.2A Additional Forces due to Cross members subjected to vertical loading in
beams with U-Frame restraints
When the compression flange of a beam is restrained laterally between points of
support by a system of U-frames with cross members subject to vertical loading, a
force FL should be added to Fs as defined in Clause 9.12.4.1A.
where:
FL =

FL =

θd 2
3
δ t lR
2.5δ R + +
2 3EI c
θd 2
3

2δ R + δ t +
FL =

where:

lR
3EI c

for an end support to a beam with several
internal U-frames;
Equation A37
for a support to a beam with a single
internal U-frame;
Equation A38
for an internal support.

θd 2
3

δR
l
+ δt + R
2
12EI c

Equation A39

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
θ

RT/CE/C/025
Issue: 1
Date: February 2001
Page 40 of 66

is defined in Clause 9.12.2.3A and is to be calculated for the most adverse
distribution on cross beams as follows:
For beams with multiple U-frames in a span, θ is the difference between the
rotation of the U-frame adjacent to an end support and the mean of the
rotations at the end support and the second U-frame from the support
respectively.
For beams with only one internal U-frames in a span, θ is the difference
between the rotation of the internal frame and the mean of the rotations at
the supports at each end of the span.
For an internal support, θ is the difference between the rotation at the
support and the mean of the coincident rotations at the U-frames on each
side of the support.

d 2 ,δ R ,lR and I c are defined in Clause 9.6.5A;
δt
is defined in Clause 9.6.3A.
When a beam is continuously restrained by the deck, so that its effective length is
determined in accordance with BS 5400: Part 3 Clause 9.6.6, FL may be taken as:
FL =
where:

f c (lw1 + lw 2 )
2

Equation A40

fc
is as derived in Clause 9.12.3.2A;
lw1 ,lw 2 are the half wavelengths of buckling of the beam on each side of the support
under consideration derived in accordance with Clause 9.12.2.2A.
U-frames other than those at the supports assumed to be part of the support Uframe under Clause 9.6.1A, should be checked against force Fc in accordance with
Clause 9.12.2.3A together with the appropriate proportion of force FL under this
Clause.
9.12.4.3A Stiffness
Delete the existing BS 5400: Part 3 Clause 9.12.4.2 and substitute the following:
The assumed stiffness of restraints including bearing stiffeners as limited by the
stability of the beam against overturning at supports against rotation about the
longitudinal axis should be compatible with the assumptions for effective length in
Clause 9.6A.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 41 of 66

9.13.5.3A Buckling of Effective Stiffener Section
In BS 5400: Part 3 Clause 9.13.5.3:
Replace “ σ ys ” with “ σ y ”in the first expression.
Delete the definitions for σ ls , Z x and σ ys and substitute the following definitions:
σ ls

is taken as equal to the value of σ c determined from:
(a)

for double sided stiffeners, curve D of BS 5400: Part 3 Figure 37; or

(b)

for single sided stiffeners, curve B of BS 5400: Part 3 Figure 37 when
the web plate is in compression or curve D when the stiffener
outstand is in compression:

where:
σy

λ=

ls
rse

Zx

is the elastic section modulus of the effective section about the
centroidal axis parallel to the web with reference to the extreme
fibres under maximum compressive stress;
is the nominal yield stress value, as defined in Clause 9.3.1A for the

σy

355

;

web plate, σ yw or for the stiffener, σ ys , whichever is the lesser.
Add to end of existing BS 5400: Part 3 Clause 9.13.5.3:
When assessing the adequacy of a transverse web stiffener allowance should be made
for initial departures from straightness, ∆ sx , measured in accordance with BS 5400:
Part 6 over a gauge length taken as g as defined in Clause 9.8A, σ ls should be
calculated from the equations in BS 5400: Part 3 Appendix G12 with η taken as:
λ −15 
y
η = 0.0083(λ −15) + 
[1.2 ∆ sx − 0.0016a] 2 but not less than zero
rse
 λ 
Equation A41
where:
y
∆ sx

is the distance from the neutral axis of the effective stiffener to the extreme
fibre under consideration;
is taken as positive when the bowing is in a direction away from the extreme
fibre under consideration.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 42 of 66

9.14.4A Strength of Bearing Stiffeners
9.14.4.4A Buckling Resistance of Unstiffened Webs
The buckling resistance PD of an unstiffened web over a bearing should be taken as:
σ c b eff t w

PD =

Equation A42

γ mγ f 3

where :
σc
is the ultimate compressive stress about an axis along the centre line of the
web obtained from σ c σ y in accordance with BS 5400: Part 3 Figure 37

beff

Curve C. In using Figure 37, le should be determined taking account of the
lateral and rotational restraint of the flange;
is the effective breadth of web obtained from:
beff =

(d 2 + s 2 ) but not greater than the width available (refer to BS 5400:
Part 3 Figure 27);

d
s

is the overall depth of the beam;
is the bearing length.

10A. DESIGN OF COMPRESSION MEMBERS
Where specified recommendations for the geometry of members including battened
and laced compression members given in BS 5400: Part 3 Clause 10 are not complied
with, reference may be made to BD 56/96: The Assessment of Steel Highway Bridges
and Structures or other recognised literature or research for consideration as a
departure from standards in the application for Approval in Principle.
10.3.1A Unstiffened Outstand
Delete the existing definition for σ y ′ and substitute following definition:
σy′

is the lesser of the nominal yield stress of the material or such lower value of
yield stress as would be necessary to meet the strength criteria of the
subsequent Clauses.

10.3.3A Circular Hollow Section
Delete the expression and substitute:
60

355
σ y′

Expression A43

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 43 of 66

10.3.4A Assessment of Sections not Complying with Shape Limitations
Outstands not complying with Clause 10.3.1A or BS 5400: Part 3 Clause 10.3.2
should be assessed in accordance with BS 5400: Part 3 Clause 9.3.2. Circular hollow
sections not complying with Clause 10.3.3A should be assessed in accordance with
Clause 9.3.6A. As a result a lower value of yield stress should be determined such
that compliance with the strength criteria of BS 5400: Part 3 Clause 10.6 and
Clause 10.3.1A, BS 5400: Part 3 Clause 10.3.2 or Clause 10.3.3A as applicable is
achieved. This lower value of yield stress should be used in all subsequent assessment
of strength in accordance with Clause 9.3.1A.
10.6.1.1A Strength
Add to end of existing BS 5400: Part 3 Clause 10.6.1.1:
Where in assessing the adequacy of a compression member allowance is made for
initial departures from straightness, ∆ s , measured in accordance with BS 5400:
Part 6, over a gauge length g equal to the clear length of the compression member,
σ c should be calculated from the equation in BS 5400: Part 6 Appendix G16 with η
taken as:
η=

y
λ −15 
α(λ −15)+ 
[1.2 ∆ s − 0.00012G ] 2 but not taken less than zero
r
 λ 
Equation A44

10.7.2A Evaluation of Stresses
Add to end of existing BS 5400: Part 3 Clause 10.7.2(c):
In assessment of the adequacy of a compression member with longitudinal stiffeners
where allowance is to be made for measured initial departures from straightness, ∆i
should be determined separately for the X-X and Y-Y axes using the expression:
∆i =

1.2∆ s

Equation A45

where ∆ s is the departure from straightness measured in accordance with
BS 5400: Part 6 over a gauge length g equal to the distance between applicable points
of restraint.
11A. TENSION MEMBERS
Where specified recommendations for the geometry of members including those
such as battens are not complied with, reference may be made to BD 56/96 or other
recognised literature or research for consideration as a departure from standards in
the application for Approval in Principle.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 44 of 66

11.1A General
Add to end of existing BS 5400: Part 3 Clause 11.1:
Nominally straight members subjected to axial tension or to combined tension and
bending should be assessed as described below:


Where acting in compression the member should be assessed in accordance
with Clause 10 unless sufficient redundancy or an alternative load path exists.
For these cases such compression may be ignored.



Where the requirements for battens, lacing, perforated plates, back to back
members are not met, the ability of the members and their relevant fixings to
resist the load effects to which the members are subjected should be assessed.

11.3.2A Effective Area
Add to end of existing BS 5400: Part 3 Clause 11.3.2:
For assessment the value of k 2 may be taken as follows:
1.2
1.15
1.1
1.0

where the member is BS 4360 grade 43, BS 15: 1906, BS 15: 1948 or
pre 1905 steel
where the member is wrought iron
where the member is BS 4360 grade 50 or BS 968 steel
where the member is BS 4360 grade 55 or Thirty Oak steel

Alternatively where the member is steel or wrought iron not complying with
BS 4360, BS 15: 1906, BS 15: 1948 or BS 968 and σ y and σ ult are the nominal yield
stress and ultimate stress derived in accordance with BS 5400: Part 3 Clauses 6.2 and
6.3 respectively, k 2 may be determined as:
k2 =

σ

1.0 + 0.5 ult −1.2  but not taken greater than 1.2.
 σy


Equation A46

11.3.5A Pin Connected Members
Add to end of existing BS 5400: Part 3 Clause 11.3.5:
Where this recommendation is not met, a check should be made that tearing will not
occur beyond the pin hole.
11.4A Thickness at Pin Holes
Add to end of existing BS 5400: Part 3 Clause 11.4:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 45 of 66

Where this recommendation is not met a check should be made that local buckling
will not occur beyond the pin hole.
12A. TRUSSES
12.1A General
Add to end of existing BS 5400: Part 3 Clause 12.1:
Bending effects may be ignored in BS 5400: Part 3 Clauses 12.2.2 and 12.2.3 where:
(i)

the truss is fully triangulated;

(ii)

the centroids of the members intersect at the joint being considered;

(iii)

all significant loadings excluding the self weight of the members are applied at
the joints;

(iv)

the joints use untensioned bolts or rivets such that any secondary bending can
be relieved by joint movement.

12.4.1A General
Add to end of existing BS 5400: Part 3 Clause 12.4.1:
A compression chord may be considered to be effectively braced provided that the
restraint system complies with the recommendations of Clause 9.12.1A with the
chord treated as a flange.
12.4.2A Lateral Restraint by Deck to Compression Chord
For assessment BS 5400: Part 3 Clause 12.4.2 may be ignored.
12.5.1A Effective Length
Delete the existing BS 5400: Part 3 Clause 12.5.1 and substitute the following:
Where there is no intermediate lateral restraint to a compression chord, the
effective length may be derived as for a beam in accordance with Clause 9.6.3A with
k e =1.
Where the lateral restraint to a chord is provided by U-frames comprising cross
members and web members (see Figure A41) the effective length may be determined
in accordance with Clause 9.6.5A with δ R taken as follows:
δR =

1
1
1
+∑
δv
δi

Equation A47

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 46 of 66

where:
δv

δi

is the deflection for a U-frame component with a vertical truss web member,
given by:
3
2
d1 uBd 2
δv =
+
+ fd 2
Equation A48
3EI1 EI 2
is the deflection for a U-frame component with an inclined truss web member,
given by:
3
2
d 3 uBd 2
2
δi =
+
+ fd 2 + θs
Equation A49
3EI 3
EI 2
where:
d3
I3
f

θ

is the length of the diagonals measured as the distance sloping from the
centroid of the chord to the top face of the cross member of the
U-frame as shown in Figure A41;
is the second moment of area of the diagonals forming an arm of a
U-frame about its axis perpendicular to the plane of the U-frame;
is the flexibility of the joints between the cross member and the truss
member, expressed in radians per unit moment. Values of f may be
taken from Figure A42;
may be taken as zero when the bottom truss chords are fully
restrained against lateral deflection throughout their lengths by an
integral deck. Otherwise, θ may conservatively be taken as:
sB
θ =
for an end diagonal;
Equation A50
2nEI 4
sB
θ =
for an intermediate diagonal.
Equation A51
nEI 4
where:
I4
is the second moment of area of the cross member about its
vertical axis;
m
may be taken as:

n
s

a)

1.5 for a Warren or a Pratt Truss;

b)

4.0 for a modified Warren Truss type (1) or (2) as
shown in Figure A41 having a cross member at each
bottom chord joint with web members;

should be taken as 1.0 when the connection between the chord
and cross member is rigid in plan or zero when the connection
between chord and cross member is flexible in plan;
is the spacing of cross members forming effective U-frames.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

d3

Page 47 of 66

d3

Warren truss

Modified Warren truss (1)

d3
d3
Modified Warren truss (2)

Pratt truss

Y

Y
R

R

FR

FR

d3
DIAGONAL
LENGTH

d1 d
2

I

1

I2
B

Y

Y

Figure A41
Truss Types and Lateral Restraint by U-Frame

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

Page 48 of 66

Connection
f
-4
through
x10
top or bottom RAD/kNm.
flange

(Or top cleat
unstiffened)

0.5
Vertical
stiffener

Extended
end plate

Connection
to both
flanges

0.2

Connection to
bottom flange
Gusset

Gusset
Weld to
gussets

0.1

Connected

Troughing

Bars through
stiffeners

All crests cleated
and troughs bolted

0.5

Connect crests to vertical
stiffeners <2 bolts.

0.2

Stiffener

0.8
Cross Girders not at stiffeners

0.4
Cross Girders cleated
to stiffener

Figure A42
U-Frame Joints and ‘f’ Values

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001

Rebar through
stiffeners

Page 49 of 66
f -4
x10
RAD/kNm.

0.5

Cleats to
web

Stiffeners
not at crests

0.95
Fabricated
troughs

if decking plate
not connected
to vertical
stiffener

Floor plate NOT
connected to vertical
stiffeners

0.3

* Floor plate
connected

H.S.F.G. bolted
connection

0.2*

H.S.F.G. bolted
connection

0

Welded

H.S.F.G. bolted
connection

H.S.F.G. bolted
connection

Figure A42 (continued)
U-Frame Joints and ‘f’ Values
A U-frame restraint should be taken into account at each connection of a web
member with the compression chord. At any restraint position, more than one web
member may be connected, and the members may be diagonal or vertical. The Uframe restraint assumed may include all the web members at each position, or may
conservatively neglect the more flexible web members, such as the tension diagonals

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 50 of 66

in a Pratt truss. Web members should be included only when the members are
adequately connected to the cross members either directly or by stiffening.
Where a cross member acts with components of more than one U-frame the
moment of area I 2 should be proportioned between the components concerned.
Where more than one type of intermediate U-frame occurs alternately, such as in a
modified Warren truss, (1) in Figure A41, when all the web members are taken into
account, the average value of δ R may be assumed.
12.5.2A Restraints to Compression Chords
Delete the existing BS 5400: Part 3 Clause 12.5.2 and substitute the following:
Restraints to compression chords should comply with the relevant recommendations
of Clause 9.12A with chords treated as flanges.
In calculating FR in accordance with Clause 9.12.2.2A, S may be taken as 1.0 and
λ LT = le ry where ry is the radius of gyration of the chord about the Y-Y axis (see
Figure A41).
12.6.1A General
In BS 5400: Part 3 Clause 12.6.1:
In the second paragraph, line 3, substitute “12.6.2” with the relevant
recommendations of Clause 9.12.1A, with the chords treated as flanges”.
In the last line replace BS 5400: Part 3 Clause 12.5.3 with Clause 12.5.2A.
Delete the last sentence beginning with “U-frames.....12.5.3”.
12.6.2A Forces on Bracing
For assessment BS 5400: Part 3 Clause 12.6.2 may be ignored.
12.6.3A Lateral Bracing not providing Adequate Restraint
Add additional Clause 12.6.3 to BS 5400: Part 3.
Where any of the provisions of Clause 12.6.1A are not met, one of the following
options should be used to assess the Bridge:
(i)

lateral bracing should be ignored and assumed to provide no restraint with
the member capacities reduced accordingly;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 51 of 66

(ii)

the structure capacity should be reduced such that the provisions of
Clause 12.6.1A are met;

(iii)

a rigorous non-linear analysis should be carried out of the complete system to
verify the capacity of the members and adequacy of the bracings.

12.7A Curved Members
Add to end of existing BS 5400: Part 3 Clause 12.7:
Where members do not comply with recommendations (a) to (d) consideration
should be given to:
(1)

the forces and stresses according to BS 5400: Part 3 Clause 9.5.7;

(2)

the effects of the change in neutral axis position due to curvature;

(3)

the buckling resistance of the section if it does not satisfy the criteria for a
compact section;

(4)

the adequacy of flanges to resist the radial component of the flange force.
Assuming the axial force in the flange is distributed uniformly across the
width, the line load radial force per unit width across the flange per unit length
of the flange may be expressed as:
σ f t fo
Rf
σf tf
Rf

in a flange outstand, or

in a plate panel between longitudinal stiffeners and/or webs

where σ f , t f , t fo and R f are as defined in BS 5400: Part 3 Clause 9.5.7.1.
14A. CONNECTIONS
14.3.5A Connection of Restraints to Parts in Compression
Add to end of existing BS 5400: Part 3 Clause 14.3.5:
Where the connection cannot resist the forces in (a) and (b) above, the intermediate
restraint should be ignored. Alternatively, the system may be checked making due
allowance for the maximum restraint that can be provided, see Clause 12.6.3A.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 52 of 66

14.4A Splices
14.4.1.1A General
Add to end of existing BS 5400: Part 3 Clause 14.4.1.1:
The following assumptions may be made for assessment:
(a)

where both surfaces of the spliced parts are provided with covers, axial
stresses only should be assumed.

(b)

where only one surface is provided with a cover, bending effects should be
considered at the serviceability limit state, but may be ignored at the ultimate
limit state. For the calculation of bending effects the line of action of the axial
force in the splice may be assumed located along the interface between the
parent material and the cover. The effects of eccentricity should be ignored
when bending is effectively prevented by:
(i)
(ii)

the presence of surrounding or adjacent concrete or other solid infill;
or
the presence of an element which prevents bending of the parent
material or the cover. Such an element should be within a distance of
12t from the furthest fastener, where t is the thickness of the parent
material to which the cover plate is attached.

14.4.5A Obsolete Splicing Methods
Add additional Clause 14.4.5 to BS 5400: Part 3:
When assessing splices where several plates were required to build up the required
section thickness, consideration should be given to the load path through the joint to
ensure no single component is overloaded, see Figures A63 and A64.

C

P

P

Figure A63
Force Flow in Typical Triple Plate Shingle Joint

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 53 of 66

P

P
Filler plate

(a) Loose Fillers
P

P
Filler plate

(b) Tight Fillers
Figure A64
Types of Filler Plate
14.5A Connections made with Bolts, Rivets or Pins
14.5.1.5A Assessment of Non Complying Arrangements
Add additional Clause 14.5.1.5 to BS 5400: Part 3.
Where any of the limits in BS 5400: Part 3 Clauses 14.5.1.1, 14.5.1.2, 14.5.1.3 or
14.5.1.4 are not complied with, allowance should be made for a reduced strength of
the fasteners or plate where there is evidence of plate bulging, distortion near or to
fasteners, or excessive rust forcing. Reductions in strength should also be applied in
the following cases:
(a)

Where the parts joined are in compression and the distance, in the direction
of stress, S a between centres of adjacent rivets or bolts exceeds Ss the
maximum distance specified according to the requirements of BS 5400: Part 3
2

S 
Clause 14.5.1, the yield stress of the outer plies should be reduced by  s  .
 Sa 
(b)

The gauge limit in BS 5400: Part 3 Clause 14.5.1.3 may be increased to 80 mm
in determining the specified maximum spacings under BS 5400:
Part 3 Clause 14.5.1.3.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 54 of 66

14.5.2A Edge and End Distance
Add to end of existing BS 5400: Part 3 Clause 14.5.2:
Where any of the above limits are not complied with, the strength of the fastener or
plate should be reduced as follows:
(a)

For fasteners away from an edge when the spacing between two fasteners is
less than 2.5d, the strength of each should be reduced linearly to zero when
the spacing reaches 1.5d. Where a number of fasteners are close to each
other, the reduction factors should be multiplied together;

(b)

For fasteners adjacent to an edge parallel to the direction of force - the value
of k 2 in BS 5400: Part 3 Clause 14.5.3.6 should be linearly reduced from the
value 2.5 when the edge distance is 1.2d to a value of zero when the edge
distance is 0.8d;

(c)

For fasteners adjacent to an end, loaded by a force away from the edge of the
part - no reduction should be made, subject to a minimum end distance of
0.8d. For a lesser distance, the fastener should be ignored;

(d)

For fasteners adjacent to an end loaded by a force toward the edge of the part
- the value of k 2 in BS 5400: Part 3 Clause 14.5.3.6 should be reduced linearly
from the value of 1.2 when the edge distance is 1.2d to a value of zero when
the edge distance is 0.9d.

(e)

For HSFG bolts - when the spacing between fasteners is less than 2.5d, the
friction capacity should be reduced in linear proportion from a value of 100%
of the maximum capacity at 2.5d to 80% of the maximum capacity at 2.0d.
When the spacing is less than 2.0d, the fasteners should be ignored.
When the edge distance is less than 1.5d, the friction capacity should be
reduced linearly to a value of zero when the distance is 1.0d. When the edge
distance is less than 1.0d the fastener should be ignored.

14.5.3.3A Rivets Subject to Axial Tension
Add to end of existing BS 5400: Part 3 Clause 14.5.3.3:
The tensile capacity should be reduced where there is significant loss of material from
rivet heads. Where rivets are subject to tension due to live loads, σ f should be
reduced to that for countersunk rivets where the remaining effective head diameter
is 1.3 times the nominal diameter and to zero if less than 1.3 times the nominal
diameter remains.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 55 of 66

14.5.3.4A Fasteners Subject to Shear Only
In BS 5400: Part 3 Clause 14.5.3.4 replace definition of σ q by:
σq

is σ y for all fasteners except black bolts and rivets for which 0.85 σ y or
0.9 σ ult respectively may be assumed where σ ult is the ultimate tensile
strength of rivet material as given in Clause 6.1A.

14.6A Welded Connections
14.6.1A General
Add to end of existing BS 5400: Part 3 Clause 14.6.1:
For bridges known to have been welded in accordance with BS 5400 Part 6 or
BS 5135: 1974 (or 1984), the strength of the welds should be determined in
accordance with BS 5400: Part 3 Clauses 14.6.2.3 and 14.6.3.11. For bridges not
known to have been welded in accordance with BS 5400: Part 6 or with
BS 5135: 1974 (or 1984) the strengths of the welds should be derived in accordance
with (a) to (d) below:
(a)

For butt welds in compression and butt welds in tension or shear
demonstrated to comply with BS 5135: Table 18, quality A, the strengths may
be taken as defined in BS 5400: Part 3 Clause 14.6.2.3;

(b)

For butt welds in tension or shear free from surface cracks but not known to
comply with BS 5135: Table 18, quality A, the strengths may be taken as 85%
of those derived from BS 5400: Part 3 Clause 14.6.2.3;

(c)

For fillet welds in bridges constructed to BS 153: Part 1(1958 or 1972) and
free from surface cracks, the weld strengths should be taken as 90% of those
derived from BS 5400: Part 3 Clause 14.6.3.11 in the absence of
demonstration of their compliance with BS 5135: Table 19 quality A or equal
to those strengths when such compliance has been demonstrated;

(d)

For other fillet welds free from visible surface cracks, the strengths should be
calculated in accordance with BS 5400: Part 3 Clause 14.6.3.11, but replacing
the equation σ w = 0.5(σ y + 455) by either:
(i)

0.4 (400 + σ y min ) in the absence of demonstration of their compliance
with BS 5135: Table 19 quality A, or

(ii)

0.5(400 + σ y min ) when such compliance has been demonstrated:

where σ y min is the yield stress of the weaker of the parts connected by the
welds.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 56 of 66

Where any of the general or specific recommendations of this or any of the following
sub-clauses are not met, due allowance should be made in the assessment of the
strength of the welds. Further information is given in Appendix F of this Code.
14.6.2.1A Intermittent Butt Welds
Delete the existing BS 5400: Part 3 Clause 14.6.2.1 and substitute the following:
For intermittent butt welds no contribution to strength of the weld should be
assumed for a weld length equal to three times the throat thickness at each end of
any intermittent length.
14.6.2.2A Partial Penetration Butt Welds
Add to end of existing BS 5400: Part 3 Clause 14.6.2.2:
The strength of partial penetration butt welds should be calculated as for fillet welds.
The throat thickness should be taken as the depth of the web preparation less 3 mm,
or as measured consistently at site. Where the weld is unsymmetrical relative to the
parts being jointed the resulting eccentricity should be allowed for when calculating
the maximum stresses, under all loadings other than those which act along the axis of
the weld.
14.6.3.11.1A Welds Subject to Longitudinal Shear Only i.e., shear in the direction
of its length (see Figure 55(a))
Delete the existing BS 5400: Part 3 Clause 14.6.3.11.1 and substitute the following:
The stress in a weld, calculated as the longitudinal shear force per unit length PL
divided by the effective throat, g , should not exceed:
σw
γf 3γ m 3

Expression A52

where:
σw

is the yield stress of the deposited weld metal and may be taken as
0.5(σ y + 455) N/mm²;

σy

is the lesser nominal yield stress value, as defined in Clause 9.3.1A or
Clause 10.3.1A of the two parts joined.

14.6.3.11.2A Weld Subject to Transverse Force Only (force at right angles to its
length see Figure 55(b))
Delete the existing BS 5400: Part 3: Clause 14.6.3.11.2 and substitute the following:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 57 of 66

The stress in a weld, calculated as the transverse force per unit length PT1 (or PT2)
shown in Figure 55(b) divided by the effective throat g, should not exceed:
Kσ w
Expression A53
γf 3γ m 3
where:
σw

is as defined in Clause 14.6.3.11.1A;

K

depends on the angle θ between the direction of the resultant transverse
force and the throat and is given by:
K=

3
but may not be greater than 1.4
1+ 2cos 2 θ

For equal fillets between components at right angles θ = 45° and K =1.225 .
14.6.3.11.3A Welds Subject to Forces in Both Transverse and Longitudinal
Directions
Add additional Clause 14.6.3.11.3 to BS 5400: Part 3:
The following condition should be satisfied:
2

P
σw
1
2
PL + T 2 ≤
g
K
γf 3γ m 3

Equation A54

where:
PL
PT

σw

is the longitudinal shear force per unit length of the weld;
is the resultant of transverse forces per unit length of the weld (see
Figure 55(c));
is the effective throat of the weld;
is the angle between the direction of the resultant transverse force and the
throat;
is as defined in Clause 14.6.3.11.1A;

K

is as defined in Clause 14.6.3.11.2A

g
θ

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 58 of 66

p

L

p

L

Figure 55(a)
Weld subjected to longitudinal shear

Throat of
the weld

2P T2

PT1
PT1

PT2

Figure 55(b)
Weld subjected to transverse force

PT2

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron
PT

Throat of
the weld

Figure 55(c)
Resultant transverse force at weld

RT/CE/C/025
Issue: 1
Date: February 2001
Page 59 of 66

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 60 of 66

BS 5400: PART 3 APPENDIX B
DISTORTION AND WARPING STRESSES IN BOX GIRDERS
B.1A GENERAL
In BS 5400: Part 3 Appendix B Clause B.1 delete “highway” from the first sentence.
B.2A RESTRAINT OF TORSIONAL WARPING
In BS 5400: Part 3 Appendix B Clause B.2 in the definition for J, delete “B” and
replace by “W”.
In BS 5400: Part 3 Appendix B Clause B.2 in the definition for B, delete “B” and
replace by “W”.
B.3.2A
CORNER STRESS
In BS 5400: Part 3 Appendix B Clause B.3.2:
In Clause (b) replace “knife-edge load” with “concentrated load”.
In Clause (c) replace “HA loading, the effects of the uniformly distributed and knifeedge load” with “RU loading, the effects of the uniformly distributed and axle loads”.
Add the following at the end of the definition for R D :
“or from
BT
BB
DYT d  2BB   BB 
 B  D d 

+
 − VD  2 + B  YT +1
DYC BT  BT + BB   BT + BB 
 BT  DYC BT 
1+

RD =

Equation A55

where:
VD
is as defined in Clause B.4.2A;
BB , BT are defined in Clause B.2A.
B.3.4.2A

STRENGTH

Delete the existing BS 5400: Part 3 Appendix B Clause B.3.4.2 and substitute the
following:
(a)

A plate diaphragm should be capable of resisting a shear stress τD given by:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001

T
2BDt d

τD =

Page 61 of 66
Equation A56

where:
td
B

is the thickness of the diaphragm plate;
is as defined in BS 5400: Part 3 Clause 9.17.2.7, and is the average of
the widths at the top and bottom flanges;
is as defined in Clause B.2A;
is the torque due to loads applied at the diaphragms and between the
adjacent diaphragms or cross frames on either side. Any torque
applied between the diaphragm and an adjacent diaphragm or cross
frame may be apportioned by simple static analysis.

D
T

B.3.4.3A

STIFFNESS

Delete the existing BS 5400: Part 3 Appendix B Clause B.3.4.3 and substitute the
following:
A cross frame or a diaphragm should have a dimensionless stiffness S not less than the
value obtained from Table A17.
where:
2

S=

2

Gt d L p δ b K
2 Ap L D

for a plated diaphragm;

2

S=

EAb δ b K
LD Lp
2

S=

for a cross braced frame;
2

EAb L p δ b K
4L D L b

3

for a vee braced cross frame, irrespective of whether the
centre of the V is at the top or bottom flange.

Alternatively,
S=

KR
KL D

for an unbraced ring cross frame with constant section framing members;
2

S=

Pp δ b K
∆ pLD

for any type of cross frame including a ring cross frame.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 62 of 66

where:
∆p

is the change in length of the diagonal L p calculated to occur under the system
of diagonal forces Pp as shown in the Figure A65 below. This method of
deriving stiffness may be used for any type of frame including those given
below;

(D2 + B 2 )

Lp

is

Lb
Ap

is the length of the brace;
is B × D which is the surface area of the plated diaphragm;

Ab

is the area of cross section of brace;

δb

is

B

is (BT + BB ) 2 which is the average width of the box girder;

L D ,K

are as defined in Clause B.3.2;

KR

is the value of K derived by taking DYT , DYB and DYC as the flexural rigidities of
the effective framing members attached to the top and bottom flanges and
webs respectively;

which is the length of the diagonal;

4 BD
which is a unit length flexibility;
KBB L P

BT , B D and D are as defined in Clause B.2A.

BT

Pp

Pp

B

D/2
D

LW

Pp

Lp

Bn
Figure A65

D/2

Pp

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 63 of 66

Cross Frame Parameters
SINGLE TORQUE
UNIFORMLY DISTRIBUTED
TORQUE
For σ DW
For σ DB
For σ DW
For σ DB

βL D

3.0
0
0
0
2.0
0
5
0
1.5
2
10
10
1.0
20
50
100
0.8
50
100
200
0.5
500
1000
200
0.3
2000
10000
200
For intermediate values of β LD , values of S may be obtained by logarithmic
interpolation.

5
30
200
500
1000
10000
20000

TABLE A17
DIAPHRAGM STIFFNESS S
B.4.2A
CORNER STRESS
In BS 5400: Part 3 Appendix B Clause B.4.2:
In Clause (b) replace “knife-edge load” with “concentrated load”.
In Clause (c) replace “HA loading” with “RU loading” and “knife-edge” with, “axle
loads”.
Add the following at the end of the definition for VD :
or from

VD =

 DYT d  B B  
 D B  2 + B  +1
 YC T 

T 
2
3
 BT    DYT d  BB  BB   DYT  BB  
1+ +    +
 +1 1+ 2 
  
 BB    DYC BT  BT  BT   DYB  BT  

Equation A57

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 64 of 66

BS 5400: PART 3 APPENDIX D
PATCH LOADING ON WEBS
Delete the existing BS 5400: Part 3 Appendix D Clause D.1 and substitute the
following:
D.1A Beams without Longitudinal Stiffeners on Web
The limiting value of patch load P on each web in its plane should be taken as the
lesser of:
(a)

Web buckling criterion
2

t  wt
Eσ yw f 1+ 3  w
t w 
d  tf

P =

0. 5t w

(b)

Web yielding criterion

P =

(2t

f

1.5






 γm γf 3 σ f
 1− 

 σ yw

γ γ σ
σ yf σ yw Bf t w + σ yw t w w ) 1−  m f 3 f
 σ yw

2

 1

 γm γf 3

Equation A58

2

 1

 γm γf 3

Equation A59

where:
tf

is the flange plate thickness;

tw
w

is the web plate thickness;
is the width of the patch load (see BS 5400: Part 3 Clause 9.5.6 and Figure 6)
but to be taken not greater than 0.2d;
is the width of the flange plate;

Bf

d
is the depth of the web in its plane;
σyf ,σ yw are the nominal yield stresses of the material of flange and web respectively, as

σf

defined in Clause 6.2A;
is the longitudinal stress in the flange due to bending moment and/or axial

γm

force on the beam;
is taken as 1.05 for the ultimate limit state.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 65 of 66

BS 5400: PART 3 APPENDIX E
Transverse Moments in Compression Flanges: U-frame Restraints
In BS 5400: Part 3 Appendix E in line 2 replace “9.12.2.3(b) or 9.12.3.2(c)” by
“BS 5400: Part 3 Clause 9.12.3.3(b) or Clause 12.5.2A”.
Replace the definitions as follows:
Ic
d2
θ
σ fc

is as defined in BS 5400: Part 3 Clause 9.6.4.1.1.2;
is as defined in BS 5400: Part 3 Clause 9.6.4.1.3;
is as defined in BS 5400: Part 3 Clause 9.12.3.3(a) or Clause 9.12.4.2A as
applicable;
is the maximum compressive stress in the flange;

σ ci ′

is taken as follows:
(a)

if lw is less than three times the spacing of U-frames, σ ci ′ = σ ci

as

defined in Clause 9.12.2.2A or 12.5.2A;
(b)

if lw is more than four times the spacing of U-frames, or if lc has been
calculated in accordance with BS 5400: Part 3 Clause 9.6.6.2,
σ'ci= 1.25σcior;

(c)

for intermediate values of le , σci, is obtained by linear interpolation
where:
lw
L

is as defined in Clause 9.12.2.2A;
is as defined in BS 5400: Part 3 Clause 9.6.2”.

In the expression in item (b), replace “ σ f σ lc ” with “ σ f Z xc MR ” and replace
“ σ b σ lco ” with “ σ b Z xc Mult ”.
Delete the definitions for σ lc and σ lco and add the following definitions:
MR and Mult are as defined in Clause 9.8.1A;
Z xc is as defined in Clause 9.7.1A.
Add the following to the end of BS 5400: Part 3 Appendix E:
(c)

for chords in trusses see BS 5400: Part 3 Clause 10.6.2.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix A - Assessment of Steel and Wrought Iron

RT/CE/C/025
Issue: 1
Date: February 2001
Page 66 of 66

BS 5400: PART 3 APPENDIX G
Equations used for production of curves in Figures
G.7A Figure 10 - Basic Limiting Stress
Delete the existing BS 5400: Part 3 Clause G.7 and substitute the following:
2

MR
5700  
5700  22800 
= 0.51+ (1+ η) 2  − 1+ (1+ η) 2  − 2  when β > 30 , or
Mult
β  
β 
β 



MR
= 1.0 when β ≤ 30
Mult

Equation A60
Equation A61

where:
η=

 β 
0.0035(β − 30 ) +1.2
k for Figure A10 but not less than zero; or
 β − 30 

η=

 β 
0.008(β − 30 ) +1.2
k for Figure A11 but not less than zero;
 β − 30 

β=

 σ yc  Mult
λ LT 

 355  Mpe


;



MR , Mult and σ y are as defined in Clause 9.8.1A;
Mpe

is defined in Clause 9.7.1A.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 100

CONTENTS
1B. SCOPE ....................................................................................................................................5
2B. DEFINITIONS AND SYMBOLS ........................................................................................5
2.1B Definitions .......................................................................................................................5
2.1.1B General.....................................................................................................................5
2.1.2B Partial Factors .........................................................................................................5
2.1.3B Materials...................................................................................................................5
2.2B Symbols ............................................................................................................................5
3B. LIMIT STATE PHILOSOPHY ...........................................................................................10
3.1B General...........................................................................................................................10
3.2B Deflection ......................................................................................................................11
4B. GENERAL..............................................................................................................................11
4.1B Limit State Criteria ......................................................................................................11
4.1.1B Serviceability Limit States ...................................................................................11
4.1.2B Ultimate Limit States ...........................................................................................11
4.1.3B Other Considerations .............................................................................................12
4.2B Loads, Load Combinations and Partial Factors γfL and γf3 ....................................12
4.2.1B Loads.......................................................................................................................12
4.2.2B Serviceability Limit State.....................................................................................12
4.2.3B Ultimate Limit State.............................................................................................12
4.3B Properties of Materials ...............................................................................................12
4.3.1B General...................................................................................................................12
4.3.2B Material Properties ..............................................................................................13
4.3.3B Values of γm ............................................................................................................16
4.4B Analysis of Structure ...................................................................................................17
4.4.1B General...................................................................................................................17
4.4.2B Analysis for Serviceability Limit State ..............................................................17
4.4.3B Analysis for Ultimate Limit State ......................................................................18
4.5B Analysis of Section .......................................................................................................18
4.5.1B Serviceability Limit State.....................................................................................18
4.5.2B Ultimate Limit State.............................................................................................19
4.6B Deflection ......................................................................................................................19
4.7B Fatigue ............................................................................................................................19
4.8B Combined Global and Local Effects .........................................................................19
4.8.1B General...................................................................................................................19
4.8.2B Analysis of Structure ...........................................................................................19
4.8.3B Analysis of Section ...............................................................................................20
5B. REINFORCED CONCRETE .............................................................................................20
5.1B General...........................................................................................................................20
5.1.1B Introduction ..........................................................................................................20
5.1.2B Limit State Assessment of Reinforced Concrete ..........................................20

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 100

5.1.3B Loads.......................................................................................................................21
5.1.4B Strength of Materials ...........................................................................................21
5.2B Structures and Structural Frames.............................................................................21
5.2.1B Analysis of Structures..........................................................................................21
5.2.2B Redistribution of Moments ................................................................................21
5.3B Beams.........................................................................................................................22
5.3.1B General...................................................................................................................22
5.3.2B Resistance Moment of Beams............................................................................23
5.3.3B Shear Resistance of Beams.................................................................................26
5.3.4B Torsion...................................................................................................................30
5.3.5B Longitudinal Shear................................................................................................33
5.3.6B Vertical Deflection of Beams .............................................................................33
5.3.7B Crack Control in Beams.....................................................................................33
5.4B Slabs ................................................................................................................................33
5.4.1B Moments and Shear Forces in Slabs .................................................................33
5.4.2B Resistance Moments of Slabs.............................................................................33
5.4.3B Resistance to In-plane Forces............................................................................34
5.4.4B Shear Resistance of Slabs....................................................................................34
5.4.5B Deflection of Slabs ...............................................................................................37
5.4.6B Crack Control in Slabs........................................................................................38
5.4.7B Torsion in Slabs ....................................................................................................38
5.5B Columns.........................................................................................................................38
5.5.1B General...................................................................................................................38
5.5.2B Moments and Forces in Columns .....................................................................39
5.5.3B Short Columns Subject to Axial Load and Bending about the
Minor Axis ............................................................................................................40
5.5.4B Short Columns Subject to Axial Load and either Bending about the
Major Axis or Bi-axial Bending.........................................................................42
5.5.5B Slender Columns ..................................................................................................43
5.5.6B Shear Resistance of Columns ............................................................................45
5.5.7B Crack Control in Columns ................................................................................46
5.6B Reinforced Concrete Walls.......................................................................................46
5.6.1B General...................................................................................................................46
5.6.2B Forces and Moments in Reinforced Concrete Walls ...................................46
5.6.3B Short Reinforced Walls Resisting Moments and Axial Forces ...................47
5.6.4B Slender Reinforced Walls...................................................................................47
5.6.5B Shear Resistance of Reinforced Walls .............................................................47
5.6.6B Deflection of Reinforced Walls.........................................................................48
5.6.7B Crack Control in Reinforced Walls .................................................................48
5.7B Bases ...............................................................................................................................48
5.7.1B General...................................................................................................................48
5.7.2B Moments and Forces in Bases ...........................................................................48

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 100

5.7.3B Assessment of Bases............................................................................................49
5.7.4B Deflection of Bases ..............................................................................................50
5.7.5B Crack Control in Bases.......................................................................................50
5.8B Considerations of Details...........................................................................................50
5.8.1B Constructional Details ........................................................................................50
5.8.2B Concrete Cover to Reinforcement .................................................................50
5.8.3B Reinforcement: General Considerations ........................................................52
5.8.4B Minimum Areas of Reinforcement in Members.............................................53
5.8.5B Bond, Anchorage and Bearing ...........................................................................53
5.8.6B Curtailment and Anchorage of Reinforcement .............................................57
5.8.7B Maximum Distance between Bars in Tension................................................57
5.9B Additional Considerations for Lightweight Aggregate Concrete......................60
5.9.1B General...................................................................................................................60
5.9.2B Strength of Concrete ..........................................................................................60
5.9.3B Shear Resistance of Beams.................................................................................60
5.9.4B Torsional Resistance of Beams..........................................................................60
5.9.5B Deflection of Beams ............................................................................................60
5.9.6B Shear Resistance of Slabs....................................................................................60
5.9.7B Deflection of Slabs ...............................................................................................60
5.9.8B Columns.................................................................................................................61
5.9.9B Local Bond, Anchorage Bond and Laps ...........................................................61
5.9.10B Bearing Stress inside Bends .............................................................................61
6B. PRESTRESSED CONCRETE ............................................................................................61
6.1B General...........................................................................................................................61
6.1.1B Introduction ..........................................................................................................61
6.1.2B Limit State Assessment of Prestressed Concrete.........................................62
6.1.3B Loads.......................................................................................................................62
6.1.4B Strength of Materials ...........................................................................................62
6.2B Structures and Structural Frames.............................................................................63
6.2.1B Analysis of Structures..........................................................................................63
6.2.2B Redistribution of Moments ................................................................................63
6.3B Beams .............................................................................................................................64
6.3.1B General...................................................................................................................64
6.3.2B Serviceability Limit State: Flexure....................................................................64
6.3.3B Ultimate Limit State: Flexure............................................................................64
6.3.4B Shear Resistance of Beams.................................................................................66
6.3.5B Torsional Resistance of Beams..........................................................................71
6.3.6B Longitudinal Shear................................................................................................72
6.3.7B Deflection of Beam ..............................................................................................72
6.4B Slabs ................................................................................................................................72
6.5B Columns.........................................................................................................................72
6.6B Tension Members ........................................................................................................72

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 100

6.7B Prestressing Requirements ........................................................................................73
6.7.1B Maximum Initial Prestress ..................................................................................73
6.7.2B Loss of Prestress other than Friction Losses .................................................73
6.7.3B Loss of Prestress due to Friction......................................................................76
6.7.4B Transmission Length in Pre-tensioned Members ..........................................77
6.7.5B End Blocks..............................................................................................................79
6.8B Considerations of Details...........................................................................................80
6.8.1B General...................................................................................................................80
6.8.2B Cover to Prestressing Tendons ........................................................................80
6.8.3B Spacing of Prestressing Tendons.......................................................................81
6.8.4B Longitudinal Reinforcement in Prestressed Concrete Beams....................81
6.8.5B Links in Prestressed Concrete Beams .............................................................81
7B. PRECAST, COMPOSITE AND PLAIN CONCRETE CONSTRUCTION.............81
7.1B General...........................................................................................................................81
7.1.1B Introduction ..........................................................................................................81
7.1.2B Limit State Assessment .......................................................................................81
7.2B Precast Concrete Construction ...............................................................................82
7.2.1B Framed Structures and Continuous Beams....................................................82
7.2.2B Other Precast Members.....................................................................................82
7.2.3B Supports for Precast Members .........................................................................82
7.2.4B Joints between Precast Members .....................................................................84
7.3B Structural Connections between Units ..............................................................86
7.3.1B General...................................................................................................................86
7.3.2B Continuity of Reinforcement.............................................................................87
7.3.3B Other Types of Connection ..............................................................................88
7.4B Composite Concrete Construction ........................................................................88
7.4.1B General...................................................................................................................88
7.4.2B Ultimate Limit State.............................................................................................89
7.4.3B Serviceability Limit State.....................................................................................92
7.5B Plain Concrete Walls and Abutments .....................................................................92
7.5.1B General...................................................................................................................92
7.5.2B Moments and Forces in Walls and Abutments ..............................................92
7.5.3B Eccentricity in the Plane of the Wall or Abutment.......................................93
7.5.4B Eccentricity at Right-angles to Walls or Abutments.....................................93
7.5.5B Analysis of Section ...............................................................................................93
7.5.6B Shear .......................................................................................................................94
7.5.7B Bearing....................................................................................................................94
7.6B Mass Concrete Arches ...............................................................................................94
APPENDIX B1 HISTORICAL CONCRETE GRADES

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 100

1B. SCOPE
The scope is as given in Section 7.
2B. DEFINITIONS AND SYMBOLS
2.1B Definitions
2.1.1B General
The definitions given in Section 1 apply in addition to those specifically defined in this
Appendix.
2.1.2B Partial Factors
The partial factors given in Section 2 apply.
2.1.3B Materials
2.1.3.1B Strength
Material strengths are expressed in terms of the cube strength of concrete, fcu , the
yield or proof strength of the reinforcement, fy , or the breaking stress of a
prestressing tendon, fpu .
The material strengths that may be used are either:
(a)

The characteristic strength, (the strength below which not more than 5% of all
possible test results may be expected to fall); or

(b)

The worst credible strength, (the lowest value of the strength which the
Engineer, based on experience and knowledge of the material, realistically
believes could occur). The method of determining the worst credible
strength should be agreed with the relevant Railtrack Director’s Nominee.

2.1.3.2B Characteristic Stress
The characteristic stress is the value of stress at the assumed limit of linearity on the
stress-strain curve for the material.
2.2B Symbols
Symbols that appear in this Appendix are as noted in this Clause. Additional
clarification is also given where necessary in other Clauses.
Ac
Acon

area of concrete
contact area

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Ae
Ai
Ao
Aps
As
A's
As1
As2
Asc
AsL
Ast
Asup
Asv
At

ab
acr
av
b
ba
bc
bcol
bs
bt
bw
c
cnom
Dc
d

dc
de
ds

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 100

area of fully anchored reinforcement per unit length crossing the shear
plane
cross-section area of in-situ concrete
area enclosed by the median wall line
area of prestressing tendons in the tension zone
area of tension reinforcement
area of compression reinforcement
area of compression reinforcement in the more highly compressed face
area of reinforcement in other face
area of longitudinal reinforcement (for columns)
cross-sectional area of one bar of longitudinal reinforcement provided for
torsion
cross-sectional area of one leg of a closed link
supporting area
cross-sectional area of shear reinforcement at a particular cross section
area of reinforcement in a particular direction
distance from compression face to a point at which the crack width is being
calculated
centre-to-centre distance between bars or group of bars perpendicular to
the plane of the bend
distance from the point (crack) considered to the surface of nearest
longitudinal bar
distance from the section under consideration to the face of the supporting
member
width or breadth of section
average breadth of section excluding the compression flange
breadth of compression face
width of column
width of section containing effective reinforcement for punching shear
breadth of section at level of the centroid of the tension steel
breadth of member web or rib, or edge zone
cover
nominal cover
density of lightweight aggregate concrete
effective depth to tension reinforcement
depth from the surface to the reinforcement in the more highly
compressed face
depth of concrete in compression
effective depth for a solid slab or rectangular beam; otherwise the overall
depth of the compression flange
effective depth to tension steel in prestressed member

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
dt
d2
Ec
(El)c
e
ec
ex
Fbst
Fbt
f
fcav
fci
fcp
fcu
fpb
fpe
fpt
fpu
fs
fs2
ft
fy
fyc
fyL
fyv
h
hf
hmax
hmin
hw
hx
hy
I
K
k
kt
kl

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 100

effective depth from the extreme compression fibre to either the
longitudinal bars or the centroid of the tendons, around which the stirrups
pass, whichever is the greater
depth from the surface to the reinforcement in the other face
static secant modulus of elasticity of concrete
flexural rigidity of the column cross-section
eccentricity, edge distance, or base of napierian loagarithms
eccentricity to compression face
resultant eccentricity of load at right-angles to plane of wall, or as defined
in Figure B5
tensile bursting force
tensile force due to ultimate loads in a bar or group of bars
stress
average compressive stress in the flexural compressive zone
concrete strength at (initial) transfer
compressive stress at the centroidal axis due to prestress
characteristic or worst credible concrete cube strength
tensile stress in tendons at failure
effective prestress (in tendon)
stress due to prestress
characteristic or worst credible strength of prestressing tendons
reinforcement stress
stress in reinforcement in other face
maximum principal tensile stress; tensile strength of reinforcement
characteristic or worst credible strength of reinforcement
assessment compressive strength of longitudinal steel
characteristic, or worst credible, strength of the longitudinal reinforcement
characteristic of worst credible strength of shear reinforcement
overall depth (thickness) of section (in plane of bending or buckling)
thickness of flange
larger dimension of section
smaller dimension of section
wall thickness
overall depth of the cross-section in the plane of bending Miy
overall depth of the cross-section in the plane of bending Mix
second moment of area
a factor depending on the type of duct or sheath used, the nature of its
inside surface, the method of forming it and the degree of vibration
employed in placing the concrete
a constant (with applicable subscripts)
depends on the type of tendon
depends on the concrete bond across the shear plane

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Ls
le
lex
ley
lo
Lt
M
Mcr
Mg
Mi
Mix
Miy
Mnt
Mq
Mtx
Mty
Mu
Mux
Muy
Mx, My
Mo
M1
M2
N
Nu
Nuz
nw
Pf
Ph
Pk
Po
Px

RT/CE/C/025
Issue: 1
Date: February 2001
Page 8 of 100

length of shear plane
effective height of a column or wall
effective height for bending about the major axis
effective height for bending about the minor axis
clear height of column between end restraints
transmission length
bending moment due to ultimate loads
cracking moment at the section considered
moment due to permanent loads
maximum initial moment in a column due to ultimate loads
initial moment about the major axis of a slender column due to ultimate
loads
initial moment about the minor axis of a slender column due to ultimate
loads
twisting moment per unit length in a slab adjacent to the edge zone
referred to axes perpendicular (n) and parallel (t) to the edge
moment due to live loads
total moment about the major axis of a slender column due to ultimate
loads
total moment about the minor axis of a slender column due to ultimate
loads
ultimate resistance moment
ultimate moment capacity in a short column assuming ultimate axial load
and bending about the major axis only
ultimate moment capacity in a short column assuming ultimate axial load
and bending about the minor axis only
moments about the major and minor axes of a short column due to
ultimate loads
moment necessary to produce zero stress in the concrete at the depth d
smaller initial end moment due to ultimate loads (assumed negative if the
column is bent in double curvature)
larger initial end moment due to ultimate loads (assumed positive)
ultimate axial load at section considered; number of bars in a group
ultimate resistance axial load
axial loading capacity of a column ignoring all bending
ultimate axial load per unit length of wall
effective prestressing force after all losses
horizontal component of the prestressing force after all losses
basic load in tendon
prestressing force in the tendon at the jacking end (or at tangent point near
jacking end)
prestressing force at distance x from jack

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
QA
Qk
r
rps
SA
sL
sv
T
Tu
V
Vc
Vco
Vcr
Vi
V1
Vs
Vt
Vu
Vux
Vuy
Vx
Vy
v
vu
vc
v1
vt
vtmin
vtu
x
x1
y
yo
ypo
y1
z
α
αn

RT/CE/C/025
Issue: 1
Date: February 2001
Page 9 of 100

assessment load
nominal load
internal radius of bend
radius of curvature of a tendon
assessment load effects
spacing of longitudinal reinforcement
spacing of shear links along the member
torque due to ultimate loads
ultimate torsional strength
shear force due to ultimate loads
ultimate shear resistance of concrete
ultimate shear resistance of a section uncracked in flexure
ultimate shear resistance of a section cracked in flexure
shear capacity of infill concrete
longitudinal shear force due to ultimate load
shear resistance of shear reinforcement
flexural shear force per unit width at the edge acting on a vertical plane
perpendicular to the edge
ultimate shear resistance of section
ultimate shear capacity of a section for the X-X axis
ultimate shear capacity of a section for the Y-Y axis
applied shear due to ultimate loads for the X-X axis
applied shear due to ultimate loads for the Y-Y axis
shear stress
ultimate shear stress in concrete (Halving joint)
ultimate shear stress in concrete
ultimate longitudinal shear stress per unit area of contact surface
torsional shear stress
minimum ultimate torsional shear stress for which reinforcement is
required
ultimate torsional shear stress
neutral axis depth
smaller centre line dimension of a link
distance of the fibre considered in the plane of bending from the centroid
of the concrete section
half the side of end block
half the side of loaded area
larger centre line dimension of a link
lever arm
inclination of shear reinforcement; factor to determine fpb
coefficient as a function of column axial load

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
α1
α2
β
γf1,γf2,γf3
γfL
γm
γmb
γmc
γmcw
γms
γmv
ε
εm
εs
ε1
µ
ξs
ρ
ρnet
ΣAsv
Σbd
φ

RT/CE/C/025
Issue: 1
Date: February 2001
Page 10 of 100

angle between the normal to the axis of the applied moment and the
direction of the tensile reinforcement
angle of friction at the joint
coefficient dependent on bar type
partial factors
product of γf1,γf2
partial factor for material strength
partial factor for bond strength
partial factor for concrete strength
partial factor for plain concrete wall strength
partial factor for steel strength
partial factor applied to vc
strain
average strain, cracking strain
strain in tension reinforcement
strain at level considered
coefficient of friction
depth factor
geometrical ratio of reinforcement equal to As/bd
area of transverse reinforcement in the flange as a percentage of the
minimum flange area
area of shear reinforcement
area of the critical section
size (nominal diameter) of bar or tendon)

3B. LIMIT STATE PHILOSOPHY
3.1B General
Structures should be assessed using limit state principles for the ultimate limit state
and, where required, the serviceability limit state.
In general, where a Bridge has been inspected in accordance with Section 3 and no
serviceability failures are apparent, it will not be necessary to assess cracking and
stress limits at the Serviceability Limit State. Where damage is apparent,
serviceability assessments should be considered in order to investigate the cause of
observed damage. Serviceability assessment may be required where the Bridge is
being assessed for a class of loading that it has not previously experienced. This
requirement should be agreed with Railtrack Director’s Nominee.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 100

However, if the structure does not meet serviceability requirements, it will not
necessarily preclude the application of a proposed more severe loading. The likely
consequences of the ‘failures’ should be considered. Where consideration is given to
keeping prestressed structures in service that do not meet the Class 3 criteria of
BS 5400: Part 4 Clause 6.3.2.4, the crack widths may be calculated using the
requirements for reinforced concrete sections treating the estimated prestress as an
applied load.
3.2B Deflection
The live load deflection should not be such as to significantly affect the trains using the
Bridge. Where checks are required, criteria should be agreed with the Railtrack
Director’s Nominee.
4B. GENERAL
4.1B Limit State Criteria
4.1.1B Serviceability Limit States
Under serviceability loads the Bridge should not suffer local damage that would
shorten its intended life or incur excessive maintenance costs.
When a serviceability limit state assessment is required, the predicted stresses and
crack widths should be checked against the criteria given in BS 5400: Part 4 except
that the characteristic stresses used may be those given in Clause 4.3.1B (a) of this
Appendix.
4.1.2B Ultimate Limit States
The strength of the structure should be sufficient to withstand the assessment loads,
so that collapse will not occur as a result of rupture of one or more critical sections,
by overturning or by buckling caused by elastic or plastic instability, having due regard
to the effects of sway when applicable.
The effects of creep and shrinkage of concrete, temperature difference and
differential settlement need not be considered at the ultimate limit state.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 12 of 100

4.1.3B Other Considerations
When other effects (such as deflection vibration, fatigue and durability) are to be
considered, applicable criteria should be agreed with the Railtrack Director’s
Nominee.
4.2B Loads, Load Combinations and Partial Factors γfL and γf3
4.2.1B Loads
The nominal values of loads and load combinations are given in Sections 2 and 4.
4.2.2B Serviceability Limit State
Where serviceability assessments are required applicable loading criteria should be
agreed with the Railtrack Director’s Nominee. Partial factors, γfL, for the
serviceability limit state from Section 2 should be used.
4.2.3B Ultimate Limit State
The values of the partial factor γfL are given in Section 2. In calculating the resistance
of members to vertical shear and torsion, γfL for the prestressing force should be
taken as 1.15 where it adversely affects the resistance and 0.87 in other cases. In
calculating secondary effects in statically indeterminate structures, γfL for the
prestressing force may be taken as 1.0.
The value of γf3 should be taken as 1.1 except as stated in Section 2.
4.3B Properties of Materials
4.3.1B General
Either the characteristic strength, or the worst credible strength may be used for a
material strength. In general, in analysing a structure to determine load effects, the
material properties applicable to the characteristic, or worst credible, strength
should be used, irrespective of the limit state being considered.
For the analysis of sections, the material properties to be used for the individual limit
states should be as follows:
(a)

Serviceability limit state and deflection - in the absence of tests, the elastic
modulus for concrete and steel given in Clauses 4.3.2.1B and 4.3.2.2B
respectively. The characteristic stress of concrete in compression may be
taken as 0.5fcu. Where tests are undertaken, the characteristic stress of
reinforcement may be taken as the 0.2% proof stress. In the absence of tests,
it may be taken as 0.75fy but not less than 225N/mm² where fy is greater than
250N/mm² and 0.9fy where fy is less than 250N/mm².

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
(b)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 13 of 100

Ultimate limit state - the values given in Clause 4.3.2B.

4.3.2B Material Properties
4.3.2.1B Concrete
In assessing the strength of sections at the ultimate limit state, the stress-strain curve
for normal weight concrete in Figure B1 may be taken using the value of γmc for
concrete given in Clause 4.3.3.3B. Guidance on strengths of historical concrete
grades can be found in Appendix B1.
The modulus of elasticity, Ec , to be used for elastic analysis should be applicable to the
cube strength of the concrete, and, in the absence of test data, the short term value
should be taken as (20 + 0.27 fcu) kN/mm² with fcu in N/mm² units. The effect of creep
under long term loading may be allowed for by using half of the short term modulus
of elasticity.
For lightweight concrete having an air dry density between 1400 kg/m³ and
2300 kg/m³, the values given in the previous paragraph should be multiplied by
(Dc/2300)² where Dc is the density of the lightweight aggregate concrete in kg/m³.
Poisson's ratio may be taken as 0.2. The value for the coefficient of thermal
expansion may be taken from BD 21/97: The Assessment of Highway Bridges and
Structures, Table 4/3.
4.3.2.2B Reinforcement and Prestressing Steel
The stress-strain curves may be taken as follows:
(i)

For reinforcement: Figure B2, using the value of γms given in Clause 4.3.3B and
modulus of elasticity 200 kN/mm²;

For prestressing steel: Figure B3 or Figure B4, using the value of γms given in
Clause 4.3.3B. and the modulus of elasticity from Figure B3 or Figure B4 as
applicable for tangent modulus at zero load.
Alternatively where the reinforcement or tendon type is known, the manufacturer’s
stress-strain curves may be used with the values of γms given in Clause 4.3.3B.
(ii)

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

0.67 × f cu
γm

Page 14 of 100

Parabolic curve

Stress
f
(N mm2 )

5.5

Strain

ε

f cu
γm

(k N mm 2 )

2.44 × 10

0.0035

f cu
γm

−4

NOTE 1

0.67 × f cu takes account of the ratio between the cube strength and the bending strength in a flexural member.

NOTE 2

The equation for the parabolic curve between ε = 0 and 2.44 × 10 −4


f cu
f
may be taken as f =  5000 cu

γm
γm


2


ε −  5000 ε2

 2.68 




Figure B1
Short Term Stress-Strain Curve for Normal Weight Concrete for
Assessment
fy

γm

0.8 × f y

fy

γm

 fy 

γ m + 

 2000 

Stress
f
(N mm2 )

200 (k N mm2 )
0.002

Strain

ε

Figure B2
Short Term Stress-Strain Curve for Reinforcement for Assessment

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Page 15 of 100

f pu
γm
0.8 f pu
γm
Stress
f
(N/mm²)

200 kN/mm² for wire and strand
to BS 5896 sections 2 and 3
165 kN/mm² for alloy bars to
BS 4486 and 19-wire strand
to BS 4757 section 3.

0.005

Strain e

Figure B3
Short Term Stress-Strain Curve for Normal and Low Relaxation
Prestressing Steel for Assessment
f pu
γ m

0.6f pu
γm

Stress
f
(N/mm²)

200 kN/mm² to BS 5896
175k N/mm² for 19-wire strand
to BS 4757 section 2.

0.005

Strain e

Figure B4
Short Term Stress-Strain Curve for ‘As Drawn’ Wire and ‘As Spun’
Strand for Assessment

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 16 of 100

4.3.3B Values of γm
4.3.3.1B General
For the analysis of sections, the values of γm are given in Clauses 4.3.3.2B to 4.3.3.4B
4.3.3.2B Serviceability Limit State
Where stress checks at serviceability limit state are required, the values for γm from
Table B1 should be used with the characteristic stresses given in Clause 4.3.2B above.
Type of Construction
Material

Type of Stress

Reinforced
Concrete

Prestressed
Concrete

Concrete

Triangular or neartriangular compressive
distribution (e.g. due to
bending)

1.00

1.25

Uniform or nearuniform compressive
stress distribution (e.g.
due to axial loading

1.33

1.67

Tension

Reinforcement

Prestressing tendons

1.25 pre-tensioned
1.55 post-tensioned

Compression

1.00

Tension

1.00

Tension

Not required

Table B1
Values of γm at the Serviceability Limit State
4.3.3.3B Ultimate Limit State
For both reinforced concrete and prestressed concrete, the values of γm applied to
either the characteristic strengths or worst credible strengths are summarised in
Table B2.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Page 17 of 100
Value for use with

Application

Symbol

Characteristic
Strength

Worst Credible
Strength

Reinforcement and prestressing tendons

γms

1.15

1.10*

Concrete

γmc

1.50

1.20

Shear in concrete

γmv

1.25

1.15

Bond

γmb

1.4

1.25

Plain concrete wall

γmcw

2.25

1.80

Table B2
Values of γm at the Ultimate Limit State
* May be reduced to 1.05 if measured steel depths are used in addition to the worst
credible steel strength

4.3.3.4B Fatigue
When applying Clause 4.7B, the values of γms applied to a reinforcement stress range
should be 1.00.
4.4B Analysis of Structure
4.4.1B General
The requirements for methods of analysis applicable to the determination of the
distribution of forces and deformations that should be used in ascertaining that the
limit state criteria are satisfied are described in Sections 2 and 4.
4.4.2B Analysis for Serviceability Limit State
4.4.2.1B General
Elastic methods of analysis should be used to determine internal forces and
deformations. The flexural stiffness constants (second moment of area) for sections
of discrete members or unit widths of slab elements may be based on any of the
following:
a)

concrete section - the entire member cross-section, ignoring the presence of
reinforcement;

b)

gross transformed section - the entire member cross-section including the
reinforcement, transformed on the basis of modular ratio;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
c)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 18 of 100

net transformed section - the area of the cross-section that is in compression
together with the tensile reinforcement, transformed on the basis of modular
ratio.

A consistent approach should be used which reflects the different behaviour of various
parts of the structure. It is, however, acceptable to use properties which are
intermediate between (a) and (b) when it is considered this will give a more realistic
representation of behaviour.
Axial, torsional and shearing stiffness constants, when required by the method of
analysis, should be based on the concrete section and used with (a) or (b). Reduced
torsional stiffnesses may be used when applicable in accordance with Clause 5.3.4.2B or
to achieve compatibility with (c).
Values of moduli of elasticity and shear moduli should be applicable to the
characteristic, or worst credible strength of the concrete.
4.4.2.2B Methods of Analysis and their Requirements
The method of analysis should take account of all significant aspects of behaviour that
govern the structure’s response to loads and imposed deformations.
4.4.3B Analysis for Ultimate Limit State
Elastic methods may be used to determine the distribution of forces and
deformations throughout the structure. Stiffness constants may be based on any of
those determined in accordance with Clause 4.4.2.1B. The torsional stiffness may be
reduced when Clause 5.3.4.2B applies. Other constants may also be adjusted to give
some allowance for redistribution where this will give a more realistic representation of
behaviour.
Non-linear and plastic methods of analysis (e.g. plastic hinge methods for beams, or
yield line methods for slabs) may be used may be used with the agreement of the
Railtrack Director’s Nominee.
4.5B Analysis of Section
4.5.1B Serviceability Limit State
An elastic analysis should be carried out. In-plane shear flexibility in concrete flanges
(shear lag effects) should be taken into account, by taking an effective width of flange
in accordance with Clause 5.3.1.2B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 19 of 100

4.5.2B Ultimate Limit State
The strength of critical sections, and whether the requirement of Clause 4.1.2B is
satisfied, should be assessed in accordance with Clauses 5B, 6B or 7B. In-plane shear
flexibility in concrete flanges (shear lag effects) may be ignored.
4.6B Deflection
Deflection should be calculated for the most unfavourable distributions of loading for
the member (or strip of slab) and may be derived from an elastic analysis of the
structure. The material properties, stiffness constants and calculations of deflections
may be based on the information given in Clause 4.3.2.1B or BS 5400: Part 4
Appendix A.
4.7B Fatigue
When required, the effect of repeated live loading on the fatigue strength of a Bridge
should be assessed. For reinforcing bars that are known or suspected to have been
subjected to welding, details of compliance criteria are given in BS 5400: Part 10 as
implemented by BD 9/81: Implementation of BS 5400: Part 10 (1980) Code of Practice
for Fatigue.
For unwelded non-corroded reinforcement, the fatigue life should be determined in
accordance with BS 5400: Part 10 as implemented by BD 9, using the following
parameters for the σr - N relationship:
bars ≤ 16 mm diameter;
m = 9 , k2 = 0.75 x 1027
bars > 16 diameter; m = 9 , k2 = 0.07 x 1027
The fatigue life of corroded reinforcement should be determined in accordance with
BA 38/93: Assessment of the Fatigue Life of Corroded or Damaged Reinforcing Bars.
4.8B Combined Global and Local Effects
4.8.1B General
In addition to the assessment of individual primary and secondary elements to resist
loading applied directly to them, it is also necessary to consider the loading
combination that produces the most adverse effects due to global and local loading
where these coexist in an element.
4.8.2B Analysis of Structure
Analysis of the structure may be accomplished either by one overall analysis (e.g.
using finite elements) or by separate analyses for global and local effects. In the latter
case the forces and moments acting on the element from global and local effects
should be combined as applicable.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 20 of 100

4.8.3B Analysis of Section
Section analysis for combined global and local effects should be carried out in
accordance with Clause 4.5B to satisfy the requirements of Clause 4.1B:
a)

b)

Serviceability limit state
(i)

For reinforced concrete elements, the total crack width due to
combined global and local effects should be determined in accordance
with Clause 5.8.7B.

(ii)

For prestressed concrete elements, co-existent stresses, acting in the
direction of prestress, may be added algebraically in checking the
stress limitations.

Ultimate limit state
The resistance of the section to the combination of local and global effects
should be checked using the assumptions given in Clauses 5.3.2.1B or 6.3.3.1B
as applicable allowing for the axial force. For a deck slab, however the
resistance to the combined global and local effects may be deemed to be
satisfactory if the axial force from the global effects is checked separately from
the resistance to moments.

5B. REINFORCED CONCRETE
5.1B General
5.1.1B Introduction
Methods of assessment are given below that will in general ensure that, for reinforced
concrete structures, the criteria set out in Clauses 4.1.1B and 4.1.2B are met. Other
methods may be used with the approval of Railtrack Director’s Nominee. In certain
cases the assumptions made below may not be applicable and the Assessing Engineer
should adopt a more suitable method having regard to the nature of the structure to
be assessed.
5.1.2B Limit State Assessment of Reinforced Concrete
5.1.2.1B Basis of Assessment
The limit state philosophy as set out in Clauses 3B and 4B should be adopted. For
most structures assessment is needed only at the Ultimate Limit State. Assessment
for Serviceability Limit State deflection or fatigue is required only when specifically
requested by Railtrack Director’s Nominee.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 21 of 100

5.1.2.2B Durability
In Clause 5.8.2B guidance is given on the nominal cover to reinforcement that is
necessary to provide adequate durability. In an existing structure the actual cover
may be more or less than recommended for design. Assessment for durability should
be based on the actual conditions found during inspection.
5.1.3B Loads
The assessment load effects(see Section 2) for the ultimate and serviceability limit
states are referred to as ‘ultimate loads’ and ‘service loads’ respectively.
The values of the ‘ultimate loads’ and ‘service loads’ that should be used in assessment
are derived from Section 4.
When analysing sections, the terms ‘strength’, ‘resistance’ and ‘capacity’ are used to
describe the assessment resistance of the section in accordance with Section 2.
5.1.4B Strength of Materials
5.1.4.1B Definition of Strengths
The assessment strengths of concrete and reinforcement are given by fcu/γmc and fy/γms,
respectively where γmc and γms are the partial factors for material strength given in
Clause 4.3.3B. The applicable value of γmc or γms should be substituted in all equations
in Clause 5B.
5.1.4.2B Strength of Concrete
Assessment may be based on either the original specified characteristic cube strength,
or the worst credible cube strength assessed from the estimated in-situ cube strength
in accordance with BA 44: The Assessment of Concrete Highway Bridges and Structures.
5.1.4.3B Strength of Reinforcement
Assessment may be based on either the original specified characteristic yield or proof
stress, or the worst credible yield or proof stress assessed from tests on
reinforcement samples extracted from the structure.
5.2B Structures and Structural Frames
5.2.1B Analysis of Structures
Structures should be analysed in accordance with the requirements of Clause 4.4B.
5.2.2B Redistribution of Moments
Redistribution of moments obtained by rigorous elastic analysis under the ultimate
limit state may be carried out provided the following conditions are met:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
(a)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 22 of 100

checks are made that adequate rotational capacity exists at sections where
moments are reduced, making reference to applicable test data, and in the
absence of a special investigation, the plastic rotation capacity may be taken as
the lesser of:
0.08 + 0.0035(0.5 − d c d e ) ; or
0.6φ
but not less than 0
d − dc
where:
dc
de
φ
d

is the calculated depth of concrete in compression at the ultimate limit
state;
is the effective depth for a solid slab or rectangular beam, otherwise
the overall depth of the compression flange;
is the diameter of the smallest tensile reinforcing bar;
is the effective depth to tension reinforcement.

(b)

proper account is taken of changes in transverse moments and transverse
shears consequent on redistribution of longitudinal moments;

(c)

shears and reactions used are those calculated either prior to or after
redistribution, whichever are the greater.

5.3B Beams
5.3.1B General
5.3.1.1B Effective Span
The effective span of a simply supported member should be taken as the smaller of:
(a)

the distance between the centres of bearings or other supports;

(b)

the clear distance between supports plus the effective depth;

(c)

for members resting directly on masonry, concrete or brick, the distance
between the centroids of the bearing pressure diagrams. In this case, the
bearing pressure diagrams should be determined by assuming that the
reaction is distributed linearly from a maximum at the front edge of the
support to zero at the back of the bearing area. The length of the bearing
area should not be taken as greater than the depth of the beam where the

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 23 of 100

support is of soft stone, or one-quarter of the depth of the beam where the
support is of hard stone such as granite or good quality concrete.
The effective span of a member framing into supporting members should be taken as
the distance between the shear centres of the supporting members.
The effective span of a continuous member should be taken as the distance between
centres of supports except where, in the case of beams on wide columns, the effect
of column width is included in the analysis.
The effective length of a cantilever should be taken as its length from the face of the
support plus half its effective depth except where it is an extension of a continuous
beam when the length to the centre of the support should be used.
5.3.1.2B Effective Width of Flanged Beams
In analysing structures, the full width of flanges may be taken as effective. In analysing
sections at the serviceability limit state, and in the absence of any more accurate
determination (such as that given in Section 5 and BS 5400: Part 3), the effective
flange width should be taken as the width of the web plus one-tenth of the distance
between the points of zero moment (or the actual width of the outstand if this is less)
on each side of the web. For a continuous beam the points of zero moment may be
taken to be at a distance of 0.15 times the effective span from the support.
5.3.1.3B Slenderness Limits for Beams
Adequate lateral stability of a simply supported or continuous beam is generally
present where the beam is so proportioned that the clear distance between lateral
restraints does not exceed 300 bc²/d, where d is the effective depth to tension
reinforcement and bc is the breadth of the compression face of the beam midway
between restraints.
Similarly, for cantilevers with lateral restraint provided only at the support, the clear
distance from the end of the cantilever to the face of the support should not exceed
150 bc²/d.
5.3.2B Resistance Moment of Beams
5.3.2.1B Analysis of Sections
When analysing a cross-section to determine its ultimate moment of resistance, the
following assumptions should be made:
(a)

The strain distribution in the concrete in compression and the compressive
and tensile strains in the reinforcement are derived using the assumption that
plane sections remain plane;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 24 of 100

(b)

the stress-strain curve in Figure B1 with the applicable value of γmc given in
Clause 4.3.3.3B applies for the derivation of the stresses in the concrete in
compression or, for rectangular sections and flanged, ribbed and voided
sections where the neutral axis lies within the flange. The compressive stress
should be taken as equal to 0.6 fcu/γmc over the whole compression zone. In
both cases the strain at the outermost compression fibre at failure is taken as
0.0035;

(c)

the tensile strength of concrete is ignored;

(d)

the stresses in the reinforcement are derived from either the stress-strain
curves in Figure B2 or, when available, manufacturers' stress-strain curves.
The values of γms are given in Clause 4.3.3.3B.

In the analysis of a cross-section of a beam that has to resist a small axial thrust, the
effect of the ultimate axial force may be ignored if it does not exceed 0.1 fcu times the
cross-sectional area.
5.3.2.2B Design Charts
For the analysis of beams reinforced in tension only or in tension and compression,
the design charts that form CP 110: Parts 2 and 3 (based on Figure 1, Figure 2 and
the assumptions of Clause 5.3.2.1B) may be used with applicable modifications for the
value of γm, which is defined in Clause 4.3.3B.
5.3.2.3B Assessment Formulae
The following formulae may be used to calculate the ultimate moment of resistance of
a solid slab or rectangular beam, and for a flanged beam, ribbed slab or voided slab
when the neutral axis lies within the flange.
For sections without compression reinforcement the ultimate moment of resistance
may be taken as the lesser of the values obtained from Equations B1 and B2.
Equations B3 and B4 may be used for sections with compression reinforcement.
A rectangular stress block of maximum depth 0.5d and a uniform compression stress
of 0.6 fcu/γmc have been assumed.
Mu =

(f y

γms )As z

Mu =

(0.225f cu γ mc )bd 2

Equation B2

Mu =

(0.60 f cu γmc )bx (d − 0.5 x )+ f s ′ A s ′ (d − d ′)

Equation B3

Equation B1

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

(f y

RT/CE/C/025
Issue: 1
Date: February 2001
Page 25 of 100

γ ms )As = (0.6f cu γ mc )bx + f s′ + A′s

Equation B4

where
Mu
As
A′
s

b
d
d′
fy

f s′ =

is the ultimate resistance moment;
is the area of tension reinforcement;
is the area of compression reinforcement;
is the width of the section;
is the effective depth to the tension reinforcement;
is the depth to the compression reinforcement;
is the characteristic, or worst credible strength, of the reinforcement;
fy
γms + f y 2000

x
is the depth of the neutral axis;
z
is the lever arm;
f cu
is the characteristic, or worst credible strength of the concrete;
γ mc , γ ms are the material partial factors given in Clause 4.3.3.3B.
The lever arm, z, in Equation B1 may be calculated from Equation B5:
z=

 0.84(f y γms )As 
1− (f γ )bd  d but not greater than 0.95d


cu
ms

Equation B5

When using Equations B3 and B4 for sections with compression reinforcement, the
neutral axis depth x should first be calculated from Equation B4.
If d ′ ≤ 0.429 x the ultimate resistance moment should be determined from
Equation B3.
If d ′ > 0.429 x either the compression steel should be ignored and the section treated
as singly reinforced or the ultimate resistance should be determined using
Clauses 5.3.2.1B or 5.3.2.2B as applicable.
The ultimate resistance moment of a flanged beam may be taken as the lesser of the
values given by Equations B6 and B7, where hf is the thickness of the flange.
Mu =

(f y

 hf 
γms )As  d − 
2


Equation B6

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Mu =



(0.6f cu γmc )bhf  d −


hf 

2

RT/CE/C/025
Issue: 1
Date: February 2001
Page 26 of 100
Equation B7

5.3.3B Shear Resistance of Beams
5.3.3.1B Shear Stress
The shear stress, v in N/mm², at any cross-section should be calculated from
Equation B8.
v =

V
bw d

Equation B8

where:
V
bw

is the shear force due to ultimate loads;
is the breadth of the section which, for a flanged beam, should be taken as the
rib width;
d
is the effective depth to tension reinforcement.
In no case should v exceed 0.92 f cu γmc or 7 γmc (where γmc is the partial factor
for concrete given in Clause 4.3.3.3B), whichever is the lesser, whatever shear
reinforcement is provided.
5.3.3.2B Shear Capacity
Shear reinforcement in the form of vertical links, inclined links or bent-up bars should
only be considered effective in resisting shear if the spacing of the legs of links in the
direction of the span and at the right angles to it, does not exceed the effective depth,
d, and where:
Asv (sinα + cosα )(f yv γ ms ) ≥ 0.2bw sv and α ≥ 30°

Equation B9

where:
Asv
sv
bw
α
f yv

is the cross-sectional area of shear reinforcement at a particular cross-section;
is the spacing of the shear reinforcement along the member;
is the breadth of the section which, for a flanged beam, should be taken as the
rib width;
is the inclination of the shear reinforcement to the axis of the member;
is the characteristic, or worst credible, strength of the shear reinforcement

γms

but not greater than 480 N/mm²;
is the partial factor for material strength of steel given in Clause 4.3.3.3B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 27 of 100

In the absence of effective shear reinforcement, the ultimate shear resistance, Vu of a
section is given by:
Vu =

ξ s v c bw d

Equation B10

Where effective vertical links are present, the ultimate shear resistance Vu of a
section is given by:
Vu =

ξ s v c b w d + (f y γms

) d Asv

Equation B11

sv

For vertical links to be effective, the tensile capacity of the longitudinal reinforcement
at a section, As f y γms , should be greater than:
M (V − ξ s v c b w d )
+
z
2
M, V
z

Expression B12

are the co-existent ultimate bending moment and shear force at the section
under consideration;
is the lever arm which may be taken as 0.9d;

Within an individual sagging or hogging region however, the assessed tensile capacity
need not exceed Mmax z , where Mmax is the maximum ultimate moment within that
region.
In the above equations/expressions:
ξs =

 550 


 d 

0.25

but m 0.7;

v c is the ultimate shear stress in concrete and should be calculated from:
1

vc =

1
A 3
0.24 
100 s  (f cu ) 3
bw d 
γmv 

In the Equation B13 the term 100
than 3.0.

Equation B13
As
should not be taken less than 0.15 or greater
bw d

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 28 of 100

γmv is a partial factor defined in Table B2.
The term As is that area of longitudinal tension reinforcement which continues at least
a distance equal to the effective depth, d, beyond the section being considered. At
supports however the full area of tensile reinforcement may be used provided, the
requirements of Clause 5.8.6B are met. Where both top and bottom reinforcement
are provided, the area of As used should be that which is in tension under the loading
that produces the shear force being considered.
Sections within a distance d from the support generally need not be assessed for
shear providing the shear reinforcement calculated for the section at distance d is
continued up to the support and the anchorage requirements of Clause 5.8.6B are
met. Where, however, more than half the total shear force on the member is due to
load applied within a distance d of the support, sections within d of the support
should be assessed.
Inclined links or bent up bars should be assumed to form the tension members of one
or more single systems of lattice girders in which the concrete forms the
compression members. The maximum stress in any link or bar should be taken as
f yv
. Bent-up bars should be checked for anchorage and bearing in accordance with
γ ms
Clause 5.8.5B.
5.3.3.3B Enhanced Shear Strength of Sections Close to Support
If the main reinforcement continues to the support and is provided with an effective
anchorage equivalent to 20 times one bar size, an enhancement of shear strength may
be allowed for at sections within a distance a v ≤ 3d from the face of a support, the
front edge of a rigid bearing or the centre line of a flexible bearing.
This enhancement should take the form of an increase in the allowable shear stress,
ξ s v c to ξ s v c 3d a v but should not exceed 0.92 f cu γmc or 7 γmc whichever is the
lesser.
Where the anchorage at a simply supported end is less than required above but not less
than 2.5 bar diameters, the enhancement may still be used but with the effective steel
area, As used to calculate v c taken as actual area times (anchorage length/20 bar
diameters) for plain round bars or actual area times (anchorage length/12 diameters)
for deformed bars. However, in this case, the actual steel area may be used in the
formula for v c in Clause 5.3.3.2B even if less than 0.15%.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 29 of 100

In the above, for cases where the simply supported end is resting directly on the
abutment, the centre of the support may be taken as the centroid of the bearing
pressure diagram considered in Clause 5.3.1.1B(c).
Where this Clause gives a lower shear strength than Clauses 5.3.3.1B and 5.3.3.2B, the
greater value should be used.
5.3.3.4B Bottom Loaded Beams
Where vertical load is applied near the bottom of a section, sufficient vertical
reinforcement to carry the load to the top of the section should be present in
addition to any reinforcement required to resist shear.
5.3.3.5B Alternative Approach
As an alternative to the method given in Clause 5.3.3.1B and Clause 5.3.3.2B, sections
with links may be assessed using the varying angle truss approach. The shear strength
Vu is the lesser value obtained from Equations B14(a) and B14(b) for elements with
vertical links. For elements with inclined links the shear strength Vu is the lesser value
obtained from Expressions B14(c) and B14(d).
0.9(f yv γms )(d sv )Asv cotθ

Expression B14(a)

0.72bw dv (f cu γmc ) (cotθ + tanθ)

Expression B14(b)

0.9(f yv γms )(d sv )Asv (cotθ + cotα )sinα

Expression B14(c)

0.72bw dv (f cu γmc )(cotθ + cotα ) (1+ cot 2 θ)

Expression B14(d)

where:
θ

v

is the angle of the assumed concrete struts to the horizontal taken such that
cotθ lies in the range 0.4 to 2.5 for members with constant reinforcement and
0.5 to 2.0 for members with curtailed reinforcement;
is the effectiveness factor taken as:
0.7 −

f cu
≥ 0.5
250

Other symbols in Expressions B14(a) to B14(d) have the same meaning as in Clauses
5.3.3.1B and 5.3.3.2B except that Asv should not be taken as greater than:

(0.4bw sv vf cu sinα ) (f yv γmc ) with vertical links and;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 30 of 100

[(0.4bw sv vf cusinα ) (f yv γmc )] sinα (1− cosα ) with inclined links.
If this approach is used, the flexural reinforcement at any section is capable of resisting
the bending moment at a distance 0.9d (cotθ − cotα ) 2 in the direction of increasing
moment from the section considered provided effective anchorage is present in
accordance with Clause 5.8.5.2B. However, at a simply supported end, the bond stress
in the length of reinforcement immediately over the bearing may be taken as 1.5 times
that given in Clause 5.8.5.2B.
5.3.4B Torsion
5.3.4.1B General
In some members the maximum torsional moment does not occur under the same
loading as the maximum flexural moment. In such circumstances reinforcement in
excess of that required for flexure and other forces may be considered in the
assessment of torsional resistance.
5.3.4.2B Torsionless Systems
In general, where the torsional resistance or stiffness of members has not been taken
into account in the analysis of the structure, no specific calculations for torsion are
necessary. However, it is essential that sound engineering judgement has shown that
torsion plays only a minor role in the behaviour of the structure, otherwise torsional
stiffness should be used in analysis.
5.3.4.3B Stresses and Reinforcement
Where torsion in a section substantially increases the shear stresses, the torsional
shear stress should be calculated assuming a plastic stress distribution.
Where the torsional shear stress, vt, exceeds the value vtmin, torsion reinforcement
should be present, where:
v t min = 0.082 f cu γ mc
In no case should the sum of the shear stresses resulting from shear force and torsion
(v + vt) exceed the value of the ultimate shear stress, v tu , nor, in the case of small
sections ( y1 < 550 mm), should the torsional shear stress, vt, exceed v tu y1 550 , where
y1 is the larger centre line dimension of a link and,
v tu =

0.92 f cu γmc but not greater than 7

γmc .

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 31 of 100

Torsion reinforcement should consist of rectangular closed links in accordance with
Clause 5.8.5.4B together with longitudinal reinforcement. Only reinforcement in
excess of that necessary to resist shear or bending should be considered as torsion
reinforcement.
Torsional capacity should be calculated assuming that the closed links form a thinwalled tube, the shear stresses in which are balanced by longitudinal and transverse
forces provided by the resistance of the reinforcement.
5.3.4.4B Treatment of Various Cross-sections
(a)
Box sections
The ultimate torsional strength (Tu) should be taken as the greater of:
Tu =

 AsL (f yL γ ms ) Ast (f yv γ ms )
2 Ao 


sv
 2( x1 + y1 ) 


Equation B15

2hw Ao v t min

Equation B16

and
Tu =
where:
Ao
Ast
AsL
f yv

is the area enclosed by the median wall line;
is the area of one leg of a closed link of a section;
is the area of one bar of longitudinal reinforcement;
is the characteristic, or worst credible, strength of the links;

f yL

is the characteristic, or worst credible, strength of the longitudinal

hw
sv
y1
x1

reinforcement;
is the thickness of the thinnest wall;
is the spacing of the links along the member;
is the larger centre line dimension of a link;
is the smaller centre line dimension of a link.

In Equation B15, f yv and f yL should not be taken as greater than 480 N/mm².
In addition, the limits given in Clause 5.3.4.3B should not be exceeded by the
torsional shear stress calculated from:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
vt =

T
2hw A o

RT/CE/C/025
Issue: 1
Date: February 2001
Page 32 of 100
Equation B17

where T is the torque due to ultimate loads.
(b)

Rectangular sections
The ultimate torsional resistance should be taken as the greater of the values
calculated from Equation B15 (with Ao taken as 0.8 x1y1 , where x1 and y1 are
defined above), and
2

Tu =

hmin 
h min 
 hmax −
v min
2 
3 

Equation B18

where hmin and hmax are, respectively, the smaller and larger dimensions of the
section.
In addition, the limits given in Clause 5.3.4.3B should not be exceeded by the
torsional shear stress calculated from:
vt =

(c)

2T
h min

2

hmin 

 hmax −

3 


Equation B19

T, L and I sections - Such sections should be divided into component
rectangles for purposes of torsional assessment. Any division into component
rectangles may be chosen which is compatible with the torsional
reinforcement present. The ultimate torsional resistance of each component
rectangle should be determined using Clause 5.3.4.4B(b), and the section
torsional resistance taken as the sum of the torsional resistances of the
component rectangles.
In addition, the torsional shear stress in each component rectangle should be
calculated from Equation B19 and should not exceed the limits in
Clause 5.3.4.3B.
A component rectangle should be treated as reinforced for torsion only if its
link reinforcement ties it to its adjacent rectangles.

5.3.4.5B Detailing
A section should be treated as reinforced for torsion only if the pitch of the closed
links is less than the smaller of ( x1 + y1 ) 4 or 16 longitudinal corner bar diameters and

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 33 of 100

the diameter of the longitudinal corner bars are not less than the diameter of the
links.
In areas subjected to simultaneous flexural compressive stress, the value of ∑ A sL of
reinforcement in the compressive zone used in Equation B15 may be notionally
increased by [f cav (area of section subject to flexural compression )/ (f yL γ ms )] where
f cav , is the average compressive stress in the flexural compressive zone.
For beams, the depth of the compression zone used to calculate the area of section
subject to flexural compression should be taken as twice the cover to the closed
links.
5.3.5B Longitudinal Shear
For flanged beams, the longitudinal shear resistance at the horizontal flange/web
junction and across vertical sections of the flange that may be critical should be
checked in accordance with Clause 7.4.2.3B.
5.3.6B Vertical Deflection of Beams
If required by the Railtrack Director’s Nominee, deflections may be calculated in
accordance with Clause 4.6B.
5.3.7B Crack Control in Beams
If required by the Railtrack Director’s Nominee, flexural crack widths in beams
should be calculated in accordance with Clause 5.8.7B.
5.4B Slabs
5.4.1B Moments and Shear Forces in Slabs
Moments and shear forces in slab bridges and in the top slabs of beam and slab,
voided slab and box beam bridges may be obtained from a general elastic analysis, or
such particular elastic analyses as those due to Westergaard or Pucher. Non-linear
methods may also be used. Alternatively, Johansen's yield line method may be used
to obtain the slab strength directly. When using non-elastic methods, the agreement
of Railtrack Director’s Nominee should be obtained.
The effective spans should be in accordance with Clause 5.3.1.1B.
5.4.2B Resistance Moments of Slabs
The ultimate resistance moment in a reinforcement direction may be determined by
the methods given in Clause 5.3.2B. In assessing whether the reinforcement can
resist a combination of two bending moments and a twisting moment at a point in a
slab, allowance should be made for the fact that the principal moment and

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 34 of 100

reinforcement directions do not generally coincide. The resistance in all directions
should therefore be checked.
In voided slabs, the transverse flexural strength should be calculated allowing for the
effects of transverse shear, using an analysis based on the assumption that the
transverse section acts as a Vierendeel frame.
5.4.3B Resistance to In-plane Forces
When checking whether reinforcement can resist a combination of two in-plane
direct forces and an in-plane shear force, allowance should be made for the fact that
the principal stress and reinforcement directions do not generally coincide. The
resistance in all directions should therefore be checked.
5.4.4B Shear Resistance of Slabs
5.4.4.1B Shear Stress in Solid Slabs: General
The shear stress, v, at any cross-section in a solid slab, should be calculated from:
v=

V
bd

Equation B20

where:
V
b
d

is the shear force due to ultimate loads;
is the width of slab under consideration;
is the effective depth to tension reinforcement.

V should not exceed the maximum value given in Clause 5.3.3.1B for beams.
The shear capacity should be assessed in accordance with Clauses 5.3.3.2B and
5.3.3.3B, with the following amendments:
(a)

bw should be replaced with b in all equations;

(b)

shear reinforcement should not be considered as effective in slabs less than
200 mm thick.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

(a)

1.5d x

1.5d x

by
1.5d y
y

x

Loaded
area

Critical
area

Load at corner of cantilever slab

(i)

bx

1.5d x

1.5d y

(c)

Load at edge of slab

bx

Direction of span

Critical section for
calculating shear
resistance Vc
(Critical sections (a),
(b) and (c) (i) are
assumed to have
squared corners for
rectangular and
circular loaded areas)

(b)

Load at middle of slab

Page 35 of 100

1.5d x

ex

1.5d y

(ii) Shortest straight
line which touches
loaded area

bx
ex

1.5d y
b

by

by

1.5d y

ey
Unsupported
edge

Critical
area

As

Critical
area

Unsupported
edges
As

x

Unsupported
edges

Critical
area
As

x

x

Idealized mode of
failure (only tension
reinforcement shown)
As

Parameters used to
derive Vc from Table
8 for each portion
of critical section
NOTE A s should
include only tensile
reinforcement which
is effectively anchored.

Shear resistance Vc
at critical section

3dy

bs

x

As

y

y

As

As

x

3dy
As

x

3dy

bs

bs

x

As

y

3dy
As

y

As

y

x

3dx

S jsVc bd for 4 critical portions

x

As

y
As

y

3dx

e x<3dx

x

bs

x
Shortest
straight line
which
touches
loaded
area

y

As

0.8 S jsVc bd for 3 critical portions

3dx

y

bs

e y<3dy

3dy
3dx

bs

bs

As

y

e x<3dx

0.6 S jsVc bd for 2 critical portions

As

x

bs

y

The ratio of reinforcement 100A s bsd
should be taken as the
average of the two ratios of
reinforcement in the two directions.

[( jsx + jsy ) /2 ] vc b [ ( d x + d y ) /2 ]

Figure B5
Parameters for Shear in Solid Slabs under Concentrated Loads

y

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 36 of 100

5.4.4.2B Shear Stresses in Solid Slabs under Concentrated Loads
The dispersal of load allowed in Section 4 should be taken to the top surface of the
concrete slab only and not down to the neutral axis.
The critical section for calculating shear should be taken on a perimeter 1.5d from the
boundary of the loaded area, as shown in Figure B5(a) where d is the effective depth
to the flexural tension reinforcement. Where concentrated loads occur on a
cantilever slab or near unsupported edges, the relevant portions of the critical section
should be taken as the worst case from Figure B5(a), (b) or (c). For a group of
concentrated loads, adjacent loaded areas should be considered singly and in
combination using the preceding requirements.
The ultimate punching shear capacity, Vu , is given by:
Vu =

Vc + ∑ Asv sinα(f yv γ m )

Equation B21

where:
f yv
γ ms
∑ A sv

is the characteristic, or worst credible, strength of the shear reinforcement
but not greater than 480 N/mm²;
is the material partial factor for material strength of steel given in
Clause 4.3.3B;
is the area of shear reinforcement within the area between the loaded area
and the critical perimeter, except for case c(ii) of Figure B5 when it is the area
of shear reinforcement within a distance from the load equal to the effective
depth. Shear reinforcement should however be considered to be effective
only if:
∑ Asv sinα(f yv γ ms ) ≥ 0.2 ∑ bd

α
Vc

where ∑ bd is the area of the critical section;
is the inclination of the shear reinforcement to the plane of the slab;
is the shear resistance of the concrete.

Vc should be taken as the sum of the shear resistances of each portion of the critical
perimeter (see Figure B5). The value of 100 A s (bd ) to be used, in the calculation of
v c from Clause 5.3.3.2B, should be derived by considering the effectively anchored
flexural tensile reinforcement associated with each portion as shown in Figure B5.
The ultimate punching shear capacity should also be checked on perimeters
progressively 0.75d out from the critical perimeter. The value of Asv used in
Equation B21 is the area of shear reinforcement between the perimeter under
consideration and a perimeter 1.5d within the perimeter under consideration.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 37 of 100

If a part of a perimeter cannot, physically, extend 1.5d from the boundary of the
loaded area, the part perimeter should be taken as far from the loaded areas as is
physically possible and the value of v c for that part may be increased by a factor,
1.5d av where a v is the distance from the boundary of the loaded area to the
perimeter actually considered.
When openings in slabs and footings (see Figure B6) are located at a distance less
than 6d from the edge of a concentrated load or reaction, that part of the periphery
of the critical section which is enclosed by radial projections of the openings to the
centroid of the loaded area should be considered as ineffective.
Where a hole is adjacent to the loaded area and its greatest width is less than onequarter of the side of the loaded area or one-half of the slab depth, whichever is the
lesser, the presence of the hole may be ignored.
5.4.4.3B Shear in Voided Slabs
The longitudinal ribs between the voids should be assessed in accordance with
Clause 5.3.3B as beams to resist the shear forces in the longitudinal direction
including any shear due to torsional effects.
The top and bottom flanges, acting as solid slabs, should each be capable of resisting a
part of the global transverse shear force proportional to the flange thickness. The
top flange of a rectangular voided slab should be capable of resisting the punching
effect due to concentrated loads (see Clause 5.4.4.2B). Where concentrated loads
may punch through the slab as a whole, this should also be checked.
Openings
<6d
Loaded area
Critical section

Figure B6
Openings in Slabs
5.4.5B Deflection of Slabs
If required by the Railtrack Director’s Nominee, deflections should be calculated in
accordance with Clause 4.6B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 38 of 100

5.4.6B Crack Control in Slabs
If required by the Railtrack Director’s Nominee, flexural crack widths in slabs should
be calculated in accordance with Clause 5.8.7B.
5.4.7B Torsion in Slabs
5.4.7.1B Slab Interior
The assessment of regions of slabs, other than edge zones, to resist twisting moments
should be carried out in accordance with Clause 5.4.2B.
5.4.7.2B Slab Edges
An edge zone of width equal to the overall depth of the slab should be capable of
resisting a total shear force of (Vt b e + Mnt ) when assessed in accordance with
Clause 5.3.3B,
where:
be

is taken as the width of the edge zone which may be assumed to be equal to
the slab overall depth (h).

Vt

is the flexural shear force per unit width at the edge acting on a vertical plane
perpendicular to the edge

Mnt

is the twisting moment per unit length in the slab adjacent to the edge zone
referred to axes perpendicular (n) and parallel (t) to the edge.

5.5B Columns
5.5.1B General
5.5.1.1B Definitions
A reinforced concrete column is a compression member whose greater lateral
dimension is less than or equal to four times its lesser lateral dimension, and in which
the reinforcement is taken into account when considering its strength.
A column should be considered as short if the ratio le h in each plane of buckling is
less than 12, where in the plane of buckling under consideration:
le
is the effective height of the column;
h
is the depth of the cross-section.
Otherwise the column should be considered as slender.
5.5.1.2B Effective Height of Column
The effective height, le , in a given plane may be obtained from Table B3 where lo is
the clear height between end restraints.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 39 of 100

The values given in Table B3 are based on the following assumptions:
(a)
(b)

rotational restraint is at least 4EI c lo for cases 1,2 and 4 to 6 and 8EI c lo
for case 7, where EI c is the flexural rigidity of the column cross-section;
lateral and rotational rigidity of bearings are zero.

Case 4 from Table B3 may be used for columns that are restrained at the base and
have roller bearings at the top, provided the rollers are equipped with racks to
maintain them in position.
Where a more accurate evaluation of the effective height is required or where the
end stiffness values are less than those values given in (a), the effective heights should
be derived from first principles.
The accommodation of movements and the method of articulation influences the
degree of restraint developed for columns. These factors should be assessed as
accurately as possible using engineering principles based on elastic theory and taking
into account all relevant factors such as foundation flexibility, type of bearings,
articulation system, etc.
5.5.1.3B Assessment of Strength
Clauses 5.5.2B to 5.5.7B give methods, for assessing the strength of columns at the
ultimate limit state, and are based on a number of assumptions. These methods may
be used provided the assumptions are valid for the case being considered and the
effective height is determined accurately. In addition, for columns subject to applied
bending moments the Railtrack Director’s Nominee, may, in accordance with
Clause 4.1.1B require crack widths to be calculated at the serviceability limit state.
5.5.2B Moments and Forces in Columns
The moments, shear forces and axial forces in a column should be determined in
accordance with Clause 4.4B, except that if the column is slender the moments
induced by deflection should be considered. An allowance for these additional
moments is made in the assessment requirements for slender columns set out in
Clause 5.5.5B. The bases or other members connected to the ends of such columns
should also be capable of resisting these additional moments.
Generally in columns with end moments the maximum and minimum ratios of
moment to axial load should be considered.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Case

Idealized column and buckling mode

1

Page 40 of 100

Restraints
Location

Position

Rotation

Top

Full

Full

Bottom

Full

Full

Top

Full

None

Bottom

Full

Full

Top

Full

None

Bottom

Full

None

Top

None

None

Bottom

Full

Full

Top

None

None

µo

4

Elastomeric
bearing

µo

5

1.3 µo

Bottom

Full

Full

Top

None

Full

Bottom

Full

Full

Top

None

None

1.5 µo

µo

µo

1.0 µo

1.4 µo

µo

6

µe

0.85 µo

µo

3

Effective
Height,

0.70 µo

µo

2

7

RT/CE/C/025
Issue: 1
Date: February 2000

or

µo

2.3 µo
Bottom

Full

Full

Table B3
Effective Height le for Columns
5.5.3B Short Columns Subject to Axial Load and Bending about the Minor
Axis
5.5.3.1B General
A short column should be assessed in accordance with the following
recommendations provided that the moment at any cross-section has been increased
by an additional moment caused by the actual eccentricity of the (assumed) axial load
arising from construction tolerances. If the actual eccentricity has not been
determined, the eccentricity should be taken as equal to 0.05 times the overall depth
of the cross-section in the plane of bending, but not more than 20 mm.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2000

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Page 41 of 100

5.5.3.2B Analysis of Sections
When analysing a column cross-section to determine its ultimate resistance to
moment and axial load, the following assumptions should be made:
(a)

The strain distribution in the concrete in compression and the compressive
and tensile strains in the reinforcement are derived using the assumption that
plane sections remain plane.

(b)

The stresses in the concrete in compression are either derived from the
stress-strain curve in Figure B1 with the applicable value of γ mc from
Clause 4.3.3.3B, or taken as equal to 0.6f cu γ mc over the whole compression
zone where this is rectangular or circular. In both cases, the concrete strain
at the outermost fibre at failure is taken as 0.0035.

(c)

The tensile strength of the concrete is ignored.

(d)

The stresses in the reinforcement are derived from either the stress-strain
curves in Figure B2 or, when available, manufacturers' stress-strain curves.
The values of γms are given in Clause 4.3.3.3B.

For rectangular and circular columns the following assessment methods, based on the
preceding assumptions, may be used. For other column shapes, assessment methods
should be derived from first principles using the preceding assumptions.
5.5.3.3B Design Charts for Rectangular Columns
The design charts that form CP 110: Parts 2 and 3 include charts (based on
Figure B1, Figure B2 and the assumptions in Clause 5.5.3.2B) which, with applicable
modifications for the value of γ m , may be used for the analysis of rectangular and
circular column sections having a symmetrical arrangement of reinforcement.
5.5.3.4B Assessment Formulae for Rectangular Columns
The formulae given in Equations B22 and B23 (based on a concrete stress of
0.6f cu γ mc over the whole compression zone and the assumptions in Clause 5.5.3.2B)
may be used for the analysis of a rectangular column having longitudinal
reinforcement in the two faces parallel to the axis of bending, whether that
reinforcement is symmetrical or not. Both the ultimate axial load, N, and the ultimate
moment, M, should not exceed the values of N u and Mu given by Equations B22 and
B23 for the applicable value of d c .
Nu =

(0.6f cu γ mc )bd c + f yc As1′ + f s 2 As 2

Mu =

(0.3f cu γ mc )bd c (h − d c ) + f yv As1′  − d ′  − f s 2 As 2  − d 2 

where:

Equation B22
h
2



h
2



Equation B23

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
N
M
Nu
Mu
f cu
b
dc
f yc

γ ms + f y 2000

As 2

h
d′
d2
fy

Page 42 of 100

is the ultimate axial load applied on the section considered;
is the moment applied about the axis considered due to ultimate loads
including the allowance for construction tolerance (see Clause 5.5.3B);
is the ultimate axial load capacity of the section for the particular value of dc
assumed;
is the ultimate bending capacity of the section for the particular value of dc
assumed;
is the characteristic, or worst credible, cube strength of the concrete;
is the breadth of the section;
is the depth of concrete in compression assumed subject to a minimum value
of 2d ′ ;
is the assessment compressive strength of the reinforcement (in N/mm²)
taken as:
fy

As1′
f s2

RT/CE/C/025
Issue: 1
Date: February 2000

Expression B24

is the area of compression reinforcement in the more highly compressed face;
is the stress in the reinforcement in the other face, derived from Figure B2
and taken as negative if tensile;
is the area of reinforcement in the other face which as the resultant
eccentricity of load increases and dc decreases from h to 2d´, may be
considered as being in compression, inactive, or in tension;
is the overall depth of the section in the plane of bending;
is the depth from the surface to the reinforcement in the more highly
compressed face;
is the depth from the surface to the reinforcement in the other face;
is the characteristic or worst credible strength of reinforcement.

5.5.4B Short Columns Subject to Axial Load and either Bending about the
Major Axis or Bi-axial Bending
The moment about each axis due to ultimate loads should be increased by that
moment caused by the actual eccentricity, such as that arising from construction
tolerances, of the (assumed) axial load. If the actual eccentricity has not been
determined, the construction tolerance eccentricity should be taken as equal to 0.03
times the overall depth of the cross-section in the applicable plane of bending, but not
more than 20 mm.
For square and rectangular columns having a symmetrical arrangement of
reinforcement about each axis, the section may be analysed for axial load and bending
about each axis in accordance with any one of the methods of assessment given in
Clauses 5.5.3.2B, 5.5.3.3B or 5.5.3.4B. The following relationship should be satisfied:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 43 of 100

αn

α

n
 My 
 Mx 
 M  +  M  ≤ 1.0
 ux 
 uy 

Equation B25

where:
Mx , My are the moments about the major X-X axis and minor Y-Y axis respectively
due to ultimate loads including the allowance for construction tolerances (see
preceding paragraph);
Mux
is the ultimate moment capacity about the major X-X axis assuming an
ultimate axial load capacity, N u , not less than the value of the ultimate axial
load, N ;
Muy
is the ultimate moment capacity about the minor Y-Y axis assuming an
ultimate axial load capacity, N u , not less than the value of the ultimate axial
load, N ;
αn
is given by 0.67 +1.66 Nu Nuz but not less than 1.0 and greater than 2.0 where
N uz is the axial loading capacity of a column ignoring all bending, taken as:
N uz = (0.675f cu γ mc )Ac + f yc Asc

Equation B26

where:
f cu and f yc are as defined in Clause 5.5.3.4B;
Ac
is the area of concrete;
Asc
is the area of longitudinal reinforcement.
For other column sections, assessment should be in accordance with Clause 5.5.3.2B.
5.5.5B Slender Columns
5.5.5.1B General
A cross-section of a slender column may be assessed by the methods given in
Clauses 5.5.3B and 5.5.4B for a short column but, in addition, account should be
taken of the additional moments induced in the column by its deflection. For slender
columns of constant rectangular or circular cross-section having a symmetrical
arrangement of reinforcement, the column should be able to resist the ultimate axial
load, N , together with the moments Mtx and Mty derived in accordance with Clause
5.5.5.4B. Alternatively, the simplified formulae given in Clauses 5.5.5.2B and 5.5.5.3B
may be used where applicable in this case the moment due to ultimate loads need not
be increased by the eccentricity given in Clause 5.5.3B. The minimum value of
moment should be not less than the allowance given in Clause 5.5.3B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 44 of 100

5.5.5.2B Slender Columns Bent about a Minor Axis
A slender column of constant cross-section bent about the minor Y-Y axis should be
assessed for its ultimate axial load, N, together with the moment Mty given by:
2

Mty =

Nhx  le   0.0035le 
Miy +
  1 −

1750  hx  
hx


Equation B27

where:
Miy
hx
le

is the initial moment due to ultimate loads, but not less than that
corresponding to the allowance for eccentricity as given in Clause 5.5.3B;
is the overall depth of the cross-section in the plane of bending Miy ;
is the effective height either in the plane of bending or in the plane at rightangles, whichever is greater.

For a column fixed in position at both ends where no transverse loads occur in its
height the value of Miy may be reduced to:
Miy =

0.4 M1 + 0.6 M2

Equation B28

where:
M1
M2

is the smaller initial end moment due to ultimate loads (assumed negative if
the column is bent in double curvature);
is the larger initial end moment due to ultimate loads (assumed positive).

In no case should Miy be taken as less than 0.4 M2 or such that Mty is less than M2 .
5.5.5.3B Slender Columns Bent about a Major Axis
When the overall depth of the cross-section, hy , is less than three times the width, hx,
a slender column bent about the major X-X axis should be assessed for its ultimate
axial load, N, together with the moment Mtx given by:
2

Mtx =

Nhy  le   0.0035le 
Mix +
  1−

1750  h x  
hx


Equation B29

where le and h x are as defined in Clause 5.5.5.2B;
Mix
hy

is the initial moment due to ultimate loads, but not less than that
corresponding to the allowance for eccentricity as given in Clause 5.5.3B;
is the overall depth of the cross-section in the plane of bending Mix .

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 45 of 100

Where hy is equal to or greater than three times h x the column should be
considered as biaxially loaded with the moment about the minor axis equal to that
due to eccentricity in accordance with Clause 5.5.3B.
5.5.5.4B Slender Columns Bent about both Axes
A slender column bent about both axes should be assessed for its ultimate axial load,
N , together with the moments Mtx about its major X-X axis and Mty about its minor
Y-Y axis, given by:
Nh y  lex

Mtx = Mix +
1750  h y






2

Nh  ley
Miy + x 
1750  h x





2

Mty =

 0.0035lex
1−

hy







Equation B30

 0.0035ley
1−
hx






Equation B31

where:
h x and hy are as defined in Clauses 5.5.5.2B and 5.5.5.3B respectively;
Mix
is the initial moment due to ultimate loads about the major X-X axis, including
the allowance for eccentricity in accordance with Clause 5.5.4B;
Miy
is the initial moment due to ultimate loads about the minor Y-Y axis, including
the allowance for eccentricity in accordance with Clause 5.5.4B;
lex
is the effective height of the column in respect of bending about the major
axis;
ley
is the effective height of the column in respect of bending about the minor
axis.
5.5.6B Shear Resistance of Columns
A column subject to uni-axial shear due to ultimate loads should be assessed in
accordance with Clause 5.5.3B except that the ultimate shear stress, ξ s v c , may be
multiplied by the enhancement factor given by:
1+

0.07 N
Ac

Expression B32

where:
N
Ac

is the ultimate axial load (in Newtons)
is the area of the entire concrete section (in mm²)

A column subjected to biaxial shear due to ultimate loads should satisfy the
expression:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Vx Vy
+
≤ 1.0
Vux Vuy

RT/CE/C/025
Issue: 1
Date: February 2000
Page 46 of 100

Equation B33

where:
V x ,Vy are the applied shears due to ultimate loads for the major X-X axis and minor
Y-Y axis respectively,
Vux ,Vuy are the corresponding ultimate shear capacities of the concrete and link
reinforcement for the major X-X and minor Y-Y axis respectively derived
allowing for the enhancement factor.
In calculating the ultimate shear capacity of a circular column, the area of longitudinal
reinforcement As to be used to calculate v c should be taken as the area of
reinforcement that is in the half of the column opposite the extreme compression
fibre. The effective depth should be taken as the distance from the extreme fibre
with maximum compression to the centroid of this reinforcement. The web width
should be taken as the column diameter.
5.5.7B Crack Control in Columns
If required by the Railtrack Director’s Nominee, for the purpose of calculating
flexural crack widths, a column subjected to bending should be considered as a beam
in accordance with Clause 5.8.7B.
5.6B Reinforced Concrete Walls
5.6.1B General
A reinforced wall is a vertical load-bearing concrete member whose greater lateral
dimension is more than four times its lesser lateral dimension, and in which the
reinforcement is taken into account when considering its strength.
Retaining walls, wing walls, abutments, piers and other similar elements subjected
principally to bending moments, and where the ultimate axial load is less than 0.1f cu Ac
may be treated as cantilever slabs and assessed in accordance with Clause 5.4B. In
other cases, this Clause applies.
A reinforced wall may be considered as either short or slender. In a similar manner
to columns, a wall should be considered as short where the ratio of the effective
height to thickness does not exceed 12. Otherwise the wall should be considered as
slender.
5.6.2B Forces and Moments in Reinforced Concrete Walls
Forces and moments should be calculated in accordance with Clause 4.4B except
that, if the wall is slender, the moments induced by deflection should also be
considered. The distribution of axial and horizontal forces along a wall from the loads

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 47 of 100

on the superstructure should be determined by analysis and their points of
application decided by the nature and location of the bearings. For walls fixed to the
deck, the moments should similarly be determined by elastic analysis.
Unless the actual eccentricity of load is determined, the moment per unit length in
the direction at right-angles to a wall should be taken as not less than 0.05n w h , where
nw is the ultimate axial load per unit length and h is the thickness of the wall.
Moments in the plane of a wall may be calculated from statics for the most severe
positioning of the relevant loads.
Where the axial load is non-uniform, consideration should be given to deep beam
effects and the distribution of axial loads per unit length of wall.
It will generally be necessary to consider the maximum and minimum ratios of
moment to axial load in assessing a wall.
5.6.3B Short Reinforced Walls Resisting Moments and Axial Forces
Each cross-section of the wall should be capable of resisting the applicable ultimate
axial load and the transverse moment per unit length calculated in accordance with
Clause 5.6.2B. The assumptions given in Clause 5.3.2.1B for the analysis of beam
sections apply under axial load and bending and also when the wall is subject to
significant bending only in the plane of the wall.
When the wall is subjected to significant bending both in the plane of the wall and at
right-angles to it, consideration should be given first to bending in the plane of the
wall in order to establish a distribution of tension and compression along the length
of the wall. The resulting tension and compression should be combined with the
compression due to the ultimate axial load to determine the combined axial load per
unit length of wall. This may be achieved by an elastic analysis assuming a linear
distribution along the wall.
The bending moment at right-angles to the wall should be considered and the section
checked for this moment and the resulting compression or tension per unit length at
various points along the wall length, using the assumptions given in Clause 5.3.2.1B.
5.6.4B Slender Reinforced Walls
The distribution of axial load along a slender reinforced wall should be determined as
for a short wall. The critical portion of wall should be considered as a slender
column of unit width and assessed as such in accordance with Clause 5.5.5B.
5.6.5B Shear Resistance of Reinforced Walls
A wall subject to uni-axial shear due to ultimate loads should be assessed in
accordance with Clause 5.4.4.1B except that the ultimate shear stress, ξsvc, may be
multiplied by the enhancement factors given by:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
1+

0.07 N
Ac

RT/CE/C/025
Issue: 1
Date: February 2000
Page 48 of 100
Expression B34

where:
N
Ac

is the ultimate load (in Newtons);
is the area of entire concrete section (in mm²).

A wall subject to biaxial shear due to ultimate loads should satisfy the expression:
Vx Vy
+
≤ 1.0
Vux Vuy

Equation B35

where:
V x ,Vy are the applied shears due to ultimate loads for the X-X axis and Y-Y axis
respectively;
Vux ,Vuy are the corresponding ultimate shear capacities of the concrete and link
reinforcement for the X-X axis and Y-Y axis respectively, derived allowing for
the enhancement factor given in this Clause.
5.6.6B Deflection of Reinforced Walls
Deflections of walls need not be calculated.
5.6.7B Crack Control in Reinforced Walls
If required by Railtrack Director’s Nominee, flexural crack widths in walls subject to
bending should be calculated in accordance with Clause 5.8.7B.
5.7B Bases
5.7.1B General
Where pockets have been left for precast members, allowance should be made when
calculating the flexural and shear strength of base sections, for the effects of these
pockets unless they have been grouted up using a cement mortar of compressive
strength not less than that of the concrete in the base.
5.7.2B Moments and Forces in Bases
Except where the reactions to the applied loads and moments are derived by more
accurate methods, such as an elastic analysis of a pile group or the application of
established principles of soil mechanics, the following assumptions should be made:
(a)

Where the base is axially loaded, the reactions to ultimate loads are uniformly
distributed per unit area or per pile;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
(b)

RT/CE/C/025
Issue: 1
Date: February 2000
Page 49 of 100

Where the base is eccentrically loaded, the reactions vary linearly across the
base. For columns and walls restrained in direction at the base, the moment
transferred to the base should be obtained from Clause 5.5B.

The critical section in the assessment of an isolated base may be taken as the face of
the column or wall.
The moment at any vertical section passing completely across a base should be taken
as that due to all external ultimate loads and reactions on one side of that section.
No redistribution of moments should be made.
5.7.3B Assessment of Bases
5.7.3.1B Resistance to Bending
Bases should be assessed in accordance with Clause 5.4B, and should be capable of
resisting the total moments and shears at the sections considered.
Where the width of the section considered is less than or equal to 1.5(b col + 3d ) ,
where bcol is the width of the column and d is the effective depth to the tension
reinforcement of the base, all reinforcement crossing the section may be considered
to be effective in resisting bending. For greater widths, all reinforcement within a
band of width (b col + 3d ) centred on the column may be considered to be effective and
the area of effective reinforcement outside this band should be taken as the lesser of:
(a)

the actual area of reinforcement outside the band, and

(b)

50% of the area of reinforcement within the band.

Pile caps may be assessed either by bending theory or by truss analogy taking the
apex of the truss at the centre of the loaded area and the corners of the base of the
truss at the intersections of the centre lines of the piles with the tensile
reinforcement.
In pile caps assessed by truss analogy, the effective area of reinforcement at a section
should be taken as the lesser of:
(a)

the total area at the section, and

(b)

1.25 times the area of reinforcement in the strips linking the pile heads.

Pile caps may only be assessed as beams if the reinforcement is uniformly distributed
across the section under consideration.
5.7.3.2B Shear
The assessment shear force is the algebraic sum of all ultimate vertical loads acting on
one side of or outside the periphery of the critical section. The shear strength of
bases in the vicinity of concentrated loads is governed by the more severe of the
following two conditions.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 50 of 100

(a)

Shear along a vertical section extending across the full width of the base, at a
distance equal to the effective depth from the face of the loaded area assessed
in accordance with Clause 5.4.4.1B.

(b)

Punching shear around the loaded area assessed in accordance with
Clause 5.4.4.2B.

The shear strength of pile caps is governed by the more severe of the following two
conditions.
(a)

Shear along any vertical section extending across the full width of the cap.
The recommendation of Clause 5.4.4.1B apply, except that the enhancement
of the shear resistance in accordance with Clause 5.3.3.3B for sections close
to supports should be applied only to strips of width not greater than twice
the pile diameter centred on each pile. Where av is taken as the distance
between the face of the column or wall and the nearer edge of the piles it
should be increased by 20% of the pile diameter. In applying Clause 5.4.4.1B
the allowable ultimate shear stress should be taken as the average over the
whole section;

(b)

Punching shear around loaded areas, where the requirements of
Clause 5.4.4.2B apply. When considering case (c)(ii) of Figure B5, the
allowable ultimate shear stress may be enhanced, in accordance with
Clause 5.3.3.3B, over a width not greater than twice the pile diameter centred
on the corner pile.

5.7.3.3B Bond and Anchorage
The recommendations of Clause 5.8.6B apply to reinforcement in bases.
5.7.4B Deflection of Bases
The deflection of bases need not be considered.
5.7.5B Crack Control in Bases
If required by the Railtrack Director’s Nominee, crack widths may be calculated in
accordance with Clause 5.8.7B taking into account the type of base and method of
assessment, in accordance with Clause 5.7.3.1B.
5.8B Considerations of Details
5.8.1B Constructional Details
When the reduced partial factor for material strength of steel of 1.05, given in Clause
4.3.3.3B, is adopted, measured reinforcement covers and effective depths should be
used.
5.8.2B Concrete Cover to Reinforcement
'Nominal' cover is that dimension generally used in design and indicated on the
drawings. However, design to BS 5400: Part 4 recognises that the actual cover may
be up to 5 mm less than the nominal cover.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 51 of 100

In the absence of drawings or other information, nominal cover may be determined
using the conditions below for Bridges known to be designed to BS 5400: Part 4.
The nominal cover should be taken as not less than either the size of the bar plus
5 mm or maximum aggregate size plus 5 mm. For a bundle of bars the nominal cover
should be assumed equal to or greater than the size of a single bar of equivalent area
plus 5 mm.
Where not measured the nominal cover of dense natural aggregate concrete to all
reinforcement, including links, may be taken as not less than the value given in
Table B4 for particular grades of concrete and conditions of exposure.
Where surface treatment such as bush hammering has cut into the face of the
concrete, the depth of treated concrete should not be considered as contributing to
the cover.
For bridges that have not been or are not known to have been designed to BS 5400:
Part 4, the cover assumed in assessment calculations should be determined on the
basis of measurements made on site.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 52 of 100
Nominal Cover
(mm)

Environment

Extreme
Concrete surfaces exposed to:
abrasive action by sea water
or
water with a pH ≤ 4.5

Examples

Marine Structures

Concrete Grade
25
30 40 50+
+

+

65† 55

+

§

50† 40

+

45† 35

30

45

35

25

Parts of structure in contact with
moorland water

Very severe
Concrete surfaces directly affected
by:
de-icing salts
or
sea water spray

Walls and structure supports
adjacent to a highway
Parapet edge beams adjacent to a
highway
Concrete adjacent to the sea

Severe
Concrete surfaces exposed to:
driving rain
or
alternative wetting and drying

Wall and structure supports
remote from a highway
Bridge deck soffits
Buried parts of structure

Moderate
Concrete surfaces above ground
level and fully sheltered against all of
the following:
rain,
de-icing salts,
sea water spray
Concrete surfaces permanently
saturated by water with a pH >4.5

Surface protected by bridge deck
water-proofing or by permanent
formwork
Interior surface of pedestrian
subways, voided superstructures
or cellular abutments
Concrete permanently under
water

30

Table B4
Nominal Cover to Reinforcement under Particular Conditions of
Exposure used in Design to BS 5400: Part 4
+

§

Concrete grade could cause inadequate durability.
Concrete should have been air entrained where the surface is liable to freezing whilst
wet and this cover was used.
For parapet beams only, grade 30 concrete should have been air entrained and the
nominal cover should have been 60 mm if grade 30 concrete was used.

5.8.3B Reinforcement: General Considerations
5.8.3.1B Groups of Bars
Subject to the reductions in bond stress, bars arranged as pairs in contact or in
groups of three or four bars bundled in contact should be considered as effective only
if the following conditions are satisfied:
(a)

the bundle is restrained by links;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 53 of 100

(b)

the bars in a bundle terminate at different points spaced at least 40 times the
bar size apart except for bundles stopping at a support;

(a)

bars in pairs or bundles of three are lapped one bar at a time, but the laps
staggered so that in any cross-section there are no more than four bars in a
bundle.

5.8.4B Minimum Areas of Reinforcement in Members
There is no minimum area of reinforcement requirement.
5.8.5B Bond, Anchorage and Bearing
5.8.5.1B Geometrical Classification of Deformed Bars
For the purposes of this Appendix there are two types of deformed bars, as follows:
Type 1 A plain square twisted bar or a plain chamfered square twisted bar, each with
a pitch of twist not greater than 18 times the nominal size of the bar.
Type 2 A bar with transverse ribs with a substantial uniform spacing not greater than
0.8 φ (and continuous helical ribs where present), having a mean area of ribs (per unit
length) above the core of the bar projected on a plane normal to the axis of the bar,
of not less than 0.15 mm²/mm where φ is the size (nominal diameter) of the bar.
Other bars may be classified as Types 1 or 2 from the results of the performance
tests described in BS 5400: Part 7.
5.8.5.2B Anchorage Bond
Bond failure is prevented provided the tension or compression in any bar at any
section due to ultimate loads is developed on each side of the section by the
embedment length or other end anchorage. The anchorage bond stress, assumed to
be constant over this effective anchorage length, is taken as the force in the bar
divided by the product of the effective anchorage length and the effective perimeter
of the bar or group of bars in accordance with Clause 5.8.5.3B. The anchorage bond
stress should not exceed the value of:
β

f cu
γmb

Expression B36

where:
β
f cu
γmb

is a coefficient dependent on bar type given in table B5;
is the characteristic, or worst credible concrete cube strength;
is a partial factor equal to 1.4, unless the worst credible concrete strength is
used, in which case it is equal to 1.25.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2000

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Page 54 of 100

β
Bar Type
Plain bars
Type 1:deformed bars
Type 2:deformed bars
Fabric

Bars in
Tension

Bars in
Compression

0.39
0.56
0.70
0.91

0.49
0.70
0.88
1.13

Table B5
Values of Bond Coefficient β
5.8.5.3B Effective Perimeter of a Bar or Group of Bars
The effective perimeter of a single bar may be taken as 3.14 times its nominal size.
The effective perimeter of a group of bars should be taken as the sum of the effective
perimeters of the individual bars multiplied by the applicable reduction factor given in
Table B6.
Number of Bars
in a group
2
3
4

Reduction Factor
0.8
0.6
0.4

Table B6
Reduction factor for Effective Perimeter of a Group of Bars
5.8.5.4B Anchorage of Links
A link may be considered to be fully anchored if records show that it passes round
another bar through an angle of 90° and continues beyond for a minimum length of
eight times its own size, or through 180° and continues for a minimum length of four
times its own size. If the radius of any bend in the link is less than twice the radius of
the test bend defined in BS 4449, or BS 785 prior to 1969, it should not be
considered to be fully anchored. Where full anchorage of links is not achieved, its
effective size should be taken as the equivalent bar diameter that the anchorage
provides.
5.8.5.5B Laps and Joints
Continuity of reinforcement may be assumed if connection has been made using any
of the following jointing methods:
(a)

lapping bars, see Clause 5.8.5.6B;

(b)

butt welding, see Clause 7.3.2.4B;

(c)

sleeving complying with Clause 7.3.2.2B;

(d)

threading of bars complying with Clause 7.3.2.3B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 55 of 100

The strength of joints made using the methods given in (c) and (d) and any other
method not listed should be verified by test evidence.
5.8.5.6B Lap Lengths
When bars are lapped, the length of the lap should at least equal the anchorage length
(derived from Clause 5.8.5.2B) required to develop the stress in the smaller of the
two bars lapped.
The required minimum lap length calculated as above should have been increased for
bars in tension by a factor of 1.4 if any of the following conditions apply:
(a)

the cover to the lapped bars from the top of the section as cast is less than
twice the bar size;

(b)

the clear distance between the lap and another pair of lapped bars is less than
150 mm;

(c)

a corner bar is lapped and the cover to either face is less than twice the bar
size.

Where conditions (a) and (b) or conditions (a) and (c) apply the required minimum
lap length should have been increased by a factor of 2.0.
The required minimum lap length for bar reinforcement under any condition should
not be less than 15 times the size of the smaller of the two bars lapped. Where the
minimum lap length is not present the effective size of the smaller bar at the section
should be determined as being L/15 where L is the lap length provided.
5.8.5.7B Hooks and Bends
Hooks, bends and other reinforcement anchorages should have been provided in
such form, dimension and arrangement as to avoid overstressing the concrete.
Hooks and bends can be considered fully effective if in accordance with BS 4466.
The effective anchorage length of a hook or bend should be measured from the start
of the bend to a point four times the bar size beyond the end of the bend, and may be
taken as the lesser of 24 times the bar size or:
(a)

for a hook, eight times the internal radius of the hook;

(b)

for a 90° bend, four times the internal radius of the bend.

In no case should the radius of the bend be less than twice the radius of the test bend
defined in BS 4449, or BS 785 prior to 1969. However, bends should be of sufficient
size to ensure that the bearing stress at the mid-point of the curve does not exceed
the value given in Clause 5.8.5.8B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 56 of 100

For a hooked bar to be effective at a support, the beginning of the hook should be at
least four times the bar size inside the face of the support.
The effective anchorage length of a hook or bend that does not satisfy Paragraphs 1,3
and 4 of this Clause should be taken as not greater than the actual length of bar from
the start of the bend to a point four times the bar size beyond the end of the bend.
5.8.5.8B Bearing Stress inside Bends
The bearing stress inside a bend, in a bar that does not extend or is not assumed to
be stressed beyond a point four times the bar size past the end of the bend, need not
be checked.
The bearing stress inside a bend in any other bar should be calculated from:
Bearing Stress =

Fbt rφ

Equation B37

where:
Fbt
r
φ

is the tensile force due to ultimate loads in a bar or group of bars;
is the internal radius of the bend;
is the size of the bar or, in a bundle, the size of a bar of equivalent area.

The bearing stress should not exceed;
5.63
γmc

L  ab 
f cu  
L1  φ 

1
3

Expression B38

where:
ab

ab
φ
L1
L

is the centre to centre distance between bars or group of bars perpendicular
to the plane of the bend. For a bar or group of bars adjacent to the face of
the member ab is taken as the cover plus φ ;
should not exceed 8;
is the length of the bar measured inside the bend and bearing on to the
concrete;
is the thickness of concrete member in the plane of the bend, but not greater
than 3L1 ;

Values of the partial factor, γ mc , are given in Clause 4.3.3.3B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 57 of 100

5.8.6B Curtailment and Anchorage of Reinforcement
Curtailment lengths and anchorages of bars should be assessed either by rigorous
analysis at the curtailment or anchorage point for the worst load case in accordance
with Clause 5.8.5.2B, or by use of the recommendations below.
Bars should be considered effective at a distance from their end equal to the effective
depth of the member or 12 times the size of the bar, whichever is the greater. In
addition, where reinforcement is stopped in a tension zone, one of the following
conditions should be satisfied:
(a)

the bars extend an anchorage length applicable to their assessment strength
(f y γms ) from the point at which they are no longer required to resist
bending; or

(b)

the shear capacity at the section where the reinforcement stops is greater
than 1.5 times the shear force actually present; or

(c)

the continuing bars at the section where the reinforcement stops provide
double the area required to resist the moment at that section.

At a simply supported end of a member, a tension bar should only be considered fully
effective if anchored by one of the following:
(a)

an effective anchorage equivalent to 12 times the bar size beyond the centre
line of the support - no bend or hook should begin before the centre of the
support, or

(b)

an effective anchorage equivalent to 12 times the bar size plus d 2 from the
face of the support, where d is the effective depth to tension reinforcement of
the member - no bend should begin before d 2 from the face of the support.

Where these conditions are not met, the effective size of the tension bar at the
support may be taken as 1/12 of the effective anchorage present beyond the centre
line of the support.
Where the simply supported end is resting directly on the abutment and/or where
the short shear span enhancement is being used, the effective reinforcement area
used may be taken as given in Clause 5.3.3.3B.
5.8.7B Maximum Distance between Bars in Tension
When required by the Railtrack Director’s Nominee, crack widths, should be
calculated as follows:
(a)
For solid rectangular sections, stems of T-beams and other solid sections
shaped without re-entrant angles, the crack widths at the surface (or at a
distance c nom from the outermost bar) should be calculated from:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Crack width =

3acr ε m
1+ 2(acr − c ) (h − d c )

RT/CE/C/025
Issue: 1
Date: February 2000
Page 58 of 100
Equation B39

where:
acr
c nom
c

dc
h
εm

is the distance from the point (crack) considered to the surface of the
nearest bar that controls the crack width;
is the required nominal cover to the outermost reinforcement given in
Table B4;
is the effective cover to the reinforcement that controls the width of
the cracks under consideration and should be taken as the lesser of
(i)

actual cover to this reinforcement; and

(ii)

perpendicular distance from this reinforcement to a surface at
a distance c nom from the outermost bars.

is the depth of the concrete in compression (if d c is zero crack widths
should be calculated using Equation B42);
is the overall depth of the section;
is the calculated strain at the level where cracking is being considered,
allowing for the stiffening effect of the concrete in the tension zone; a
negative value of ε m indicates that the section should be uncracked.

The value of ε m should be not greater than ε1 and be obtained from the
equation:
εm =

 3.8bt h(a′ − d c )   Mq
ε1 − 
 1− M
ε
A
h

d
(
)
 s s
 
c
g

 −9 
10 




Equation B40

where:
ε1
bt
a′

Mg
Mq
εs
As

is the calculated strain at the level where cracking is being considered,
ignoring the stiffening effect of the concrete in the tension zone;
is the width of the section at the level of the centroid of the tension
steel;
is the distance from the compression face to the point at which the
crack width is being calculated;
is the moment at the section considered due to permanent loads;
is the moment at the section considered due to live loads;
is the calculated strain at the centroid of reinforcement, ignoring the
stiffening effect of the concrete in the tension zone;
is the area of tension reinforcement. Where the axis of the moment
and the direction of the tensile reinforcement resisting that moment

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 59 of 100

are not normal to each other (such as in a skew slab), As should be
taken as:
4
∑ ( At cos α1 )

Expression B41

where:
At
α1

(b)

is the area of reinforcement in a particular direction;
is the angle between the normal to the axis of the moment and
the direction of the tensile reinforcement, At, resisting that
moment.

For flanges in overall tension, including tensile zones of box beams,
rectangular voided slabs and, when subjected to longitudinal bending, circular
voided slabs, the crack width at the surface (or at a distance c nom from the
outermost bar) should be calculated from:
crack width = 3a cr ε m

Equation B42

where ε m is obtained from equation B40.
For flanges of circular voided slabs subjected to transverse bending, the crack
width at the surface (or at a distance c nom from the outermost bar) should be
calculated from the following equation:
crack width = 1.2ε m (hf ρ net ) c φ

Equation B43

where:
hf
σ net
φ

 3.8bt hf   Mq  −9 
10 
ε1 − 
Equation B44
 1−

 ε s As   Mg 

Where global and local effects are calculated separately, in accordance with
Clause 4.8.3B the value of ε m may be obtained by algebraic addition of the
strains calculated separately. The crack width should be calculated in
accordance with (b) but may, in the case of deck slab where a global
compression is being combined with a local moment, be obtained using (a),
and calculating d c on the basis of the local moment only.
εm =

(c)

is the minimum flange thickness;
is the area of transverse reinforcement in the flange as a percentage of
the minimum flange area;
is the diameter of the outermost transverse bar;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 60 of 100

5.9B Additional Considerations for Lightweight Aggregate Concrete
5.9.1B General
Lightweight aggregate concrete may generally be assessed in accordance with the
requirements of Clauses 4B and 5.1B to 5.8B inclusive. Clauses 5.9.2B to 5.9.11B
below relate specifically to reinforced lightweight aggregate concrete of strength 25
N/mm² or above. Only the requirements of Clause 7.5B (plain concrete walls) apply
to concretes below a strength of 25 N/mm².
For lightweight aggregate concrete, the properties for any particular type of aggregate
should be to be established far more accurately than for most naturally occurring
materials and, when the aggregate type can be identified, specific data should be
obtained from the aggregate producer or other source.
All the properties of lightweight aggregate concrete to be used should be supported
by applicable test data.
5.9.2B Strength of Concrete
Clause 5.1.4.2B applies.
5.9.3B Shear Resistance of Beams
The shear resistance of lightweight aggregate concrete beams should be established in
accordance with Clauses 5.3.3.1B to 5.3.3.3B except that the value of v c calculated
from the expression given in Clause 5.3.3.2B should be multiplied by 0.9 and the
maximum allowable value of v referred to in Clauses 5.3.3.1B and 5.3.3.3B should be
multiplied by 0.8.
5.9.4B Torsional Resistance of Beams
The torsional resistance of lightweight aggregate concrete beams should be
established in accordance with Clause 5.3.4B except that the values of v t min and v tu
calculated from the expressions given in Clause 5.3.4.3B should be multiplied by 0.8.
5.9.5B Deflection of Beams
Deflection of lightweight aggregate concrete beams may be calculated using a value of
Ec as described in Clause 4.3.2.1B.
5.9.6B Shear Resistance of Slabs
The shear resistance of lightweight aggregate concrete slabs should be established in
accordance with Clause 5.4.4B, except that v c and the maximum allowable value of v
should be modified in accordance with Clause 5.9.3B.
5.9.7B Deflection of Slabs
Deflection of lightweight aggregate concrete slabs may be calculated using a value of
Ec as described in Clause 4.3.2.1B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 61 of 100

5.9.8B Columns
5.9.8.1B General
The requirements of Clause 5.5B apply to lightweight aggregate concrete columns
subject to the conditions in Clauses 5.9.8.2B and 5.9.8.3B.
5.9.8.2B Short Columns
The ratio of effective height, µe, to thickness, h, for a short column should not exceed
10.
5.9.8.3B Slender Columns
The divisor 1750 in Equations B27, B29, B30 and B31 in Clause 5.5.5B should be
replaced by 1200.
5.9.9B Local Bond, Anchorage Bond and Laps
Anchorage bond stresses and laps lengths in reinforcement for lightweight aggregate
concrete members should be assessed in accordance with Clause 5.8.6B except that
the bond stresses should not exceed 80% of those given in Clause 5.8.5.2B.
For concrete with foamed slag or similar aggregates, bond stresses should be less
than the maximum values in the preceding paragraph for reinforcement that was
known to have been in a horizontal position during casting. Acceptable bond stresses
should be obtained from test data.
5.9.10B Bearing Stress inside Bends
The requirements of Clause 5.8.5.8B apply to lightweight aggregate concrete, except
that the bearing stress should not exceed two-thirds of the allowable value given by
the expression in Clause 5.8.5.8B.
6B. PRESTRESSED CONCRETE
6.1B General
6.1.1B Introduction
Methods of assessment for prestressed concrete construction for compliance with
the recommendations set out in Clause 4B are given below. Other methods may be
used provided such methods can be shown to be satisfactory for the type of structure
or member considered. In certain cases the assumptions included below may be
inapplicable and the Engineer should adopt a more suitable method having regard to
the nature of the structure in question.
Assessment of prestressed concrete construction where any of the following are
incorporated in the structure is not included:
(a)

unbonded tendons, where prestress is solely transmitted through
compression of the ends of the member and these tendons are not grouted
within the ducts;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 62 of 100

(b)

external tendons (a tendon is considered external if, after it was stressed and
incorporated in the permanent works but before protection, it was outside
the concrete section);

(c)

lightweight aggregate concrete.

6.1.2B Limit State Assessment of Prestressed Concrete
6.1.2.1B Basis of Assessment
Usually a structure needs to be assessed only at the ultimate limit state and for
deflection under working loads. A serviceability limit state assessment for stresses
and cracking is required only when specifically requested by the Railtrack Director’s
Nominee.
6.1.2.2B Durability
Clause 6.8.2B gives recommendations for the nominal cover to prestressing tendons
that in design are assessed to provide adequate durability.
6.1.2.3B Other Limit States and Considerations
The recommendations of Clause 4.1.3B apply.
6.1.3B Loads
The assessment load effects(see Section 2) for the ultimate and serviceability limit
states are referred to as ‘ultimate loads’ and ‘service loads’ respectively.
The values of the ‘ultimate loads’ and ‘service loads’ that should be used in assessment
are derived from Section 4.
When analysing sections, the terms ‘strength’, ‘resistance’ and ‘capacity’ are used to
describe the assessment resistance of the section, see Section 2.
Consideration should be given to the construction sequence and to the secondary
effects due to prestress, particularly for a serviceability limit state assessment.
6.1.4B Strength of Materials
6.1.4.1B Definition of Strengths
The symbol f cu represents either the characteristic or the worst credible cube
strength of the concrete. The symbol f pu represents either the characteristic or the
worst credible tendon strength.
The assessment strengths of concrete and prestressing tendons are given by f cu γ mc
and f pu γ ms respectively, where γ mc and γ ms are the applicable partial factors for
material strength given in Clause 4.3.3.3B. The applicable value of γ mc or γ ms should
be substituted in all equations/ expressions contained within Clause 6B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 63 of 100

6.1.4.2B Strength of Concrete
Assessment may be based on either the specified characteristic cube strength, or the
worst credible cube strength assessed as the lower bound to the estimated in-situ
cube strength determined in accordance with BS 6089.
6.1.4.3B Strength of Prestressing Tendons
Assessment may be based on either the specified characteristic strength, or the worst
credible strength assessed from tests on tendon samples extracted from the
structure.
6.2B Structures and Structural Frames
6.2.1B Analysis of Structures
Complete structures and complete structural frames may be analysed in accordance
with the recommendations of Clause 4.4B but, when applicable the methods given in
Clause 6.3B may be used for the assessment of individual members.
The relative stiffness of members should generally be based on the assumptions for
the concrete section as described in Clause 4.4.2.1B.
6.2.2B Redistribution of Moments
Redistribution of moments obtained by rigorous elastic analysis under the ultimate
limit state may be used provided the following conditions are met:
(a)

Checks are made to ensure that adequate rotational capacity exists at sections
where moments are reduced, making reference to applicable test data. In the
absence of a special investigation, the plastic rotation capacity may be taken as
the lesser of:
d 

0.008 + 0.035 0.5 − c 
de 

or

Expression B45

10
d − dc
but not less than 0
where:
dc
de
d

is the calculated depth of concrete in compression at the ultimate limit
state (in mm);
is the effective depth for a solid slab or rectangular beam, otherwise
the overall depth of the compression flange (in mm);
is the effective depth to tension reinforcement (in mm).

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 64 of 100

(b)

Account is taken of changes in transverse moments and transverse shears
consequent on redistribution of longitudinal moments.

(c)

Shears and reactions used in assessment are taken as those calculated either
prior to or after redistribution, whichever are the greater.

6.3B Beams
6.3.1B General
6.3.1.1B Definitions
The definitions and limitations of the geometric properties for prestressed beams are
as given for reinforced concrete beams in Clause 5.3.1B.
6.3.2B Serviceability Limit State: Flexure
When a serviceability limit state assessment is required, the necessary criteria should
be agreed with the Railtrack Director’s Nominee.
6.3.3B Ultimate Limit State: Flexure
6.3.3.1B Analysis of Sections
When analysing a cross-section to determine its ultimate strength the following
assumptions should be made:
(a)

The strain distribution in the concrete in compression is derived from the
assumption that plane sections remain plane.

(b)

The stress-strain curve in Figure B1 with the applicable value of γmc given in
Clause 4.3.3.3B applies for the derivation of the stresses in the concrete in
compression or, for rectangular sections and flanged, ribbed and voided
sections where the neutral axis lies within the flange, the compressive stress is
taken as equal to 0.6 fcu/γmc over the whole compression zone. In both cases
the strain at the outermost compression fibre at failure is taken as 0.0035.

(c)

The tensile strength of concrete is ignored.

(d)

The strains in bonded prestressing tendons and in any additional
reinforcement, whether in tension or compression, are derived from the
assumption that plane sections remain plane. In addition, the tendon should
have an initial strain due to prestress after all losses.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
(e)

RT/CE/C/025
Issue: 1
Date: February 2000
Page 65 of 100

The stresses in bonded prestressing tendons, whether initially tensioned or
un-tensioned, and in additional reinforcement, are derived either from the
applicable stress-strain curve in Figures B2, B3 and B4 or, when available,
manufacturers' stress-strain curves. The values of γ ms are given in
Clause 4.3.3.3B. An empirical approach for obtaining the stress in the tendons
at failure is given in Clause 6.3.3.3B.

6.3.3.2B Design Charts
The design charts in CP 110: Part 3 include charts, based on Figures 1, 3 and 4, and
the assumptions given in Clause 6.3.3.1B, which, with applicable modifications for γ ms
which is defined in Clause 4.3.3B, may be used for the assessment of rectangular
prestressed beams.
6.3.3.3B Assessment Formula
In the absence of an analysis based on the assumptions given in Clause 6.3.3.1B, the
resistance moment of a rectangular beam, or of a flanged beam in which the neutral
axis lies within the flange, may be obtained from:
Mu =

f pb Aps (d − 0.5 x )

Equation B46

where:
Mu
f pb
x
d
Aps

is the ultimate moment of resistance of the section;
is the tensile stress in the tendons at failure;
is the neutral axis depth;
is the effective depth to tension reinforcement;
is the area of the prestressing tendons in the tension zone.

The tensile stress, fpb, should not be greater than 1.0 N/mm² and may be calculated
from:

(f

f pb
pu

γ ms )

f A 

=  α − pu ps 
f cu bd 


Equation B47

where:
α
γ ms

may be taken as 1.3 for pre-tensioning, and 1.15 for post-tensioning with
effective bond;
is the partial factor for the tendons given in Clause 4.3.3.3B.

The neutral axis depth, x , may be calculated from:
x =

f pb Aps γ mc
0.6f cu b

Equation B48

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 66 of 100

where γ mc is the partial factor for concrete given in Clause 4.3.3.3B.
Prestressing tendons and additional reinforcement in the compression zone should be
ignored in strength calculations when using this method.
6.3.3.4B Non-rectangular Sections
Non-rectangular beams should be analysed using the assumptions given in
Clause 6.3.3.1B.
6.3.4B Shear Resistance of Beams
6.3.4.1B General
Calculations for shear are only required for the ultimate limit state. In a haunched
box beam, the component of the flange forces perpendicular to the longitudinal axis
of the beam calculated from an elastic section analysis under the relevant load case
may be subtracted algebraically from the applied shear force.
At any section, the ultimate shear resistance is the sum of the resistance of the
concrete alone, Vc , calculated in accordance with Clauses 6.3.4.2B and 6.3.4.3B, and
of the shear reinforcement, Vs, calculated in accordance with Clause 6.3.4.4B.
For vertical links to be effective, the tensile capacity of the longitudinal steel at a
section should be greater than:
M (V − ξ s v c bw d )
+
z
2

Expression B49

where:
M, V

are the co-existent ultimate bending moment and shear force at the section
under consideration;
z
is the lever arm which may be taken as 0.9d;
ξ s ,v c ,bw and d are as defined in Clause 5.3.3.2B.
The tensile capacity of the longitudinal steel is:

[As ( t )f pu( t ) + As(u )f yL (u ) ] γms

Expression B50

where As ( t ) , f pu ( t ) , As ( u ) and f yL ( u ) are as defined in Clause 6.3.4.3B. However within an
individual sagging or hogging region, the tensile capacity need not exceed Mmax z
where Mmax is the maximum ultimate moment within that region.
At a section where the applied moment, M, does not exceed the cracking moment,
Mcr , calculated in accordance with Clause 6.3.4.2B, Vc may be taken as equal to the

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 67 of 100

uncracked value, Vco , in accordance with Clause 6.3.4.2B. In all other cases Vc should
be taken as the lesser of the uncracked value, Vco , and the cracked value, Vcr ,
calculated in accordance with Clause 6.3.4.3B.
For a cracked section the conditions of maximum shear with co-existent bending
moment and maximum bending moment with co-existent shear should both be
considered.
Within the transmission length of pre-tensioned members, in accordance with
Clause 6.7.4B, the shear resistance of a section should be taken as the greater of the
values calculated from:
(a)

Clause 5.3.3B except that in determining the area As , the area of tendons
should be ignored unless the tendons are rigid bars; and

(b)

Clauses 6.3.4.2B to 6.3.4.4B, using the applicable value of prestress at the
section considered, assuming a linear variation of prestress over the
transmission length.

6.3.4.2B Sections Uncracked in Flexure
A section may be assumed to be uncracked in flexure if the applied moment does not
exceed the cracking moment, Mcr given by:
Mcr =

(0.49

)

f cu γmc + f pt I y

Equation B51

where:
f pt is the stress due to prestress only at the tensile fibre distance y from the centroid
of the concrete section that has a second moment of area I. The value of f pt should
be derived from the prestressing force after all losses have occurred, multiplied by
the applicable value of γfL , given in Clause 4.2.3B. Values of the partial factor γmc are
given in Clause 4.3.3.3B.
It may be assumed that the ultimate shear resistance of a section uncracked in
flexure, Vco , corresponds to the occurrence of the maximum principal tensile stress
f t , at the centroidal axis of the section, given by:
ft =

0.32 f cu γmc

Equation B52

In the calculation of Vco , the value of f cp should be derived from the prestressing
force after all losses have occurred, multiplied by the applicable value of γ fL given in
Clause 4.2.3B. The value of Vco is given by:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Vco =

2

0.67bh f t + f cp f t

RT/CE/C/025
Issue: 1
Date: February 2000
Page 68 of 100
Equation B53

where:
ft
f cp
b

h

is 0.32 f cu γmc and is taken as positive;
is the compressive stress at the centroidal axis due to prestress, taken as
positive;
is the breadth of the member which for T, I and L beams, should be replaced
by the breadth of the rib, bw. Where the position of a duct coincides with the
position of maximum principle tensile stress, such as at or near the junction of
flange and web near a support, the value of b should be reduced by the full
diameter of the duct if ungrouted and by two-thirds of the diameter if
grouted;
is the overall depth of the member.

In flanged members, where the centroidal axis occurs in the flange, the principal
tensile stress should be limited to f t at the intersection of the flange and web. For
such members, the algebraic sum of the stress due to the bending moment under
ultimate loads and the stress due to prestress at this intersection should be used in
calculating Vco .
For a section with inclined tendons, the component of prestressing force (multiplied
by the applicable value of γ fL ) normal to the longitudinal axis of the member should
be algebraically added to Vco . This component should be taken as positive where the
shear resistance of the section is increased.
For flanged sections the actual maximum principal tensile stress may be less than given
by Equation B53. The shear strength, Vco in such cases should be taken as equal to that
under which the actual maximum principal tensile stress is equal to 0.32 f cu γmc 2.
6.3.4.3B Sections Cracked in Flexure
The ultimate shear resistance of a section cracked in flexure, Vcr , should be
calculated using Equation B54 when the factored effective prestress, f pe , exceeds
0.6 f pu . When f pe is less than 0.6 f pu , the shear strength may still be taken as given by
Equation B54. Alternatively, the value given by Equation B55 may be used if this is
greater.
Vcr =

0.045bd f cu γmc +

Mcr
M
 −d


 V 2

but not less than 0.12bd f cu γmc

Equation B54

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

(1− 0.55f

Vcr =

pe

f pu ) v c bd s +

Mo
 M − ds 


V 2

RT/CE/C/025
Issue: 1
Date: February 2000
Page 69 of 100

Equation B55

but not less than 0.12bd s f cu γ mc
where:
is the distance from the extreme compression fibre to the centroid of the
tendons at the section considered but not less than 0.625h;
is the partial factor for concrete given in Clause 4.3.3.3B;
is the cracking moment defined in Equation B51;
are the shear force and bending moment (both taken as positive) at the
section considered due to ultimate loads;
is the moment necessary to produce zero stress in the concrete at the
depth d as given by:

d

γmc
Mcr
V ,M
Mo

Mo =

fpt I/y

in which fpt is the stress due to prestress only at the depth d, distance y from
the centroid of the concrete section that has a second moment of area I.
The value of fpt should be derived from the prestressing forces after all losses
have occurred, multiplied by the applicable value of γfL given in Clause 4.2.3B.
Mo should not be taken as greater than Mcr ;
is the factored effective prestress that is equal to the effective prestress after
all losses have occurred, multiplied by the applicable value of γfL , given in
Clause 4.2.3B;
is obtained from Clause 5.3.3.2B;
(required to obtain vc) should be taken as the actual area of steel in the
tension zone, irrespective of its characteristic strength;
is the distance from the compression face to the centroid of the steel
area, As .

f pe

vc
As
ds

For cases where both tensioned and untensioned steel are contained in As , f pe f pu
may be given by:
f pe
f pu

=

where:

Pf
As ( t ) f pu ( t ) + As ( u ) f yL ( u )

Equation B56

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Pf
As ( t )
As ( u )
f pu ( t )
f yL ( u )

RT/CE/C/025
Issue: 1
Date: February 2000
Page 70 of 100

is the effective prestressing force after all losses;
is the area of tensioned steel;
is the area of untensioned steel;
is the characteristic strength or the worst credible strength of the
tensioned steel;
is the characteristic strength or the worst credible strength of the
untensioned steel.

For sections cracked in flexure and with inclined tendons, the component of
prestressing force normal to the longitudinal axis of the member should be ignored.
However, in a haunched section the component of prestress normal to the (inclined)
longitudinal axis of the member may be considered. This component should not be
taken as greater than it would be if the tendons were parallel to the flange, that is, the
tension flange ignoring the effect of prestress.
6.3.4.4B Shear Reinforcement
Types of shear reinforcement and the criterion for the amount of shear
reinforcement required to be present for it to be considered effective are defined in
Clause 5.3.3.2B. In addition links should be considered as effective only if their
spacing both along a beam and laterally does not exceed dt, nor four times the web
thickness for flanged beams.
When the above criteria are met, the shear resistance of vertical links is given by:
Vs =

Asv (f yv γms ) d t sv

Equation B57

where dt is the depth from the extreme compression fibre either to the centroid of
the tendons or to the longitudinal bars, tendons, or groups of tendons in the tension
zone around which the links are anchored in accordance with Clause 5.8.6.5B,
whichever is greater.
All other terms in the equation for Vs are defined in Clause 5.3.3.2B.
Sections within a distance d from the support need not be assessed for shear
providing the shear reinforcement calculated for the section at a distance d is
continued up to the support.
Inclined links or bent up bars should be assumed to form the tension members of
lattice girders as described in Clause 5.3.3.2B.
6.3.4.5B Maximum Shear Force
The shear force, V , due to ultimate loads, should not exceed the stress
0.36 (0.7 − f cu 250 ) γmc multiplied by bd s , where b is as defined in Clause 6.3.4.2B and
d s is defined in Clause 6.3.4.3B. Where a section is uncracked in flexure according

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 71 of 100

to Clause 6.3.4.2B d s = d t which is defined in Clause 6.3.4.4B. γmc is the partial
factor for concrete given in Clause 4.3.3.3B.
6.3.4.6B Segmental Construction
In post-tensioned segmental construction, the shear force due to ultimate loads
should not be greater than:
0.7 γfL Ph tanα 2

Equation B58

where:
γfL
Ph
α2

is the partial factor for the prestressing force, taken as 0.87;
is the horizontal component of the prestressing force after all losses;
is the angle of friction at the joint. Tan α 2 may be taken as 0.7 for a smooth
interface and 1.4 for a roughened or castellated interface. If there is any
doubt regarding the type of interface, tan α 2 should be taken as 0.7.

The method of assessment of match cast joints with shear keys should be agreed with
the Railtrack Director’s Nominee.
6.3.4.7B Alternative Approach
As an alternative to the method given in Clauses 6.3.4.1B to 6.3.4.5B, beams may be
assessed using the varying angle truss approach described in Clause 5.3.3.5B in which
case the vertical component of prestress may be deducted algebraically from the
applied shear force.
6.3.5B Torsional Resistance of Beams
6.3.5.1B General
In some members, the maximum torsional moment does not occur under the same
loading as the maximum flexural moment. In such circumstances reinforcement and
prestress in excess of that required for flexure and shear may have been used to
resist torsion.
6.3.5.2B Stresses and Reinforcement
Calculations of torsion are only required for the ultimate limit state. The torsional
shear stresses should be calculated assuming a plastic shear stress distribution.
Calculations for torsion should be in accordance with Clause 5.3.4B with the
following modifications:
(i)

when prestressing steel is used as transverse torsional steel, or as longitudinal
torsional steel, the stress assumed in assessment should be the lesser of
(f pe + 460 γms ) or f pu γms , where γms is the partial factor for material strength
of steel given in Clause 4.3.3.3B;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 72 of 100

(ii)

the compressive stress in the concrete due to prestress should be taken into
account separately in accordance with Clause 5.3.4.5B;

(iii)

in calculating (v + vt) for comparison with vtu (in accordance with
Clause 5.3.4.3B), v should be calculated from Equation B8 regardless of
whether Clause 6.3.4.2B or Clause 6.3.4.3B is critical in shear.

6.3.5.3B Other Assessment Methods
Alternative methods of assessing members subjected to combined bending, shear and
torsion may be used provided that the method used can be shown to satisfy the
ultimate limit state requirements.
6.3.6B Longitudinal Shear
For flanged beams, the longitudinal shear resistance across vertical sections of the
flange that may be critical should be checked in accordance with Clause 7.4.2.3B.
6.3.7B Deflection of Beam
If required by the Railtrack Director’s Nominee, deflections may be calculated by a
method applicable to the level of prestress in the member and the level of loading.
6.4B Slabs
The analysis of prestressed slabs should be in accordance with Clause 5.4.1B provided
that due allowance is made for moments due to prestress. The assessment should be
in accordance with Clause 6.3B.
The assessment of shear should be in accordance with Clause 6.3.4B.
For assessment of shear stresses under concentrated loads, the ultimate shear
resistance of a section uncracked in flexure, Vco , may be taken as corresponding to
the occurrence of a maximum principal tensile stress of f t = 0.32 f cu γ mc at the
centroidal axis around the critical section that is assumed at a perimeter h/2 from the
loaded area. The shear resistance of any shear reinforcement, Vs , should be assessed
in accordance with Clause 6.3.4.4B.
6.5B Columns
Prestressed concrete columns, where the mean stress in the concrete section
imposed by the tendons is less than 2.5 N/mm², may be assessed as reinforced
columns in accordance with Clause 5.5B. In all other cases the full effects of the
prestress should be considered.
6.6B Tension Members
The tensile strength of tension members should be based on the assessment strength
f pu γms of the prestressing tendons and the strength developed by any additional
reinforcement. The additional reinforcement may usually be assumed to be acting at

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 73 of 100

its assessment stress f y γ ms . In special cases it may be necessary to check the stress
in the reinforcement using strain compatibility.
6.7B Prestressing Requirements
6.7.1B Maximum Initial Prestress
The initial prestress should be assessed from record drawings, available site data or
original design calculations. In the absence of such information, the likely nominal
value of the initial prestress should be assessed from the standards current at the
time of the design.
6.7.2B Loss of Prestress other than Friction Losses
6.7.2.1B General
Allowance should be made, when calculating the forces in tendons, for losses of
prestress resulting from:
(a)
(b)
(c)
(d)

relaxation of the steel comprising the tendons;
the elastic deformation and subsequent shrinkage and creep of the concrete;
slip or movement of tendons at anchorages during anchoring;
other causes in special circumstances, such as when steam curing has been
used with pre-tensioning.

If experimental evidence on performance is not available, account should be taken of
the properties of the steel and the concrete when calculating the losses of prestress
from these Clauses. For a wide range of structures, the recommendations given in
this Clause should be used. It should be recognised, however, that these
requirements are necessarily general and approximate.
6.7.2.2B Loss of Prestress due to Relaxation of Steel
The loss of force in the tendon allowed for in the assessment should be the maximum
relaxation after 1000 hours duration, for a jacking force equal to that which is
estimated was imposed at transfer, as given by the applicable British Standard or
manufacturer's data.
In special cases, such as tendons at high temperature or subjected to large lateral
loads as in deflected tendons, greater relaxation losses may be present and specialist
literature should be consulted.
6.7.2.3B Loss of Prestress due to Elastic Deformation of the Concrete
Calculation of the immediate loss of force in the tendons due to elastic deformation
of the concrete at transfer may be based on the values for the modulus of elasticity of
the concrete given in Clause 4.3.2.1B. The modulus of elasticity of the tendons may
be obtained from Clause 4.3.2.2B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 74 of 100

For pre-tensioning, the loss of prestress in the tendons at transfer should be
calculated on a modular ratio basis using the stress in the adjacent concrete.
For members with post-tensioning tendons that were not stressed simultaneously,
there would have been a progressive loss of prestress during transfer due to the
gradual application of the prestressing force. The resulting loss of prestress in the
tendons should be calculated on the basis of half the product of the modular ratio and
the stress in the concrete adjacent to the tendons, averaged along their length.
Alternatively, the loss of prestress may be computed exactly based on the sequence
of tensioning if that is known.
In making these calculations, it may usually be assumed that the tendons are located
at their centroid.
6.7.2.4B Loss of Prestress due to Shrinkage of the Concrete
The loss of prestress in the tendons due to shrinkage of the concrete may be
calculated from the modulus of elasticity for the tendons given in Clause 4.3.2.2B,
assuming the values for shrinkage per unit length given in Table B7.
Shrinkage per Unit Length
System

Humid Exposure
(90% rh)

Normal Exposure
(70% rh)

Pre-tensioning: transfer at between 3 and
5 days after concreting

100 x 10 -6

300 x 10-6

Post-tensioning: transfer at between 7 and
14 days after concreting

70 x 10 -6

200 x 10-6

Table B7
Shrinkage of Concrete
For other ages of concrete at transfer, for other conditions of exposure, or for
massive structures, some adjustment to these values is necessary. Reference should
be made to BS 5400: Part 4 Appendix C or to specialist literature, details of which are
given in Appendix F.
6.7.2.5B Loss of Prestress due to Creep of the Concrete
The loss of prestress in the tendons due to creep of the concrete should be
calculated on the assumption that creep is proportional to stress in the concrete for
stress of up to one-third of the cube strength at transfer. The loss of prestress is
obtained from the product of the modulus of elasticity of the tendons, given in
Clause 4.3.2.2B and the creep of the concrete adjacent to the tendons. Usually it is
sufficient to assume, in calculating this loss, that the tendons are located at their
centroid.
For pre-tensioning at between 3 days and 5 days after concreting and, for humid or
dry conditions of exposure, where the cube strength at transfer was greater than

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 75 of 100

40 N/mm², the creep of the concrete per unit length should be taken as 48 x 10-6 per
N/mm². For lower values of cube strength at transfer the creep per unit length
should be taken as 48 x 10-6 x40/fci per N/mm².
For post-tensioning at between 7 days and 14 days after concreting, and for humid or
dry conditions of exposure, where the cube strength at transfer was greater than
40N/mm², the creep of the concrete per unit length should be taken as 36 x 10-6 per
N/mm². For lower values of cube strength at transfer, the creep per unit length shall
be taken as 36 x 10-6 x 40/fci per N/mm².
The concrete strength at transfer, f ci , (in N/mm²) which should be taken from
contract record drawings, available site data or original design calculations. In the
absence of such information, the likely nominal value should be assessed from the
standards current at the time of the design.
Where the maximum stress anywhere in the section at transfer exceeded one-third
of the cube strength of the concrete at transfer, the value of the creep per unit length
used in calculations may be increased. When the maximum stress at transfer was half
the cube strength at transfer, the values for creep should be taken as 1.25 times those
given above. At intermediate stresses, the values should be interpolated linearly.
In applying these requirements, which are necessarily general, reference should be
made to BS 5400: Part 4 Appendix C or specialist literature, details of which are given
in Appendix F, for more detailed information on the factors affecting creep.
6.7.2.6B Loss of Prestress during Anchorage
In post-tensioning systems, allowance should be made for any movement of the
tendon at the anchorage that would have occurred when the prestressing force was
transferred from the tensioning equipment to the anchorage.
6.7.2.7B Losses of Prestress due to Steam Curing
Where steam curing was employed in the manufacture of prestressed concrete units,
changes in the behaviour of the material at higher than normal temperatures needs to
be considered. In addition, where the ‘long-line' method of pre-tensioning was used
there may be additional losses as a result of bond developed between the tendon and
the concrete when the tendon was hot and relaxed. Since the actual losses of
prestress due to steam curing are a function of the techniques used by the various
manufacturers, specialist advice should be sought.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 76 of 100

6.7.3B Loss of Prestress due to Friction
6.7.3.1B General
In post-tensioning systems there will have been movement of the greater part of the
tendon relative to the surrounding duct during the tensioning operation. If the
tendons were in contact with either the duct or any spacers provided, friction would
have caused a reduction in the prestressing force as the distance from the jack
increased. In addition, friction would have developed in the jack itself and in the
anchorage through which the tendon passed.
In the absence of site data, the stress variation likely to be expected along the tendon
profile should be assessed in accordance with Clauses 6.7.3.2B to 6.7.3.5B in order to
obtain the prestressing force at the critical sections considered in assessment.
6.7.3.2B Friction in the Duct due to Unintentional Variation from the Specified
Profile
Whether the desired duct profile was straight or curved or a combination of both,
there will have been slight variations in the actual line of the duct, which may have
caused additional points of contact between the tendon and the sides of the duct, and
so produced friction. The prestressing force, Px ,at any distance x from the jack may
be calculated from:
Px =
P0 e − Kx
Equation B59
where:
P0
e
K

is the prestressing force in the tendon at the jacking end;
is the base of Napierian logarithms (2.718);
is the constant depending on the type of duct, or sheath employed, the nature
of its inside surface, the method of forming it and the degree of vibration
employed in placing the concrete.

When Kx ≤ 0.2 , e − Kx may be taken as ( 1− Kx )
The value of K per metre length in Equation B59 should generally be taken as not less
than 33 x 10-4, except where strong rigid sheaths or duct formers were used closely
supported to prevent displacement during the concreting operation, In which case
the value of K may be taken as 17 x 10-4. Other values may be used provided they
have been established by tests to the satisfaction of the Engineer at time of
construction and their use for assessment is agreed by the Railtrack Director’s
Nominee.
6.7.3.3B Friction in the Duct due to Curvature of the Tendon
When a tendon is curved, the loss of tension due to friction is dependent on the angle
turned through and on the coefficient of friction between the tendon and its
supports. The prestressing force, Px , at any distance x along the curve from the
tangent point may be calculated from:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Px =

P0 e

RT/CE/C/025
Issue: 1
Date: February 2000

−µx rps

Page 77 of 100

Equation B60

where:
Po
rps

is the prestressing force in the tendon at the tangent point near the jacking
end;
is the radius of curvature

When µx rps ≤ 0.2, e

−µx rps

may be taken as (1− µx rps ) .

When the combined effects of Clause 67.3.2B and this Clause result in
(Kx + µx rps )≤ 0.2, e −(Kx +µx rps ) may be taken as [1− (Kx + µx rps )].
Values of µ may be taken as:
0.55
0.30
0.25

for steel moving on concrete;
for steel moving on steel;
for steel moving on lead.

The value of µ may be reduced where special precautions were taken during
construction and where results are available to justify the value assumed. For
example, a value of µ = 1.0 has been observed for strand moving on rigid steel
spacers coated with molybdenum disulphide. Such reduced values should be used
only if construction records confirm that precautions were used.
6.7.3.4B Friction in Circular Construction
Where circumferential tendons have been tensioned by means of jacks, the losses
due to friction may be calculated from the equation in Clause 6.7.3.3B, but the value
of µ may be taken as:
0.45
0.25
0.10

for steel moving in smooth concrete;
for steel moving on steel bearers fixed to the concrete;
for steel moving on steel rollers.

6.7.3.5B Lubricants
Where lubricants were specified and lower values of µ than those given in
Clauses 6.7.3.4B and 6.7.3.5B were obtained by trials prior to construction, the lower
values may be used for assessment.
6.7.4B Transmission Length in Pre-tensioned Members
The transmission length is defined as the length over which a tendon is bonded to
concrete to transmit the initial prestressing force in a tendon to the concrete.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 78 of 100

The transmission length depends on a number of variables, the most important being:
(a)
(b)
(c)
(d)
(e)
(f)

the degree of compaction of the concrete;
the strength of the concrete;
the size and type of tendon;
the deformation (such as crimp) of the tendon;
the stress in the tendon; and
the surface condition of the tendon.

The transmission lengths of the tendons towards the top of a unit may be greater
than those at the bottom.
A sudden release of tendons may also cause a considerable increase in the
transmission lengths.
Where the initial prestressing force was not greater than 75% of the characteristic
strength of the tendon and where the concrete strength at transfer was not less than
30 N/mm², the transmission length, Lt , may be taken as follows:
kt φ
Lt =
Equation B61
f ci
where:
f ci

Lt
φ
kt

is the concrete strength at transfer (in N/mm²) which should be assessed from
record drawings, available site data or original design calculations. In the
absence of such information, the likely nominal value should be assessed from
the standards current at the time of the design:
is the transmission length (in mm);
is the nominal diameter of the tendon (in mm);
is a coefficient dependent on the type of tendon, to be taken as:
600 for plain, indented and crimped wire with a total wave height less than
0.15φ;
400 for crimped wire with a total wave height greater than or equal to 0.15φ;
240 for 7-wire standard and super strand;
360 for 7-wire drawn or compacted strand.

The development of stress from the end of the unit to the point of maximum stress
should be assumed to be linear over the transmission length.
If the tendons have been prevented from bonding to the concrete near the ends of
the unit by the use of sleeves or tape, the transmission lengths should be taken from
the ends of the de-bonded portions.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 79 of 100

6.7.5B End Blocks
The end block (also known as the anchor block or end zone) is defined as the highly
stressed zone of concrete around the termination points of a pre-tensioned or posttensioned tendon. It extends from the point of application of prestress (that is the
end of the bonded part of the tendon in pre-tensioned construction) or the
anchorage in post-tensioned construction to that section of the member at which
linear distribution of stress is assumed to occur over the whole cross-section.
The following aspects should be considered in assessing the strength of end blocks:
(a)
(b)
(c)

bursting forces around individual anchorages;
overall equilibrium of the end block;
spalling of the concrete from the loaded face around anchorages.

In considering each of these aspects, particular attention should be given to factors
such as:
(a)
(b)
(c)
(d)
(e)
(f)

shape, dimensions and position of anchor plates relative to the crosssection of the end block;
the magnitude of the prestressing forces and the sequence of
prestressing;
shape of the end block relative to the general shape of the member;
layout of anchorages including asymmetry, group effects and edge
distances;
influence of the support reaction;
forces due to curved or divergent tendons.

The following recommendations are applicable to a circular, square or rectangular
anchor plate, symmetrically positioned on the end face of a square or rectangular
post-tensioned member.
The bursting tensile forces in the end blocks, or end regions of post-tensioned
members should be assessed on the basis of the load in the tendon at the ultimate
limit state.
The bursting tensile force, Fbst , existing in an individual square end block loaded by a
symmetrically placed square anchorage or bearing plate, may be derived from:
Fbst =

Pk (0.32 − 0.3y po y o )

where:
Pk
yo
y po

is the load in the tendon;
is half the side of end block;
is half the side of loaded area.

Equation B62

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 80 of 100

The force, Fbst , is distributed in a region extending from 0.2 y o to 2 y o from the loaded
face of the end block. Reinforcement present may be assumed to sustain the bursting
tensile force working up to a stress of f y γms .
In rectangular end blocks, the bursting tensile force Fbst in the two principle directions
should be assessed from Equation B62.
When circular anchorage or bearing plates are present, the side of the equivalent
square area should be derived.
Where groups of anchorages or bearing plates occur, the end blocks should be
divided into a series of symmetrically loaded prisms and each prism treated as above.
When assessing the end block as a whole, it is necessary to check that the groups of
anchorages are appropriately tied together by reinforcement.
Special attention should be paid to end blocks having a cross-section different in
shape from that of the general cross-section of the beam. Reference should be made
to the specialist literature.
Compliance with the preceding requirements will generally ensure that the bursting
tensile forces along the load axis can be sustained. Alternative methods of
assessment which use higher values of Fbst PK and allow for the tensile strength of
concrete may be more applicable in some cases, particularly where large
concentrated tendon forces are involved.
Consideration should also be given to the spalling tensile stresses that occur in end
blocks where the anchorage or bearing plates are highly eccentric. These stresses
reach a maximum at the loaded face.
6.8B Considerations of Details
6.8.1B General
The considerations in Clauses 6.8.2B to 6.8.5B are intended to supplement those for
reinforced concrete given in Clause 5.8B.
6.8.2B Cover to Prestressing Tendons
6.8.2.1B General
The covers given in Clauses 6.8.2.2B and 6.8.2.3B, other than those for curved ducts,
are those that are currently considered to be necessary to provide adequate
durability.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 81 of 100

6.8.2.2B Pre-tensioned Tendons
The requirements of Clause 5.8.2B concerning cover to reinforcement may be taken
to be applicable. The ends of individual pre-tensioned tendons do not normally
require concrete cover.
6.8.2.3B Tendons in Ducts
The cover to any duct should be not less than 50 mm.
Requirements for the cover to curved tendons in ducts are given in BS 5400: Part 4
Appendix D.
6.8.3B Spacing of Prestressing Tendons
6.8.3.1B Tendons in Ducts
Recommendations for the spacing of curved tendons in ducts are given in BS 5400:
Part 4 Appendix D.
6.8.4B Longitudinal Reinforcement in Prestressed Concrete Beams
Reinforcement in prestressed concrete members may be considered to enhance the
strength of the sections.
6.8.5B Links in Prestressed Concrete Beams
Links present in a beam may be considered as shear reinforcement, in accordance
with Clause 6.3B or to resist bursting tensile stresses in the end zones of posttensioned members, in accordance with Clauses 6.3.4B and 6.7.4B.
7B. PRECAST, COMPOSITE AND PLAIN CONCRETE
CONSTRUCTION
7.1B General
7.1.1B Introduction
Additional considerations that arise in assessment when precast members or precast
components are incorporated into a structure or when a structure in its entirety is of
precast concrete construction are detailed below. The assessment of plain concrete
walls and abutments is also described.
7.1.2B Limit State Assessment
7.1.2.1B Basis of Assessment
The limit state philosophy set out in Clause 4B applies equally to precast and in situ
construction. In general, therefore, the relevant methods of assessment for
reinforced concrete given in Clause 5B and those for prestressed concrete given in
Clause 6B apply also to precast and composite construction. Sub-clauses in
Clause 5B or 6B that do not apply are either specifically worded for in situ
construction or are modified below.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 82 of 100

7.1.2.2B Connections and Joints
The strength of connections is of fundamental importance in precast construction and
should be carefully considered in assessment.
In the assessment of beam and slab ends on corbels and at supports, particular
attention should be given to the detailing of overlaps and anchorages and all
reinforcement adjacent to the contact faces. Reinforcement detailing should be
assessed in accordance with Clause 5.8.7B.
7.2B Precast Concrete Construction
7.2.1B Framed Structures and Continuous Beams
When the continuity of reinforcement or tendons through the connections and/or
the interaction between members is such that the structure behaves as a frame, or
other rigidly interconnected system, the analysis, re-distribution of moments and
assessment of individual members, may all be in accordance with Clause 5B or 6B, as
applicable.
7.2.2B Other Precast Members
All other precast concrete members should be assessed in accordance with the
applicable requirements of Clauses 5B, 6B or 7.5B. Connections should be assessed
in accordance with Clause 7.3B.
7.2.3B Supports for Precast Members
7.2.3.1B Concrete Corbels
A corbel is a short cantilever beam in which the principal load is applied such that the
distance av between the line of action of the load and the face of the supporting
member does not exceed the effective depth and the depth, at the outer edge of the
bearing is not less than one-half of the depth at the face of the supporting member.
The shear capacity at the face of the supporting member should be assessed in
accordance with Clause 5.3.3.3B, but using the modified definition of av given above.
The adequacy of the main tension reinforcement in a corbel should be assessed on
the assumption that the corbel behaves as a simple strut and tie system and making
due allowance for horizontal forces. The tensile force which the main reinforcement
can develop may be limited by any one of the following:


the yield of the reinforcement;



the anchorage of the reinforcement in the supporting member;



the anchorage at the front face of the corbel.

Any part of the area of the bearing that projects beyond the straight portion of the
bars forming the main tension reinforcement should be ignored when proportioning
the strut and tie system, and when checking bearing stresses in accordance with
Clause 7.2.3.3B.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2000

The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Page 83 of 100

7.2.3.2B Width of Supports for Precast Units
The width of supports for precast units should be sufficient to provide proper
anchorage of tension reinforcement in accordance with Clause 5.8.6B.
7.2.3.3B Bearing Stresses
The compressive stress in the contact area should not exceed 0.6 fcu/γmc under the
ultimate loads. When the members are made of concretes of different strengths, the
lower concrete strength is applicable.
Higher bearing stresses are acceptable where suitable measures have been taken to
prevent splitting or spalling of the concrete, such as the provision of well-defined
bearing areas and additional binding reinforcement in the ends of the members.
Bearing stresses due to ultimate loads should be limited to:
3(f cu γmc )
1+ 2 Acon Asup

Expression B63

where:
Acon
Asup

is the contact area;
is the supporting area.

For rectangular bearings (see Figure B7):
Asup =

(b x + 2 x )(by + 2 y ) and x ≤ b x , and y ≤ by

x
Supporting area
bx
Contact area

x
y

by

y

Figure B7
Bearing Areas
where:
b x ,by are the dimensions of the bearing in the x, y directions respectively;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
x ,y

RT/CE/C/025
Issue: 1
Date: February 2000
Page 84 of 100

are the dimensions from the boundary of the contact area to the boundary of
the support area.

For lightweight aggregate concrete the bearing stresses due to ultimate loads should
be limited to two-thirds of those for normal weight aggregate concrete given by the
above formula.
Higher bearing stresses due to ultimate loads should be used only where justified by
tests.
7.2.3.4B Horizontal Forces or Rotations at Bearings
The presence of significant horizontal forces at bearings can reduce the load-carrying
capacity of the supporting and supported members considerably by causing
premature splitting or shearing. These forces may be due to creep, shrinkage and
temperature effects or result from misalignment, lack of plumb or other causes.
When these forces are likely to be significant, it is necessary to check that either:
(a)
(b)
(c)

sliding bearings are present; or
suitable lateral reinforcement is present in the top of the supporting
member; and
continuity reinforcement is present to tie together the ends of the
supported members.

Where, owing to large spans or other reasons, large rotations are likely to occur at
the end supports of flexural members, suitable bearings capable of accommodating
these rotations should be present. In the absence of such bearings, bearing stresses
could be increased due to concentration of the reaction towards one edge of a
bearing and/or flexure of the supported member could result, depending on the type
of bearing actually present.
7.2.4B Joints between Precast Members
7.2.4.1B General
The critical sections of members close to joints should be assessed under the worst
combinations of shear, axial force and bending effects caused by the co-existing
ultimate vertical and horizontal forces. The evaluation of the effects should take due
account of any fixity imposed by the joints.
7.2.4.2B Halving Joint
For the type of joint shown in Figure B8(a), the maximum vertical ultimate load, Fv ,
should not exceed v u bd o .
where:
vu
b

is the lesser of 0.92 f cu γmc or 7 γ mc in N/mm²;
is the breadth of the beam;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
do

RT/CE/C/025
Issue: 1
Date: February 2000
Page 85 of 100

is the depth to the horizontal reinforcement in the halving joint.

The capacity of a halving joint may be determined by considering the two following
strut and tie systems and summing the capacities of the two systems, and in
accordance with the recommendations of BA 39/93: Assessment of Reinforced Concrete
Half-joints.
The system, shown in Figure B8(b), involves the inclined reinforcement which
intersects the line of action of Fv . The inclined reinforcement may take the form of
bent-up bars or links.
For bent-up bars, the bearing stresses inside the bends should be checked to
determine whether the stress in the bars should be limited to less than f y γms in
accordance with Clause 5.8.5.8B.
For links, their anchorage in the compression face of the beam should be in
accordance with Clause 5.8.5.4B, whilst in the tension face the horizontal component,
Fh , of the link force is transferred to the main reinforcement. The links may be
considered to be fully anchored in the tension face if the anchorage bond stress of the
main reinforcement due to the force Fh does not exceed twice the anchorage bond
stresses given in Clause 5.8.5.2B.
The strut and tie system shown in Figure B8(c) involves the vertical reinforcement in
the full depth section adjacent to the halving joint, and requires the horizontal
reinforcement in the halving joint to be in excess of that required to resist the
horizontal ultimate load.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

Inclined links

RT/CE/C/025
Issue: 1
Date: February 2000
Page 86 of 100

Vertical links

d0

Horizontal reinforcement

Main tension
reinforcement

(a)

C o m p r e s s ive strut
Reinforcement tie

(b)

(c)
Figure B8
Halving Joint

7.3B Structural Connections between Units
7.3.1B General
7.3.1.1B Structural Requirements of Connections
When assessing the connections across joints between precast members the overall
stability of the structure should be considered.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 87 of 100

7.3.1.2B Assessment Method
Connections should, where possible, be assessed in accordance with the generally
accepted methods applicable to reinforced concrete (see Clause 5B), prestressed
concrete (see Clause 6B) or structural steel. Where, by the nature of the
construction or material used, such methods are not applicable, the adequacy of the
connection should be proved by tests.
7.3.2B Continuity of Reinforcement
7.3.2.1B General
The assumptions made in analysing the structure and assessing critical sections should
reflect the degree of continuity of reinforcement through a connection. The
following methods are capable of achieving continuity of reinforcement:
(a)
(b)
(c)
(d)

lapping bars;
butt welding;
sleeving;
parallel threading of bars and tapered threads.

The strength of the joints in (c) and (d) and any other method not listed should be
assessed on the basis of test evidence.
7.3.2.2B Sleeving
The following three principal types of sleeve jointing may be found:
(a)
(b)
(c)

grout or resin filled sleeves;
sleeves that mechanically align the square-sawn ends of two bars to allow the
transmission of compressive forces only;
sleeves that are mechanically swaged to the bars.

7.3.2.3B Threading
The following methods for joining threaded bars may be found:
(a)
(b)
(c)
(d)

parallel threaded ends of bars are joined by a coupler having left - and right hand threads;
one set of bars is welded to a steel plate that is drilled to receive the threaded
ends of the second set of bars, which are fixed to the plate by means of nuts;
threaded anchors cast into a precast unit to receive the threaded ends of
reinforcement;
taper threaded bars joined by the use of internally taper threaded couplers.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 88 of 100

The structural assessment of special threaded connections should refer to testing in
accordance with BS 5400: Part 1, (including behaviour under fatigue conditions
where relevant) which may have been carried out when the structure was designed.
Where tests have shown the tensile strength of the threaded connection to be
greater than or equal to the characteristic strength of the parent bars, the strength of
the joint may be based on the specified characteristic strength of the joined bars
divided in each case by the applicable γms partial factor.
7.3.2.4B Welding of Bars
Where bars are known or suspected to be welded the fatigue strength of the bars
should be assessed in accordance with Clause 4.7B.
7.3.3B Other Types of Connection
The load carrying capacity of any other type of connection should have been justified
by test evidence when the Bridge was designed. For resisting shear and flexure
suitable connections are those types which are made by prestressing across the joint.
7.4B Composite Concrete Construction
7.4.1B General
The requirements of this Clause apply to flexural members consisting of precast
concrete units acting in conjunction with added concrete where the contact surface is
capable of transmitting longitudinal shear. The precast units may be of either
reinforced or prestressed concrete.
In general, the analysis and assessment of composite concrete structures and
members should be in accordance with Clause 5B or 6B, modified where applicable
by Clauses 7.4.2B and 7.4.3B. Particular attention should be given in the assessment
to the effect of the method of construction and whether or not props were used.
The relative stiffness of members should be based on the concrete, gross transformed
or net transformed section properties as described in Clause 4.4.2.1B. If the
concrete strengths in the two components of the composite member differ by more
than 10 N/mm², allowance for this difference should be made in assessing stiffness and
stresses.
Differential shrinkage between the added concrete and precast concrete members
should be considered in analysing composite members for the serviceability limit
states in accordance with Clause 7.4.3B. It need not be considered for the ultimate
limit state.
Precast units, incorporating pre-tensioned tendons, assessed as continuous members
with continuity obtained with reinforced concrete cast in situ over the supports, the
compressive stresses due to prestress in the ends of the units may be assumed to
vary linearly over the transmission length of the tendons in assessing the strength of
sections.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 89 of 100

7.4.2B Ultimate Limit State
7.4.2.1B General
Where the cross-section of composite members and the applied loading increased
during construction in stages (such as when a precast prestressed unit initially
supporting self-weight and the weight of added concrete and subsequently acting
compositely for live loading), the entire load may be assumed to act on the final
cross-section.
7.4.2.2B Vertical Shear
The assessment of the resistance of composite sections to vertical shear should be in
accordance with Clause 5.3.3B for reinforced concrete (except that in determining
the area As , the area of tendons within the transmission length should be ignored)
and Clause 6.3.4B for prestressed concrete, modified where applicable as follows:
(a)

(b)

For I, M, T, U and box beam precast prestressed concrete units with an in situ
reinforced concrete top slab cast over the precast units (including pseudo box
construction), the shear resistance should be based on either of the following:
(i)

the precast unit acting alone assessed in accordance with
Clause 6.3.4B;

(ii)

the composite section assessed in accordance with Clause 6.3.4B. In
this case, section properties should be based on those of the
composite section, with due allowance for the different grades of
concrete where applicable.

For inverted T beam precast prestressed concrete units with transverse
reinforcement placed through holes in the bottom of the webs of the units,
completely infilled with concrete placed between and over the units to form a
solid deck slab, the shear resistance should be taken as the sum of Vi and Vp
where:
Vi

Vp

(c)

is the shear capacity of the infill concrete assessed in accordance with
Clause 5.3.3B with the breadth taken as the distance between adjacent
precast webs and the depth as the mean depth of infill concrete, or the
mean effective depth to the longitudinal reinforcement where this is
provided in the infill section.

is the shear capacity of the precast prestressed section assessed in
accordance with Clause 6.3.4B with the breadth taken as the web
thickness and the depth as the depth of the precast unit.
In applying Clause 6.3.4.4B, dt should be derived for the composite section.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 90 of 100

7.4.2.3B Longitudinal Shear
The longitudinal shear force, V1 , per unit length of a composite member, whether
simply supported or continuous, should be calculated at the interface of the precast
unit and the in situ concrete and at any vertical planes that may be critical in
longitudinal shear (as shown by planes 2-2 or 2´-2´ in Figure B9). An elastic method
may be adopted using properties of the composite concrete section, in accordance
with Clause 4.4.2.1B, with due allowance for different grades of concrete where
applicable.

2 2'

1

In-situ concrete
1

2

2'

Precast beam

Figure B9
Potential Planes
Vi

should not exceed the lesser of the following:

(a)

( k1f cu γmc )L s

Expression B64

(b)

(v1 γ mv )Ls + (0.8 Ae f s γms )

Expression B65

where:
k1

f cu
γmc
γms
Ls
v1

is a constant depending on the concrete bond across the shear plane under
consideration, which may be taken as 0.24 for monolithic construction or
surface type 1, or 0.14 for surface type 2. These values should be reduced by
25% for lightweight aggregate concrete construction;
is the characteristic, or worst credible, strength of the weaker of the two
concretes each side of the shear plane but should not be taken as >45 N/mm².
is the partial factor for concrete given in Clause 4.3.3.3B;
is the partial factor for steel given in Clause 4.3.3.3B;
is the breadth of the shear plane under consideration;
is the longitudinal shear stress in the concrete for the plane under
consideration, and should be taken as:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

γmv
Ae
fs

RT/CE/C/025
Issue: 1
Date: February 2000
Page 91 of 100

for monolithic construction:

0.05 f cu but not less than 1.13 N/mm² and
not greater than 1.56 N/mm²;

for surface Type 1:

0.04 f cu but not less than 0.8 N/mm² and
not greater than 1.28 N/mm²;

for surface Type 2:

0.019 f cu but not less than 0.38 N/mm² and
not greater than 0.63 N/mm².

All values should be reduced by 25% for lightweight aggregate concrete
construction;
is the partial factor for material strength for shear given in Clause 4.3.3.3B;
is the area of reinforcement per unit length crossing the shear plane under
consideration; reinforcement assumed to resist co-existent bending and
vertical shear, in accordance with Clause 7.4.2.2B, may be included;
is the stress at the ultimate limit state in the steel reinforcement of area Ae .
The stress may be assumed to be the characteristic, or worst credible
strength, f y , if the reinforcement Ae is fully anchored, in accordance with
Clause 5.8.6B. Otherwise f s should be taken as a fraction of f y in proportion
to the ratio of the anchorage available to that required by Clause 5.8.6B. The
value of f s should be such that Ae f s b is not greater than 10 N/mm² where b
is the width of the interface under consideration.

For composite beam and slab construction, reinforcement crossing the shear plane
should be considered as effective only if its spacing does not exceed the lesser of the
following:
(a)

six times the minimum thickness of the in situ concrete flange and;

(b)

900 mm.

The types of surface are defined as follows:
Type 1 - The contact surface of the concrete in the precast members was prepared as
described in either (1) or (2) as applicable:
(1)

when the concrete had set but not hardened the surface was sprayed with a
fine spray of water or brushed with a stiff brush, just sufficient to remove the
outer mortar skin and expose the larger aggregate without disturbing it;

(2)

the surface skin and laitance were removed by sand blasting or the use of a
needle gun, but not by hacking.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 92 of 100

Type 2 - The contact surface of the concrete in the precast member was jetted with
air and/or water to remove laitance and all loose material. (This type of surface is
known as ‘rough as cast'.)
The type of surface should be assessed from record drawings, available site data or
original design calculations. In the absence of such information, surface Type 2 should
be assumed.
For inverted T-beams defined in Clause 7.4.2.2B(b) no longitudinal shear strength
check is required.
As an alternative to the use of conventional elastic analysis for checking longitudinal
shear, the following approach may be used provided the effective area of steel
crossing the interface exceeds 0.15 times 460/fs % of the concrete area:
The force required according to the ultimate flexural analysis in the part of the critical
section in flexure which is outside the interface should be calculated. This force
should be not greater than the total interface shear strength available over the length
of beam between the critical section in flexure and the point of contraflexure under
the relevant load case.
This approach is more consistent with the flexural analysis and gives higher strengths
when the interface is locally inadequate to the conventional check.
7.4.3B Serviceability Limit State
When a serviceability limit state assessment is required, the method of checking
should generally be as given in BS 5400: Part 4. However, where flexural tensile
stresses in the in situ concrete exceed the permitted values, stresses in the precast
concrete may be calculated using section properties determined ignoring the in situ
concrete that is in tension.
7.5B Plain Concrete Walls and Abutments
7.5.1B General
A plain concrete wall or abutment is a vertical load bearing concrete member whose
greatest lateral dimension is more than four times its least lateral dimension and
which is assumed to be without reinforcement when considering strength.
The requirements given in Clauses 7.5.2B to 7.5.10B refer to the assessment of a plain
concrete wall that has a height not exceeding five times its average thickness.
7.5.2B Moments and Forces in Walls and Abutments
Moments, shear forces and axial forces in a wall should be determined in accordance
with Clause 4.4B.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 93 of 100

The axial force may be calculated on the assumption that the beams and slabs
transmitting forces into it are simply supported.
The resultant axial force in a member may act eccentrically due to vertical loads not
being applied at the centre of the member or due to the action of horizontal forces.
Such eccentricities should be treated as indicated in Clauses 7.5.3B and 7.5.4B.
The minimum moment in a direction at right-angles to the wall should be taken as not
less than that produced by considering the ultimate axial load per unit length acting at
an eccentricity of 0.05 times the thickness of the wall.
7.5.3B Eccentricity in the Plane of the Wall or Abutment
For a single member, the eccentricity may be calculated from statics alone. Where a
horizontal force is resisted by several members, the amount allocated to each
member should be in proportion to its relative stiffness provided the resultant
eccentricity in any individual member is not greater than one-third of the length of
the member. Where a shear connection between vertical edges of adjacent members
can withstand the calculated forces, an applicable elastic analysis may be used.
7.5.4B Eccentricity at Right-angles to Walls or Abutments
The load transmitted to a wall by a concrete deck may be assumed to act at one-third
the depth of the bearing area from the loaded face. Where insitu concrete decks
span onto either side of the member, the common bearing area may be assumed to
be shared equally by each deck.
The resultant eccentricity of the total load on a member unrestrained in position at
any level should be calculated making full allowance for the eccentricity of all vertical
loads and the overturning moments produced by any lateral forces above that level.
The resultant eccentricity of the total load on a member restrained in position at any
level may be calculated assuming that immediately above a lateral support the
resultant eccentricity of all the vertical loads above that level is zero.
7.5.5B Analysis of Section
Loads of a purely local nature, for example, bearings or column bases, may be
assumed to be immediately dispersed, provided the local stress under the load does
not exceed that given in Clause 7.5.7B. Where the resultant of all the axial loads acts
eccentrically in the plane of the member, the ultimate axial load per unit length of
wall, nw , should be assessed on the basis of an elastic analysis, assuming a linear
distribution of load along the length of the member, and a tensile resistance of
concrete of:
0.12

f cu
γmc

Expression B66

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 94 of 100

Consideration should first be given to the axial force and bending in the plane of the
wall to determine the distribution of tension and compression along the wall. The
bending moment at right-angles to the wall should be considered and the section
assessed for this moment and the compression or tension per unit length at various
positions along the wall. Where the eccentricity of load in the plane of the member is
zero, a uniform distribution of nw may be assumed.
For members restrained in position, the axial load per unit length of member, nw , due
to ultimate loads should be such that:
nw ≤ (0.675f cu γmcw )(h − 2e x )

Equation B67

where:
nw
h
ex
f cu
γmcw

is the maximum axial load per unit length of member due to ultimate loads;
is the overall thickness of the section;
is the resultant eccentricity of load at right-angles to the plane of the member,
see Clause 7.5.2B, (minimum value 0.50h);
is the characteristic, or worst credible, concrete strength;
is a material partial factor defined in Clause 43.3.3B.

7.5.6B Shear
The resistance to shear forces in the plane of the member may be assumed to be
adequate, provided the horizontal shear force due to ultimate loads is less than either
one-quarter of the vertical load, or the force to produce an average shear stress of
0.45 N/mm² over the whole cross-section of the member where f cu is at least
25 N/mm². Where f cu is less than 25 N/mm², a figure of 0.3 N/mm² should be used.
7.5.7B Bearing
Bearing stresses due to ultimate loads of a purely local nature, as at girder bearings,
should be limited in accordance with Clause 7.2.3.3B.
7.6B Mass Concrete Arches
Mass concrete arches may be analysed according to the rules for masonry arches
given in Section 6. Where methods other than MEXE are used, the compressive
strength of the concrete should be taken as 0.6 f cu .

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 95 of 100

APPENDIX B1 : HISTORICAL CONCRETE GRADES
This Appendix summarises historical data relating to concrete strengths that has
been gathered during various assessment projects. The data are for advice and
guidance only.
It should be noted that depending on the particular Standard or Specification, the
strength of concrete is not always the same for a given classification or concrete
mix. Furthermore, depending on the particular Specification, as the class increases
from A to C, the concrete strength may increase or decrease. Caution should
therefore be exercised when using this data. It is always preferable to use the
actual Specification for the particular structure where it is identified and available.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000

The Engineer's Year Book, 1923, edited by H R Kempe and W Hanneford Smith
1923
Pg 443
28-day Cube Strength
Ref
Nominal Mix
Works Test
lb/in2
N/mm2
(lb.:c.ft.:c.ft.)
1:2:4
1800
12.4

Concrete Construction, Charles E Reynolds, 2nd Edition, 1945, Pg. 257 Table XIX
Reinforced Concrete Designers' Handbook, Chas E Reynolds, 3rd Edition, 1945, Table No. 23
1945
Ministry of Transport Ordinary Grade Concrete
28-day Cube Strength
Ref
Nominal Mix
Works Test
N/mm2
lb/in2
(lb.:c.ft.:c.ft.)
IV
1:2:4
2250
15.5
2
1
1 : 1 /3 : 3 / 3
2580
17.8
III
1 : 1½ : 3
2700
18.6
I
1:1:2
3600
24.8
LCC By-laws Ordinary Grade Concrete
28-day Cube Strength
Ref
Nominal Mix
Works Test
lb/in2
N/mm2
(lb.:c.ft.:c.ft.)
III

1:2:4
2
1
1 : 1 /3 : 3 / 3

2250
2450

15.5
16.9

II
I

1 : 1½ : 3
1:1:2

2550
2925

17.6
20.2

LCC By-laws High Grade Concrete
28-day Cube Strength
Ref
Nominal Mix
Works Test
lb/in2
N/mm2
(lb.:c.ft.:c.ft.)
IIIA

1:2:4
2
1
1 : 1 /3 : 3 / 3

2850
3155

19.7
21.8

IIA
IA

1 : 1½ : 3
1:1:2

3300
3750

22.8
25.9

Ministry of War Transport
Memorandum No. 577
1945 (Reprinted 1949)
Section 6
28-day Cube Strength
Ref
Nominal Mix
Works Test
lb/in2
N/mm2
(lb.:c.ft.:c.ft.)
A
150 : 2 : 4
3600
24.8
B
120 : 2 : 4
3300
22.8
C
90 : 2 : 4
2850
19.7

Approximate Equivalent
Nominal Mix by volume
3
assuming 90 lb/ft cement
1 : 1.2 : 2.4
1 : 1½ : 3
1:2:4

Mixes A,B,C Note:
correspond to mixes II, III, IV for High Grade
concrete in the Code of Practice for Reinforced Concrete.

Design and Construction of Reinforced Concrete Bridges, A W Legat, G Dunn, W A Fairhurst, 1948
Specification for a Typical Bridge Contract
1948
Pg. 467, Table A
28-day Cube Strength
Ref
Nominal Mix
Works Test
lb/in2
N/mm2
A
B
C
D

1:2:3
1 : 1½ : 3
1:1:2
1:3:5

3150
3300
3750
1800

21.7
22.8
25.9
12.4

First Report on Prestressed Concrete, Institution of Structural Engineers
1951
Clause 4.(f)(i)

Page 96 of 100

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 97 of 100

Design and Construction of Reinforced Concrete Bridges, A W Legat, G Dunn, W A Fairhurst, 1948
Specification for a Typical Bridge Contract
1948
Pg. 467, Table A
28-day Cube Strength
Ref
Nominal Mix
Works Test
2
2
lb/in
N/mm
21.7
A
1:2:3
3150
22.8
B
1 : 1½ : 3
3300
25.9
C
1:1:2
3750
12.4
D
1:3:5
1800

First Report on Prestressed Concrete, Institution of Structural Engineers
1951
Clause 4.(f)(i)
28-day Cube Strength
Works Test
2
2
lb/in
N/mm
Pre-tensioned steel
6000 min. 41.4 min.
generally

CP 114 - Reinforced Concrete in Buildings
1957
Table 1

Nominal Mix
1:1:2
1 : 1½ : 3
1:2:4

28-day Cube Strength
Works Test
2
2
lb/in
N/mm
31.0
4500
25.9
3750
20.7
3000

CP 115 - Prestressed Concrete in Buildings
1959
Clause 207

Pre-tensioned
Post-tensioned

28-day Cube Strength
Works Test
2
2
lb/in
N/mm
6000 min. 41.4 min.
4500 min. 31.0 min.

CP 114:Part 2
1969
Table 1
(with amendments up to 1973)
28-day Cube Strength
Nominal Mix
Works Test
2
2
lb/in
N/mm
1:1:2
30.0
1 : 1½ : 3
25.5
1:2:4
21.0

CP 115
1969
Clause 207
(with amendments up to 1977)
28-day Cube Strength
Works Test
2
2
lb/in
N/mm
Pre-tensioned
40.0 min.
Post-tensioned
30.0 min.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 98 of 100

British Rail Midland Region - Drawing Office Handbook Section 5.2
1960's
28-day Cube Strength
Ref
Nominal Mix
Works Test
2
lb/in2
N/mm
27.6
A
1 : 1½ : 3
4000
20.7
B
1:2:4
3000
10.3
C
1:3:6
1500
D
1:4:8
Not specified

British Rail Midland Region - Specification
1960's
28-day Cube Strength
Ref
Nominal Mix
Works Test
2
2
lb/in
N/mm
27.6
A
1 : 1½ : 3
4000
20.7
B
1:2:4
3000
10.3
C
1:3:6
1500
D
1:4:8
Not specified
Prestressed Concrete
A1

7000

48.3

Standard Beam Sections for Pretressed Concrete Bridges, Prestressed Concrete Development Group
(C&CA)
1961
28-day Cube Strength
Works Test
2
2
lb/in
N/mm
Pre-tensioned steel
6000 min. 41.4 min.
generally

Ministry of Transport
Memorandum No. 785
1961
Nominal Mix
Ref
(lb.:c.ft.:c.ft.)
A
B
C

150 : 2 : 4
120 : 2 : 4
90 : 2 : 4

[Source: Concrete Bridge Design, R E Rowe, 1962]

28 day Cube Strength
Works Test
2
lb/in2
N/mm
29.0
4200
25.9
3750
20.7
3000

Approximate Equivalent
Nominal Mix by volume
assuming 90 lb/ft 3 cement
1 : 1.2 : 2.4
1 : 1½ : 3
1:2:4

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures

RT/CE/C/025
Issue: 1
Date: February 2000
Page 99 of 100

Ministry of Transport
Specification for Road & Bridge Works 3rd Edition
1963
Table A & Table B
28-day Cube Strength
Class
Works Test
2
lb/in2
N/mm
29.0

4200
29.0
A 1½
4200
25.9

3750
25.9
B 1½
3750
20.7

3000
20.7
C 1½
3000
16.5
D 1½
2400
16.5
D3
2400
E 1½
Not specified
E3
Not specified

3
X /8

Y 3/ 8

Z 1½

7500
7500
6000
6000
4200
4200

CP 116 - Precast Concrete
1965

Grade
A
B
C*
D**
E

51.7
51.7
41.4
41.4
29.0
29.0

Table 1

28-day Cube Strength
Works Test
2
lb/in2
N/mm
20.7
3000
25.9
3750
31.0
4500
41.4
6000
51.7
7500
* Lowest grade for post-tensioned steel
** Lowest grade for pre-tensioned steel

CP 116
1969
Table 1
(with amendments up to 1977)
28-day Cube Strength
Grade
Works Test
lb/in2
N/mm2
A
21.0
B
25.5
C*
30.0
D**
40.0
E
50.0
(Clause 208)

General Specification for Concrete, British Rail Civil Engineering Handbook No. 21, 1965
[Source: Notes for Designers on the Use of the General Specification for Concrete,
BRB Civil Engineering Department, Technical Note No.3, February 1966]
1965
Standard Mixes
Table 1 & Appendix 2
28-day Cube Strength
Class
Works Test
2
lb/in2
N/mm
Notes:
3
20.7
SAH /8
3000
S = Standard mix
20.7
SAH ¾
3000
Y = Designed mix
20.7
SAH 1½
3000
A-E = Strength grade
25.9
SBH 3/8
3750
H = High workability
25.9
SBH ¾
3750
25.9
SBH 1½
3750
31.0
SCH ¾
4500
31.0
SCH 1½
4500

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix B – Assessment of Concrete Structures
Designed Mixes
Class
3

YA /8 - 1½
3

YB /8 - 1½

RT/CE/C/025
Issue: 1
Date: February 2000
Page 100 of 100

Table 2 & Appendix 2
28-day Cube Strength
Works Test
Application
2
lb/in2
N/mm
20.7
3000
Reinforced concrete
25.9
3750
Reinforced concrete

YC 3/8 - 1½

4500

31.0

Reinforced concrete

YD ¾ - 1½

6000

41.4

RC and infill concrete to precast beams

YE 3/8 - 1½

7500

51.7

YF ¾ - 1½

8000

55.2

RC, precast T beams, precast rectangular hog back
beams and jointing to transversely stressed beams
Precast rectangular beams

YAP

3000

20.7

Bored piles

YCP

4500

31.0

Bored piles (sulphate resisting)

British Rail Southern Region - Contract Specification for Norwood High St Bridge
1968
S = Standard mix
Y = Contractor designed mix
A-E = Concrete grades to CP116 (see above)
LWT = Lightweight concrete
RH = Rapid hardening

Technical Memorandum BE20
Prestressed Concrete for Highway Structures
1969
Prestressed concrete normally:
Class 7500
Class 6000
Prestressed concrete
exceptionally:
Class 9000

52.5 N/mm2
45 N/mm2
60 N/mm2

Interim Memorandum IM8 (superseded BE20 + IM3)
Prestressed Concrete for Highway Structures
1970
Prestressed concrete normally:
52.5 N/mm2
45 N/mm2

Technical Memorandum BE10 (superseded Memo 577/2)
Prestressed Concrete for Highway Structures
1970
Reinforced concrete normally:
30 N/mm2

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 12

CONTENTS
APPENDIX C
1C. SCOPE....................................................................................................................................2
4.3C Limit State Recommendations....................................................................................2
4.3.1C General ....................................................................................................................2
4.3.2C Serviceability Limit State ......................................................................................2
5C. ANALYSIS OF STRUCTURE - SERVICEABILITY LIMIT STATE...............................3
5.1.1C Distribution of Bending Moments and Vertical Shear Forces......................3
5.2.3C Effective Breadth of Concrete Flange ...............................................................3
5.2.6C Control of Cracking in Concrete ......................................................................4
5.3C Longitudinal Shear.........................................................................................................4
5.3.1C General ....................................................................................................................4
5.4C Temperature Effects and Shrinkage Modified by Creep .......................................6
5.4.1C General ....................................................................................................................6
6.1.2C Deck Slabs Forming the Flanges of Composite Beams..................................6
6.2C Analysis of Sections.......................................................................................................6
6.2.1C General ....................................................................................................................6
6.2.2C Bending Resistance of Compact Sections.........................................................7
6.2.3C Bending Resistance of Non Compact Sections ...............................................7
6.2.4C Analysis of Slender Cross Sections....................................................................7
6.3.4C Shear Connectors .................................................................................................7
7.3C Composite Box Girders Effective Breadth ..............................................................7
7.7C Composite Plate ............................................................................................................7
8.0C CASED BEAMS AND FILLER BEAM CONSTRUCTION ........................................8
8.2C Limit State Recommendations....................................................................................8
8.3.1C Transverse Moments in Filler Beam Deck .......................................................8
8.4C Analysis of Sections.......................................................................................................8
8.4.1C Serviceability Limit State ......................................................................................8
8.4.2C Ultimate Limit State ..............................................................................................8
8.5C Longitudinal Shear.........................................................................................................9
8.5.1C General ....................................................................................................................9
8.5.2C Cased Beams ..........................................................................................................9
8.5.3C Filler Beams...........................................................................................................10
8.6C Temperature and Shrinkage Effects.........................................................................11
14C. CONCRETE INFILLED TROUGH CONSTRUCTION .........................................11

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 12

1C. SCOPE
Delete the existing BS 5400: Part 5 Clause 1 and substitute the following:
This Appendix augments Section 5 and Section 7 when steel or wrought iron
components of bridges are interconnected with concrete bridge components, and act
compositely.
All references to structural steelwork should be read as also applicable to wrought
iron except where stated otherwise.
4.3C Limit State Recommendations
4.3.1C General
Delete the existing BS 5400: Part 5 Clause 4.3.1 and substitute the following:
4.3.1.1C Steelwork and Wrought Iron
All structural steelwork and wrought iron in composite beams should be assessed in
accordance with Section 5 and Appendix A in relation to the ultimate limit state and
in addition the serviceability limit state where required by Appendix A. The effects of
temperature and shrinkage modified by creep should be assessed in accordance
Clause 5.4C.
4.3.1.2C Concrete and Reinforcement
The concrete and reinforcement should be assessed to the ultimate limit state in
accordance with Section 7 and Appendix B.
Slabs, which are part of a composite beam, should also be assessed to the ultimate
limit state in accordance with Section 7 and Appendix B. The serviceability limit state
should be checked when coexistent stresses occur under local and global effects in
accordance with BS 5400: Part 5 Clause 5.2.4.1.
Where EUDL live loading is being considered, coexistent stresses are only likely to be
critical in the vicinity of a transverse member supporting the slab.
4.3.1.3C Shear Connection
The shear connection should be assessed for the ultimate limit state in accordance
with Clause 6.3.4C and additionally where relevant to Appendix D for fatigue.
Assessment for the serviceability limit state is only necessary where the spacing
between shear connectors exceeds the recommendations of BS 5400: Part 5 Clause
5.3.3.1.
4.3.2C Serviceability Limit State
Delete the existing BS 5400: Part 5 Clause 4.3.2 and substitute the following:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 12

A serviceability limit state is reached when any of the following conditions occur:
(a)

The stress in structural steel reaches the applicable limit given in Section 5.
For most underbridges this is not likely to govern, see Appendix A;

(b)

Where stresses in concrete or reinforcement reach the applicable limits given
in Section 7.
This will only be critical where local and global effects are added together in
accordance with BS 5400: Part 5 Clause 5.2.4.1;

(c)

Spacing of connectors exceeds the recommendations of BS 5400: Part 5
Clause 5.3.3.1, in which case a check on the serviceability recommendations
for the shear connection is required.

5C. ANALYSIS OF STRUCTURE - SERVICEABILITY LIMIT STATE
Recommendations given in the following Clauses are only required for serviceability
limit state checks as required by Clause 4.3.1C.
5.1.1C Distribution of Bending Moments and Vertical Shear Forces
Delete the existing BS 5400: Part 5 Clause 5.1.1.1 to 5.1.1.3 and substitute the
following:
Analysis should be carried out to the same assumptions used for the analysis for the
ultimate limit state in accordance with BS 5400: Part 5 Clause 6.1.4.
5.2.3C Effective Breadth of Concrete Flange
Delete the existing BS 5400: Part 5 Clauses 5.2.3.1 to 5.2.3.8 and Tables 2, 3, and 4
and replace by Clauses 5.2.3.1C to 5.2.3.3C:
5.2.3.1C General
In calculating the stresses in a flange, and in the absence of rigorous analysis, the effect
of in-plane shear flexibility (shear lag) should be allowed for by assuming an effective
breadth of flange in accordance with BS 5400: Part 3 Clause 8.2 or Clause 9.15.2.2 as
applicable.
5.2.3.2C Effective Breadth of Cracked Flange
For a concrete flange in tension that is assumed to be cracked the mean effective
breadth ratio ψ obtained in accordance with Clause 5.2.3.1C should be modified by
adding (1− ψ ) 3 .
5.2.3.3C Width Over Which Slab Reinforcement is Effective
Only reinforcement within the effective breadth of the concrete slab parallel to or
within 20o of parallel to the span of the steel beam should be assumed to be effective
in analysing cross sections.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 12

5.2.4.3C Co-existent Stresses
Delete the existing BS 5400: Part 5 Clause 5.2.4.3 and substitute the following:
In calculating combined stresses in a deck slab, which also forms the flange of a
composite beam, the global longitudinal bending stress across the deck width may be
calculated in accordance with BS 5400: Part 3 Appendix A Clause A.6.
5.2.5.1C Steel Section - General
Delete the existing BS 5400: Part 5 Clause 5.2.5.1 and substitute the following:
The serviceability limit state should be checked in accordance with Appendix A. In
carrying out serviceability limit state checks consideration should be given to the
effects detailed in BS 5400: Part 5 Clauses 5.2.5.2 to 5.2.5.3 and Clause 5.2.5.4C.
5.2.5.4C Slab Cast in Specified Sequence
Delete the existing BS 5400: Part 5 Clause 5.2.5.4 and substitute the following:
Where the deck slab is known to have been cast in a specified sequence, the dead
load stresses may be calculated on the composite section in accordance with BS 5400:
Part 5 Clause 12.1 using the effective breadth calculated from BS 5400: Part 3
Clause 8.2. Where the sequence of casting is unknown, the slab should be assumed
to have been cast in one operation.
5.2.6C Control of Cracking in Concrete
For assessment BS 5400: Part 5 Clauses 5.2.6.1 to 5.2.6.4 including Tables 5 and 6
may be ignored.
5.3C Longitudinal Shear
5.3.1C General
Delete the existing BS 5400: Part 5 Clause 5.3.1 and substitute the following:
Longitudinal shear per unit length q, of a composite beam, whether simply supported
or continuous, should be calculated on the basis of elastic theory using the properties
of the transformed composite cross section calculated assuming the concrete flange
to be uncracked and unreinforced. The effective breadth of the concrete flange may
be assumed to be constant over any span and may be taken as the quarterspan value
for uniformly distributed loading given in BS 5400: Part 3 Clause 8.2.
Where the second moment of area of the composite section varies significantly along
the length of any span, account should be taken of the variation of stiffness in
calculating the longitudinal shear flow.
Calculation of horizontal shear for the serviceability limit state is only required for
the assessment of fatigue or when the longitudinal spacing of connectors exceeds the
recommendations of BS 5400: Part 5 Clause 5.3.3.1.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 12

5.3.2.1C Nominal Strengths of Shear Connectors Embedded in Normal Density
Concrete
Add the following paragraph to that below BS 5400: Part 5 Table 7:
Nominal static strengths of bolt and rivet heads may be calculated from Equation C1.
Means of preventing separation of the concrete should be present such as by
encasement or other mechanical devices, unless tests are carried out to demonstrate
that adequate means of preventing separation are present.
Nominal Static Strength =

2.0 A1f cu
γmc γb

Equation C1

where:
γb

may be taken as 1.25 for bolts/bolt heads or other connectors with
predominantly vertical surfaces resisting the horizontal shear, and as 2.0 for
rivet heads or for other connections with predominantly inclined surfaces
resisting the horizontal shear;

A1

is the face area of the connector in the direction of horizontal shear.

5.3.3.4C Uplift on Shear Connectors
In BS 5400: Part 5 Clause 5.3.3.4(b), insert the following after “bracing”:
“or from the forces generated at the corners when the slab acts as part of a Uframe”.
Delete the remainder of BS 5400: Part 5 Clause 5.3.3.4 from “the effect of axial
tension .......” and insert “ where a stud connector is subject to both shear Q and
tension due to uplift Tu the equivalent shear Q max for checking the connector should
be taken as:
Q max =

Q2 +

2

Tu
3

Equation C2

5.3.3.5C Design Procedure: General
Delete the existing BS 5400: Part 5 Clause 5.3.3.5 and substitute the following:
Shear connectors need not be checked for the serviceability limit state except for
fatigue in accordance with Appendix D or where the longitudinal spacing exceeds the
recommendations of BS 5400: Part 5 Clause 5.3.3.1.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 12

5.4C Temperature Effects and Shrinkage Modified by Creep
5.4.1C General
Delete the existing BS 5400: Part 5 Clause 5.4.1 paragraph 1, lines 3 to 9 from
“serviceability limit state in composite beams ............” to “ the serviceability limit
state.”, and insert:
“for the beam section when serviceability checks are to be carried out in accordance
with Appendix A. In such checks account should be taken of the longitudinal shear
forces arising from these effects.”.
5.4.2.1C Temperature Effects to be considered
Delete the existing BS 5400: Part 5 Clause 5.4.2.1 paragraph 2, lines 7 and 8 from
“and assuming the concrete slab to be of effective breadth as given in Table 8” in and
insert:
“No account need be taken of shear lag. Concrete may be assumed to be uncracked,
except that, for calculating longitudinal bending stresses due to the secondary effects
in (c) above, the concrete in tension may be ignored”.
Delete Table 8.
6.1.2C Deck Slabs Forming the Flanges of Composite Beams
Delete the existing BS 5400: Part 5 Clause 6.1.2 second and third paragraphs and
replace with:
“Local and global effects should be considered separately and should not be combined
for the ultimate limit state except for a deck slab spanning longitudinally between a
series of cross beams and which is composite with the longitudinal members and
where the dispersed width of the local loading is greater than 75% of the effective
width of the slab forming part of the composite longitudinal members.”
6.2C Analysis of Sections
6.2.1C General
Delete the existing BS 5400: Part 5 Clause 6.2.1 and substitute the following:
The strength of composite sections should be assessed using a plastic or an elastic
modulus as specified in Appendix A for calculating the bending resistance of the beam.
If a plastic modulus is used, the transformed width of the concrete flange should be
obtained from:
 0.4f cu
 σ yc γm

( width of concrete flange )
where:






Expression C3

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures
f cu
σ yc
γm

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 12

is the characteristic concrete cube strength (refer to Section 7);
is the nominal yield strength of the compression flange of the steel member
(refer to Section 5);
is the partial factor for material strength for the steel member (refer to
Section 5).

If an elastic modulus is used, the stresses in the concrete and reinforcement should
not exceed the limits given in Section 7 and Appendix B.
6.2.2C Bending Resistance of Compact Sections
For assessment BS 5400: Part 5 Clause 6.2.2 may be ignored.
6.2.3C Bending Resistance of Non Compact Sections
For assessment BS 5400: Part 5 Clause 6.2.3 may be ignored.
6.2.4C Analysis of Slender Cross Sections
For assessment BS 5400: Part 5 Clause 6.2.4 may be ignored.
6.3.3.3C Interaction between Longitudinal Shear & Transverse Bending
In BS 5400: Part 5 Clause 6.3.3.3(b), paragraph 2, line 6 - delete “6.3.2(a)”, insert
“6.3.3.2(a)”.
6.3.4C Shear Connectors
Delete the existing BS 5400: Part 5 Clause 6.3.4 and substitute the following:
Assessment of shear connectors should be considered at the ultimate limit state in
accordance with BS 5400: Part 5 Clause 6.3.1. The static strength per connector
0.8Pu
should be taken as
where Pu is the nominal strength (as defined in
γm
Clause 5.3.2.1C). γm should be taken as 1.10.
7.3C Composite Box Girders Effective Breadth
In BS 5400: Part 5 Clause 7.3 delete references to “Clauses 5 and 6 as appropriate”,
and insert “BS 5400: Part 3 Clause 8.2.”.
7.7C Composite Plate
Add to the end of the existing BS 5400: Part 5 Clause 7.7 the following new
paragraph:
The longitudinal shear forces due to local loading in the regions of a composite plate
supported by cross members may be determined by considering the plate as an
equivalent simply supported beam spanning between these cross frames; the width of
the equivalent beam, b, supporting the load should be taken as:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures
b=

RT/CE/C/025
Issue: 1
Date: February 2001

4x
+u
3

Page 8 of 12
Equation C4

where:
x
u

is the distance from centroid of the loaded area to the nearest cross frame;
is the length of the loaded area, which is parallel to cross frame.

8.0C CASED BEAMS AND FILLER BEAM CONSTRUCTION
8.2C Limit State Recommendations
Delete the existing BS 5400: Part 5 Clause 8.2 and substitute the following:
Except where specific recommendations are given in BS 5400: Part 5 Clause 8, cased
beams and filler beam decks should be assessed for the ultimate limit state only.
8.3.1C Transverse Moments in Filler Beam Deck
Delete the existing BS 5400: Part 5 Clause 8.3.1 and substitute the following:
Transverse moments should be calculated using the load distribution as follows:
(a)

the decks spans simply supported between filler beams taking a static
distribution of loads between the filler beams including for dispersal of railway
live loading through track and ballast in accordance with Section 4.

(b)

Alternatively, or where the assessment does not meet the recommendations
on the basis of static distribution in (a) above, an elastic distribution analysis
should be carried out of the filler beam construction assuming that the
concrete is uncracked and unreinforced with the filler beams assumed
composite. Where filler beam decks occur in half through bridges Appendix
A Clauses 9.6.5A, 9.12.2A and BS 5400: Part 3 Clauses 9.6.6 and 9.12.3
should be used to calculate U-frame forces.

For filler beam decks less than 300 mm in depth where there is no encasement above
the top flange nor below the bottom flange (as in former type ‘A’ decks) only static
distribution as in (a) should be assumed. No composite behaviour should be taken,
but the steel beam may be assumed to be compact.
8.4C Analysis of Sections
8.4.1C Serviceability Limit State
For assessment BS 5400: Part 5 Clause 8.4.1 may be ignored.
8.4.2C Ultimate Limit State
Delete the existing BS 5400: Part 5 Clause 8.4.2 and substitute the following:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 9 of 12

The moments of resistance of cased and filler beams should be assessed at the
ultimate limit state in accordance with Clause 6.2C. The effects of shear lag in filler
beam decks may be neglected. The steel beam may be considered as compact.
Vertical shear should be assumed to be resisted by the steel section, and in addition
by the concrete if reinforced by vertical stirrups and assessed in accordance with
Section 7. Alternatively, the vertical shear may be assumed to be resisted by the steel
section with the addition of the shear resistance of the section of reinforced concrete
between adjacent filler beam flange outstands, provided the proportions of shear can
be resisted at the end connections.
8.5C Longitudinal Shear
8.5.1C General
Delete the existing BS 5400: Part 5 Clause 8.5.1 and substitute the following:
The longitudinal shear force per unit length between the concrete and steel beam
should be calculated by elastic theory in accordance with Clause 5.3.1C, except that
in positive (sagging) moment region of cased beams and in filler beams, concrete in
tension should be neglected. Shear lag effects may be neglected in filler beam decks.
The shear force to be transferred should be that applicable to the area of concrete
and steel reinforcement in compression.
8.5.2C Cased Beams
Delete the existing BS 5400: Part 5 Clause 8.5.2 and substitute the following:
The longitudinal shear resistance should be taken as the lesser of Expressions C6 to
C8 below:
f b Lb

Expression C6

k1sL s + 0.7 Ae f ry

Expression C7

k 2 L s f cu

Expression C8

where:
is the local bond stress taken as 0.7 N/mm²;
fb
Lb
is the bond perimeter;
k1 , k 2 , s , L s , Ae , f ry and f cu are defined in BS 5400: Part 5 Clause 6.3.3.2;
Ls
is taken as the length of plane 5-5 in BS 5400: Part 5 Figure 6(d);
is given by:
Lb
Fully encased
2d w + btf bbf ;
Soffit exposed
2d w + btf ;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 10 of 12

where:
dw
btf
bbf

is the web depth;
is the top flange width;
is the bottom flange width.

Alternatively, the steel section, assumed compact, may be assumed to carry the entire
loading.
8.5.3C Filler Beams
Delete the existing BS 5400: Part 5 Clause 8.5.3 and substitute the following:
The longitudinal shear resistance should be taken as the lesser Expressions C9 to C12
below:
f b Lb +1.4 Aw f ry
f b Lb +

2t w d b σ y
sb

Expression C9
Expression C10

k1sL s + 0.7 Ae f ry

Expression C11

k 2 L s f cu

Expression C12

where:
fb
is the local bond stress taken as 1.0 N/mm²;
Lb
is as defined in Clause 8.5.2C;
k1 , k 2 , s , L s , f ry and f cu are defined in BS 5400: Part 5 Clause 6.3.3.2;
Aw
is the area of any reinforcement passing through the beam web;
tw
is the web thickness of the beam;
db
is the diameter of any reinforcing bars passing through the beam;
sb
is the spacing of reinforcement bars passing through the beam;
σy
is the yield strength of the beam web;
Ls
is taken as the total length of planes 6-6 in Figure 6(e) of this Appendix;
Ae =

At + Ab + Aw

Equation C13

where At and Ab are defined in BS 5400: Part 5 Clause 6.3.3.1.
Alternatively, the steel section, assumed compact, may be assumed to carry the entire
loading.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 12

8.6C Temperature and Shrinkage Effects
Delete the existing BS 5400: Part 5 Clause 8.6 and substitute the following:
Temperature and shrinkage effects need not be considered in filler beam
construction.

6

6

6

Lb

tw

6

6

At

Aw

6

Lb

At

Aw
sb

6

tw

6

Figure 6 (e)
Shear Plane and Transverse Reinforcement for Filler Beams
14C. CONCRETE INFILLED TROUGH CONSTRUCTION
Add the following Clause to BS 5400: Part 5:
Dispersal of live loading to troughing should be allowed in accordance with Section 4.
Troughing may generally be assumed to satisfy the compact section criteria in
accordance with Appendix A, allowing use of the plastic modulus. The elastic
modulus should be used for U-frame calculations where transverse troughing forms
cross girders.
Composite action with concrete infill may be assumed where:
(i)

Troughs are filled up to at least 75 mm above crests, as shown in Figure C10,
with concrete known to be dense and without significant evidence of slip or
separation; and

(ii)

Rivets occur along at least every alternate crest.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix C – Assessment of Composite Structures

RT/CE/C/025
Issue: 1
Date: February 2001
Page 12 of 12

The strength or rigidity determined should not exceed the calculated strength or
rigidity of the troughs by more than 30% unless justified by applicable testing.

75mm MINIMUM.

Figure C10
Trough Construction

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 1 of 12

CONTENTS
APPENDIX D
1D SCOPE.....................................................................................................................................1
1.1D General............................................................................................................................1
3D. DEFINITIONS AND SYMBOLS .......................................................................................2
3.1.17D Element...................................................................................................................2
3.1.18D Element Identification..........................................................................................2
3.2D Symbols ...........................................................................................................................2
4D GENERAL GUIDANCE ......................................................................................................2
4.1D Residual Life....................................................................................................................2
4.4D Methods of Assessment...............................................................................................3
Stage A Fatigue Assessment - Identify Fatigue Criticality by Inspection and CutOff Stress ................................................................................................................3
Stage B Fatigue Assessment - Damage Calculation to Standard Spectrum...........5
Stage C Fatigue Assessment – Damage Calculation to Particular Spectrum........5
Stage D Fatigue Assessment – Assessment using Measured Strains.........................6
5.4.D Steel Decks....................................................................................................................6
5.5.D Classification for Wrought Iron Elements ..............................................................6
6.1.4.D Calculation of Stresses ........................................................................................6
9.1.3D Load Factors ...............................................................................................................6
9.2.D Stage B Assessment - Damage Calculation to Standard Spectrum ................6
9.2.1D General....................................................................................................................6
9.2.2.D Procedure ..............................................................................................................7
9.3.D Stage C Assessment – Assessment with Damage Calculation .........................10
9.3.1D General..................................................................................................................10
9.3.5D Calculation of Residual Life ...............................................................................11
11.3.D Treatment of Low Stress Cycles ..........................................................................12
11.6.D Design σr – N relationship for wrought iron.....................................................12
1D SCOPE
1.1D General
Add to the end of the existing BS 5400: Part 10 Clause 1.1 the following:
The provisions and procedures in BS 5400: Part 10: 1980 (incorporating amendment
no 1 as issue 2, March 1999) should be followed, subject to the modifications given
below to clauses within BS 5400: Part 10 and which relate to the clause numbers of
that code.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 2 of 12

3D. DEFINITIONS AND SYMBOLS
Add the following clauses to BS 5400: Part 10 Clause 3:
3.1.17D Element
A member, part detail or connection of the structure of the bridge which is subject to
repeated fluctuations of stress under railway traffic.
3.1.18D Element Identification
A Damage Tolerant Element is an element where there is a reliable alternative load
path of adequate strength within that element, other elements or construction
including the track and its support.
All other elements shall be considered to be Safe Life Elements.
3.2D Symbols
Add the following symbols to BS 5400: Part 10 Clause 3.2:
Ya

age of the element in years at the time of assessment;

YR

residual life in years from the time of assessment which will be the remaining
period in which the element may be predicted to perform safely, with an
acceptable probability that it will not require repair, or fail due to propagation
of fatigue cracks;

γ1

factor on stress range taking account of method of analysis;

γ2

factor on stress range taking account of damage criticality related to
inspection access.

4D GENERAL GUIDANCE
4.1D Residual Life
Delete the existing BS 5400: Part 10 Clause 4.1 and substitute the following:
The acceptable reliability may be related to the capability of the bridge to carry
railway traffic in the event of failure of an element depending on whether the element
is a damage tolerant or a safe life element. The required reliability may be deemed to
be achieved by applying factors γ1 and γ 2 to the relevant stress ranges.
Allowance should be made for fatigue damage that has occurred since construction of
the bridge to the element considered and for differences between traffic spectra in
the past, present and future whenever this is practicable.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 3 of 12

4.4D Methods of Assessment
Delete the existing BS 5400: Part 10 Clause 4.4 and substitute the following:
Assessment of steel and wrought iron underbridges for the purposes of determining
fatigue endurance should be carried out in stages from (A) through to (D) as relevant
for the various elements of the structure which contribute to vertical live load
capacity. Where at any of these stages the stated criteria can be satisfied for a
particular element, fatigue endurance may be deemed as satisfactory for that element
subject to the limitations stated. Subsequent stages representing more rigorous
approaches need not be evaluated for that element. Stage D involving the use of
measured strains from site investigation should not be undertaken without due
consideration of economic factors including the costs of investigation compared with
that of remedial works.
Stage A Fatigue Assessment - Identify Fatigue Criticality by Inspection and CutOff Stress
(i)
In accordance with Section 3, Clause 3.4.2 carry out inspection of the
structure to identify elements which may be particularly susceptible to fatigue.
Note the presence such as critical features such as visible cracks, welded
repairs and notched railbearer ends. Where visible cracks have been detected
measures should be undertaken to identify whether there are fatigue related
or not. If so, then further investigations should be carried out even if they
occur in elements which have otherwise been determined as non-fatigue
critical. As a minimum requirement, the cracks should be monitored whilst
detailed assessment is made or repair/strengthening is carried out.
Consideration may be given to calculations of crack growth based upon
fracture mechanics principles. Such calculations should normally be
undertaken or supervised by specialists in this field. It should be noted that
calculations as carried out in stages A, B and C may not be valid when
significant fatigue cracks exist.
(ii)

Identify Damage Tolerant Elements as defined by 3.1.18D. They may normally
include:
Main girders and railbearers with effective span of 3 metres or less;
Stiffened or unstiffened floor plates spanning less than 3 metres;
Cross girders or trimmers supporting a single track;
Main girders of deck type bridges interconnected such that in the event of
failure of one, the remainder can sustain the required loading at the ultimate
limit state with γfL for railway live loading multiplied by 0.85, but with
γfL '1.10;

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 4 of 12

Multiple flange plates (but excluding any flange angles) which are
interconnected by rivets or bolts and not by welding.
(iii)

For all elements which contribute to vertical live load capacity, carry out the
following procedure:
(a) Apply RA railway live loading with the number of units equivalent to that
of the route (which may be in excess of the assessed rating of the bridge)
to one or two tracks so as to produce the greatest algebraic maximum
and minimum values of stress at the critical locations within the element.
Include for dynamic increment evaluated in accordance with Section 4
Table 4.5 for fatigue for a speed applicable to the applied live loading (NB:
this may be less than the line speed);
(b) Determine the maximum and minimum values of principal stress, or
vector sum stress for weld throat, σ P max and σ P min , occurring at the
critical locations within the element being assessed;
(c) Determine the maximum range of stress σ R max equal to the numerical
value of σ P max minus σ P min . For non-welded elements the stress range
should be modified in accordance with BS 5400: Part 10 Clause 6.1.3;
(d) Multiply the stress range σ R max by factors γ1 and γ 2 to give the factored
range of stress σ f max where;
σ f max =

σ R max γ1γ 2

Equation D1

where γ1 is obtained from Table D1 and γ 2 from Table D2.
Assessment Stage

Method of Analysis

A
B, C
D (measured strain)

Static or Load
3D Finite
Distribution Analysis
Element
1.00
NA
1.00
0.95
0.80
Table D1
Values of γ1

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron
Inspection
Access
Accessible
Inaccessible

Damage Tolerant Element
0.80
0.86

Page 5 of 12

Safe Life Element
0.86
1.00

Table D2
Values of γ 2
Plates which are covered on both sides, for example the areas of webs of
girders covered by flange angles and the inner plates of multiple flanges
should be considered as inaccessible, unless non-destructive testing or
other investigations are undertaken to detect any concealed cracks or
other fatigue defects. Single flange plates where are accessible on at least
one side can be assumed to be accessible.
(e) Compare the value of σ f max with the cut-off stress value σ c 0 given by
Clause 11.3D in Table D8, or Table D8 as appropriate. Where σ f max
does not exceed σ c 0 at the critical locations, the element may be
considered to be non-fatigue critical up to the loading assumed. Where
σ f max exceeds σ 0 a Stage B fatigue assessment should be undertaken;
Stage B Fatigue Assessment - Damage Calculation to Standard Spectrum
This simplified method should be used for all elements which fail Stage A and where a
numerical assessment of residual fatigue life YR is required. Stage B assessment
should be carried out in accordance with Clause 9.2D. Where YR is evaluated as less
than required or the age of the element at the time of assessment, Stage C fatigue
assessment should be carried out.
Stage C Fatigue Assessment – Damage Calculation to Particular Spectrum
This method should be used for all elements which fail Stage B and where a numerical
assessment of residual fatigue life YR is required. Stage C assessment should be
carried out in accordance with Clause 9.3D. The application of the method in any
particular case should be agreed with the Railtrack Director’s Nominee. Where the
residual fatigue life is less than required for the age of the element at the time of
assessment, Stage D may be undertaken where this can be justified as an alternative
to immediate acceptance that the bridge has inadequate fatigue resistance and
remedial work should be undertaken in the near future.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 6 of 12

Stage D Fatigue Assessment – Assessment using Measured Strains
In particular cases where the consequences of accepting that the bridge has
inadequate fatigue resistance based on a Stage C assessment, a better assessment may
be possible using measured strains. Such assessments require specialist expertise,
usually require calibration against theoretical analysis, and can be costly. Any Stage D
assessment should be agreed by the Railtrack Director’s Nominee.
5.4.D Steel Decks
Add to the existing BS 5400: Part 10 Clause 5.4 the following:
Classifications to be used in fatigue assessment of stiffened steel floors and
orthotropic steel decks are not included in this Appendix. In these cases the basis
and method of assessment should be agreed with the Railtrack Director’s Nominee.
5.5.D Classification for Wrought Iron Elements
Add the following additional Clause to BS 5400: Part 10:
For wrought iron elements the σr - N relationships given by Clause 11.6D may be
used in the absence of fatigue data or testing relating to the material and particular
details. Classifications for steel elements should not be used for wrought iron.
6.1.4.D Calculation of Stresses
Add to end of the existing BS 5400: Part 10 Clause 6.1.4.1 the following:
Due account should be taken of any corrosion losses identified in the calculation of
overall stress and local stresses at a section. The stress ranges calculated should be
multiplied by factors γ1 and γ 2 taken from Tables D1 and D2.
9.1.3D Load Factors
Add the following additional clause to BS 5400: Part 10:
The load factors γfL and γf 3 should both be taken as equal to 1.0.
9.2.D Stage B Assessment - Damage Calculation to Standard Spectrum
Delete the existing BS 5400: Part 10 Clause 9.2 and substitute the following Clauses
9.2.1D and 9.2.2D.
9.2.1D General
This method determines the residual life and should only be used where the following
conditions are satisfied:
(a)

the detail class is in accordance with BS 5400: Part 10 Table 17 or as specified
in this Appendix;

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 7 of 12

(b)

the assumed load spectrum complies with the standard load spectra for heavy,
medium or light traffic given in BS 5400: Part 10 Table 2 based on the typical
trains described in BS 5400: Part 10 Appendix E for RU loading;

(c)

on the basis of knowledge of the service history of the bridge, the assumed
loading spectrum can be assumed to have been applied throughout the
previous life of the bridge.

9.2.2.D Procedure
9.2.2.1.D
(a)
Apply 20 units of RA loading to the first track so as to produce the algebraic
maximum and minimum values of stress at the critical locations within the
element. Include for dynamic increment evaluated in accordance with Section
4 Table 4.5 for fatigue for a speed applicable at the bridge location to the
heaviest live loading normally permitted (NB: this may be less than the line
speed).
(b)

Determine the maximum range of stress σ P1 for the first track equal to the
numerical value of σ P max minus σ P min . For non-welded elements, the stress
range should be modified as given in BS 5400: Part 10 Clause 6.1.3;

(c)

Determine the maximum range of stress range σ P 2 for the second track
(where the element is subject to live load stresses for more than one track) as
described for the first track in (a) and (b).

(d)

Multiply the maximum range of stress σ P1 for the first track by factor K1 to
take account of the second track and by factors γ1 and γ2 to give the factored
range of stress σ f max where
σ f max = σ P1K1γ1γ 2

Equation D1

where:
K1
γ1
γ2

is obtained from Table D3;
is obtained from Table D1;
is obtained from Table D2.

Note: A more accurate value for K1 can be obtained using equation D2.
K1 =


 T  σ p2
(1− P )1+ 2 
 T1  σ p1








m

  σ p2
 + P 1+
  σ p1


m

 
 

 

−m

Equation D2

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

Page 8 of 12

where:
P

is the proportion of cycles of loading on the first track which are
coincident with loading of the second track;
is the annual tonnage on the first track;
is the annual tonnage on the second track;
is obtained from BS 5400: Part 10 Table 8 or from Table D8.

T1
T2
m

σp2
σ p1
+1.0
+0.9
+0.8
+0.7
+0.6
+0.5
+0.4
+0.3
+0.2
+0.1
0
-0.5
-0.8
-1.0

D, E, F, F2, G
and W
(m=3)
1.38
1.31
1.25
1.19
1.15
1.11
1.07
1.05
1.03
1.01
1.00
1.00
1.11
1.22

DETAIL CLASS
B, C and
wrought iron
(m=3.5,4)
1.36
1.29
1.24
1.18
1.14
1.10
1.07
1.04
1.03
1.01
1.00
1.00
1.08
1.18

S
(m=8)
1.51
1.44
1.37
1.30
1.23
1.17
1.11
1.07
1.04
1.01
1.00
1.00
1.01
1.08

TABLE D3
Values of K1 - Second Track Loading
Where σ p 2 is of opposite sign to σ p1 then

σp 2
should be taken as of minus
σ p1

sign.
Values of K1 in Table D3 assume P is equal to 0.10 and that T1 is equal to T2 .
(e)

Determine the fatigue spectrum using BS 5400: Part 10, Table 2. The load
proportions, k wa , should be obtained from Equation D3.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron

K wa =

K w K 2 K 3 but ≤ 1.0

Page 9 of 12

Equation D3

where:
K wa

is the proportion of 20 units of standard BSU loading which occurs for
a total of n R ×10 6 cycles in a period of 120 years;
Kw
is the proportion of RU loading as defined by BS 5400: Part 10
Clause 7.3.3 for RU loading;
K 2 , K 3 are obtained from Table D4;
nR
is as defined by BS 5400: Part 10 Clause 3.2;
Derive K wa , σ fmax for each of the group numbers corresponding to the values
of K w shown in BS 5400: Part 10 Table 2.
BENDING
K3
Heavy Traffic
Medium or
Light Traffic

length
L (m)

2
3
4
5
7
10
15
20
30
m50

SHEAR
K3
Heavy Traffic

Medium or
Light Traffic

K2

Longitudinal
members

Transverse
Members

Longitudinal
Members

Transverse
Members

K2

Longitudinal
Members

Transverse
Members

Longitudinal
Members

Transverse
Members

1.01
1.09
1.18
1.21
1.23
1.29
1.34
1.32
1.29
1.22

1.57
1.57
1.52
1.41
1.29
1.20
1.14
1.08
1.03
0.94

1.50
1.39
1.25
1.13
1.02
-

1.56
1.56
1.51
1.40
1.27
1.19
1.12
1.06
1.01
0.93

1.42
1.32
1.19
1.07
0.97
-

1.12
1.07
1.17
1.20
1.24
1.25
1.23
1.23
1.15
1.13

1.42
1.42
1.42
1.30
1.21
1.14
1.09
1.06
1.02
0.96

1.26
1.19
1.09
1.02
0.94
-

1.40
1.40
1.36
1.29
1.19
1.14
1.08
1.05
1.00
0.95

1.19
1.13
1.04
0.96
0.89
-

TABLE 4D
Values of K2 and K3
Note: The values of K2 are a proportion of static RU loading to 20 units of static RA
loading.
(f)

For each value of K wa , σ f max calculate n N using BS 5400: Part 10 Clause 11.3.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron Page 10 of 12
(g)

Determine the value of ∑ n N using BS 5400: Part 10 Clause 11.

9.2.2.2.D
Derive the residual life of the element which may be calculated as:
120  27 x10 6 
Equation D3

 −Y
n T  a

N
where T is the annual tonnage on each track taken as the greater of T1 or T2
YR =

9.3.D Stage C Assessment – Assessment with Damage Calculation
Delete the existing BS 5400: Part 10 Clauses 9.3.1, 9.3.2 and 9.3.3 and substitute the
following:
9.3.1D General
Stage C assessment involves the calculation of Miner’s Summation for the stress
histories of the various trains which are considered to use the bridge, and may be
used for any element for which the σ r − N relationship is known and for any known
load or stress spectra. Trains which are to be considered should be representative of
trains which have used the bridge or particular element as appropriate since it was
built. The traffic spectrum in any particular case should be agreed with the Railtrack
Director’s Nominee, but should consist of either:
(i)

Typical train types assumed as Figure 19 of BS 5400: Part 10. The number of
trains per annum to be as given in Table 15 of BS 5400: Part 10 for heavy,
medium or light traffic adjusted pro rata to the total annual tonnage advised
by the Railtrack Director’s Nominee and the annual tonnage of 27 x 106
stated. Train speed for each train should be as follows:
Train Types 2, 3, 4, 5 & 6
The lesser of the line speed across the bridge or 125 mph.
Train Types, 1, 7, 8 & 9
The permitted speed across the bridge for freight trains.

(ii)

Typical “real train” types with numbers of trains per annum of each type and
train speeds as advised by the Railtrack Director’s Nominee.

9.3.2D Design Spectrum
For all elements which contribute to vertical live load carrying capacity, carry out the
following procedure:

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron Page 11 of 12

(a)

Apply train loading from each train at the appropriate speed to the first track.
Include for dynamic increment evaluated in accordance with Section 4 for
fatigue for a speed applicable to each train. Each train should be traversed
across the relevant point load influence lines for the critical locations within
the elements being assessed.

(b)

Determine the stress history for each train to derive values of σ R , where σ R
is defined in BS 5400: Part 10 Clause 3.2.

(c)

For elements which are subject to live load stresses from more than one track
apply train loading to the second track as described for the first track in (a)
and (b). Account should be taken of the possibility of stress fluctuations
arising from the passage of trains on not more than two tracks, both
separately and in combination. When considering stresses in combination
from two tracks it may be assumed that up to 10% (i.e. P=0.1 as assumed in
Table D3) of cycles of loading on the first track are coincident with loading of
the second track, or unless otherwise advised by the Railtrack Director’s
Nominee. As an approximation the effects of two track loading may be
obtained by multiplying the stress histories by factor K1 as defined in
Clause 9.2.2.1D.

(d)

Multiply the values of σ R by factors γ1 and γ 2 to give the factored stress
history giving values of σ f max , where γ1 is obtained from Table D1 and γ 2
from Table D2.

(e)

Analyse the factored stress history for each train by the rainflow method (see
BS 5400: Part 10 Appendix F, example 4) to derive the respective stress
spectra. These should then be combined with the appropriate total
occurrences per annum to compile the overall spectrum. For non-welded
elements the stress range should be modified in accordance with BS 5400:
Part 10 Clause 6.1.3.

9.3.5D Calculation of Residual Life
Delete the existing BS 5400: Part 10 Clause 9.3.5 and substitute the following:
Using the design spectrum obtained in Clause 9.3.1D the residual life of the element
should be calculated using Clause 11 of BS 5400: Part 10 but taking n1 , n 2 .......... ..n n as
the numbers of repetitions per annum of the various factored stresses and taking
account of Clauses 11.3D and 11.6D. The residual life should then be calculated as:

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix D - Fatigue Assessment of Steel and Wrought Iron Page 12 of 12


1
YR = 
 n n
n
  1 + 2 + ..... n
Nn
  N1 N 2



 −Y
a




Equation D4

11.3.D Treatment of Low Stress Cycles
Add to the existing BS 5400: Part 10 Clause 11.3 the following:
Values of each stress range σ f max under Clause 4.4D, Clause 9.2D or Clause 9.3D
which are calculated as not exceeding the cut-off stress σ c 0 corresponding to N=108
may be ignored. Table D8 shows values of σ c 0 for steel and Table D9 shows values
of σ c 0 for wrought iron.
DETAIL CLASS
W
G
F2
F
E
D
C
B
S

σ c 0 (N/mm²)
16
18
22
25
30
33
51
68
65

TABLE D8
Values of σco for steel - cut-off stress
11.6.D Design σr – N relationship for wrought iron
Add the following additional Clause to BS 5400: Part 10.
The σr – N relationship given in Table D9 should be used for wrought iron elements.
Detail Class
Wrought Iron
plain section

m
4.0

K2
17.70 x 1013

σ 0 (N/mm²)
65

44

Wrought Iron
at locations of
rivets

4.0

3.73 x 1013

44

30

Table D9
Additional Data for Wrought Iron

)
σ c 0 (N/mm²

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 16

CONTENTS
APPENDIX E
1E. MODEL BRIDGE ASSESSMENT REPORT ......................................................................1
2E. TYPEFACE AND BINDING...............................................................................................1
3E. REPORT SEQUENCE AND FORMAT............................................................................1
3.1E Cover................................................................................................................................2
3.2E Distribution .....................................................................................................................2
3.3E Summary ..........................................................................................................................2
3.4E Contents ..........................................................................................................................2
3.5E Location Maps.................................................................................................................2
3.6E General Arrangement Drawing ..................................................................................2
3.7E Introduction ....................................................................................................................2
3.8E Condition Survey Summary .........................................................................................3
3.9E Results Obtained From Calculations .........................................................................5
3.10E Conclusions...................................................................................................................5
3.11E Recommendations .......................................................................................................5
3.12E Figures ............................................................................................................................5
3.13E Appendices ....................................................................................................................5
4E. CALCULATIONS.................................................................................................................6
5E. STATUTORY UNDERTAKERS’ SERVICES....................................................................6
6E. TEST RESULTS ......................................................................................................................6
APPENDIX E
1E. MODEL BRIDGE ASSESSMENT REPORT
This Appendix provides a recommended model for the contents and format of the
Bridge Assessment report. Use of this model by the assessing organisation should
ensure that all relevant information is recorded and summarised.
A Bridge Assessment report which uses this model will summarise the main Bridge
attributes and assessment findings in the first few pages. More detailed (and bulky)
information, including the calculations should be recorded as appendices.
2E. TYPEFACE AND BINDING
Whenever possible Gill Sans 12 point should be used throughout the report.
Reports should normally be bound to prevent the loss of individual sheets.
3E. REPORT SEQUENCE AND FORMAT
The following report format and contents are recommended:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 16

3.1E Cover
The cover should be one full page, and include, the title, a general photograph, the
date, unique structure number, consultants file reference and telephone number and
a confidentiality clause. An example cover layout is shown in Figure E2. Paper
bearing the name and/or logo of the assessing organisation may be used as the cover
for the report.
3.2E Distribution
A distribution list, on one full page, of recipients of the report and their addresses
should be included.
3.3E Summary
A short summary on one full page should be given by the assessing organisation
describing the assessment methods used and giving the main outcome of the Bridge
Assessment in terms of the safe load capacity for the Bridge.
The Structures Manager having reviewed the report may add an additional summary
sheet giving recommendations.
3.4E Contents
Headings and page numbers of the various sections and appendices of the report
should be listed. Each appendix should be provided with a detailed list of contents.
3.5E Location Maps
Two location plans should be included showing:



the position of the Bridge in relation to the local junctions and lines of the rail
network,
the structure, to an ordnance survey scale, indicating a north point and giving
the National Grid reference for the structure. (For multi-span structures this
should be the centre of the structure overall).

3.6E General Arrangement Drawing
An A4 or folded A3 sized general arrangement drawing. Existing record drawings
may be used if suitable.
3.7E Introduction
Stating the reason for the Bridge Assessment, the location of structure, a brief
constructional description, the normal live loading conditions (number of
tracks/carriageways and frequency of traffic) and relevant historical information.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 16

3.8E Condition Survey Summary
A description of the main relevant features of the condition of the Bridge including
information obtained during the inspection for the assessment. A limited number of
photographs (colour, 150 mm by 100 mm in size) may be included in the summary in
order to highlight features of particular importance. Close-up photographs should be
supported by general views to illustrate the location of the detail shown in close up.
The completed condition survey report, giving full details of the condition of the
Bridge obtained during the review should be included as an Appendix.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Status of Report (Interim, Draft, Final)

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 16

(Consultants Logo with address)
(Consultants Telephone number)
(If required)

Bridge Name(s)/Road Number(if appropriate)
ELR and Structure Number
Stations between
O.S. location reference

General View Photograph

Date:
Assessors Reference:

Confidentiality Clause: e.g. This report was commissioned by Railtrack PLC and
is confidential. It is not to be passed to a third party without the permission of the
commissioning Railtrack Director (Insert Zone)or his delegated representative.
Figure E2
An Example of a Bridge Assessment Report Cover Page

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 16

3.9E Results Obtained From Calculations
A statement should be made of the overall Bridge capacity (according to the
applicable assessment codes) and identifying the limiting elements. A table should be
included at the front of the report summarising the element safe load capacities and
the relevant speeds as applicable, including for abnormal road loads whether in traffic
or alone and position if alone. Where appropriate, comments should be made on the
results obtained, particularly where the calculations are sensitive to the parameters
considered.
In addition to the foregoing, the results of the assessment should be recorded on the
applicable detailed Summary Sheet shown in Tables E1 to E7, which should be
included in the Bridge Assessment report.
3.10E Conclusions
A summary (with appropriate commentary) of the main findings of the assessment.
3.11E Recommendations
Recommendations shall be commented on and/or endorsed by the signature of
Structures Manager.
3.12E Figures
Sketches and diagrams should be included as necessary.
3.13E Appendices
Typically, the following Appendices should be attached:
1.

Technical Approval in Principle and Check Certificates

Copies of signed completed forms for the assessment and check shall be included.
2.

Record Data

A list of drawings, main dimensions, including span and skew, levels, clearances and
any other relevant information on which any aspects of the Bridge Assessment are
based.
3.

Inspection for Assessment Information

Sufficient drawings and where applicable, annotated photographs, should be included
for each Bridge to record clearly and completely all information obtained from the
inspection for assessment. The drawings should show the following:
(a)

The minimum clear dimension between the soffit and the highest road level
over the road carriageways or rail level as applicable, and the location of such
dimension. The position, shape and value of any low Bridge warning plates
should be indicated.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 16

(b)

Relative soffit, carriageway and rail levels. Levels of the road surface or rail
levels (and corresponding soffit positions) at positions beneath the Bridge
faces and also where the soffit is low should be included. Levels shall, where
a ordnance bench mark (OBM) is convenient, be related to Ordnance
(Newlyn) Datum. Otherwise, levels shall be related to a well established and
permanently marked temporary bench mark (TBM).

(c)

Details of any components or details potentially susceptible to fatigue and
where available the changes from the previous report or examination.

(d)

Details of deterioration including corrosion, cracking, water seepage, spalling
of concrete, exposed reinforcement, etc. on the super-structure.

(e)

Details of any stress raising details considered fatigue susceptible, and where
available, the change from the previous report or examination.

(f)

Details of any visible distress, cracks, water seepage, lack of verticality, signs
of settlement etc of the sub-structure.

(g)

The location, type and size of services and ducts built into the structure or
buried in the overburden, whether such services or ducts affect the Bridge
Assessment or not.

(h)

The positions, types, sizes and condition of bearings and joints. Location and
types of waterproofing membranes and joints should also be indicated.

4E. CALCULATIONS
Calculations should be clear, objective, legible with a narrative approach and
provided with a detailed index. Where applicable, calculations should be annotated
to show references to clauses of relevant assessment/design standards. Diagrams
and sketches to support the calculations should be provided as necessary.
Material strengths assumed for assessment purposes should be clearly stated,
together with their derivation. Computer programmes, where used, should be
described with their titles, version numbers and validation status.
5E. STATUTORY UNDERTAKERS’ SERVICES
A list of all Statutory Undertakers and Service Owners including Railtrack contacted
regarding the presence of their services on or within the structure should be
included together with copies of received correspondence, and relevant drawings or
sketches, from these bodies.
6E. TEST RESULTS
If either load testing or materials testing has been used to obtain information used in
the assessment calculations, this appendix should contain the report of the work
completed. The format of the report of the tests should be compatible to the main
assessment report.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 16

The following pages contain the Assessment Summary Sheets for steel, wrought iron,
concrete and masonry arch structures. They may be copied and should form part of
the Assessment Report.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 8 of 16

Assessment Information Summary Sheet
Steel and Wrought Iron Structures
DESCRIPTION OF BRIDGE

LOCAL NAME

LINE OR BRANCH

MILEAGE

SPANNING ACROSS

ELR

OS GRID REF

LINE SPEED

ZONE

SECTIONAL APPENDIX RATING
OF LINE

mph

RA
CURRENT FATIGUE RATING

REAL TRAINS SPECIFIED
Yes
No
(if Yes attach details separately)

....................... X 106

HEAVY/MEDIUM/LIGHT

Bridge Configuration
Format:

Half Through

Span Data:

Through Deck

Waybeam

Beam and Decking Type

Railbearers:

Yes

No

Cross Girders:

Yes

No

Trimmer Girders:

Yes

No

Number of Spans:
Support condition:

Track Data:

Simply Supported

Continuous

Span Length(s)

metres

Skew angle(s)

degrees

Number of:
Track Support:

Longitudinal timbers

Sleeper Type:

Transverse sleepers

Timber

Ballasted Track:

Concrete

Yes

Type of Rail:

No

BH

Electrified:

113A

3rd

4th

UIC60
OHLE

Minimum Ballast Depth from Underside of Sleepers

None
mm

Radius of Track Curvature

metres

Rail Joints on Bridge:

Yes

No

Nearest Rail Joints off Bridge from C/L Bearings:
Floor Type:

Direct fixing

metres

Plate

Yes

No

Flat

Buckle Plates:

Troughing

Yes

No

Transverse

Longitudinal

Timber

Yes

No

Transverse

Longitudinal

Jack Arch

Yes

No

Transverse

Longitudinal

Open

Yes

No

Transverse

Longitudinal

Table E1
Steel and Wrought Iron Structure Summary Table

Up / Dn

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 9 of 16

Assessment Information Summary Sheet
Steel and Wrought Iron Structures cntd.
Main Girder:

Type

Plate Girder

Box Girder

Depth

Constant

Variable

Reliant on U-Frame action
Bearings:

Type:

Yes

Spreader plates

Truss

No
Linear rocker

Rollers

None

Other:
Articulation:
Materials

Known

Unknown

Date of Manufacture
Steel

Yield stress (N/mm²)

Pre 1905
After 1906
After 1948
to BS

Wrought Iron
Component Format

Rolled Section

Riveted

Welded

Main Girders
Cross Girders
Rail Bearers
Floor Plate
Assessed by:

Date:

Table E1
Steel and Wrought Iron Structure Summary Table, cntd.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 10 of 16

ASSESSMENT RESULTS SUMMARY
FOR STEEL AND WROUGHT IRON STRUCTURES
BENDING
CAPACITY KNm
REF

DEAD
x
γfl
1

20 BSU
x
γfl
2

1+ϕ

20 BSU x
γfl (1+ϕ)

MD
CAPACITY

LIVE
CAPACITY

3

(2x3)

4

(4-1)

DEAD x
γfl

20 BSU x
γfl

1+ϕ

20 BSU x
γfl (1+ϕ)

LIVE
CAPACITY

1

2

3

(2x3)

CAPACITY
RIVETS/
WELDS
4

BSU

static

+
impact

RA

static

+
impact

MAIN
GIRDERS
CROSS
GIRDERS
TRIMMER
RAIL BEARERS
FLOOR PLATE
SHEAR CAPACITY KN
REF
MAIN
GIRDERS
CROSS
GIRDERS
TRIMMER
RAIL BEARERS
OTHER
HORIZONTAL
SHEAR KN/m
REF

(4-1)

BSU

static

+
impact

MAIN
GIRDERS
CROSS
GIRDERS
TRIMMER
RAIL BEARERS
OTHER
CONNECTIONS - CROSS GIRDER/MAIN GIRDER
SHEAR kN
BENDING kN
SHEAR/
BENDING
RATIO
RAIL
BEARER/
CROSS
GIRDER kN
BEARING STIFFENERS
REACTION
M TRANS
MLONGIT
COMBINATION
CURTAILMENT OF FLANGE PLATES

Table E2
Steel and Wrought Iron Structure Assessment Results
Summary Table

RA

static

+
impact

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 11 of 16

Assessment Summary Sheet
Masonry Arch Structures
DESCRIPTION OF BRIDGE

LINE OR BRANCH

SPANNING ACROSS

LOCAL NAME

MILEAGE

ELR

OS GRID REF

LINE SPEED

ZONE

SECTIONAL APPENDIX RATING OF LINE
mph

RA

Bridge Configuration
Span Data:

Pier Data:

Track Data:

Number of Spans:
Skew span

metres

Square span

metres

Skew angle

degrees

Height

metres

Thickness

metres

Number of:
Type of Rail:

BH

Electrified:

3

rd

113A
4

th

Radius of Track Curvature:

UIC60
OHLE

None

metres

Minimum Ballast Depth from Underside of Sleepers at crown:

mm

Arch Data:
Masonry Type:
Arch Profile:

Semicircular

Segmental

Parabolic

Pointed

Number of Rings:

No.

O/A Ring Thickness:

mm

Ring Thickness used in assessment

mm

Elliptical

MEXE Assessment
Provisional Axle Capacity (QP) ………………………….
Modifying Factors: KP = ……. KS = …….. KM = …….. KC = …….. KD = …….. KV = ……..
Permissible Axle Capacity (CF) = …………………………….
Assessment by Other Method
Effective width considered …………m
Critical Spans/Piers …………………………. (multi-span structures only)
Ultimate Capacity ……………………………
ASSESSED ROUTE AVAILABILITY:

RA

Assessed by:

Date:

Table E3
Masonry Arch Summary Table

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 12 of 16

Assessment Information Summary Sheet
Concrete Structures
DESCRIPTION OF BRIDGE

LOCAL NAME

LINE OR BRANCH

MILEAGE

SPANNING ACROSS

ELR

OS GRID REF

LINE SPEED

ZONE

SECTIONAL APPENDIX RATING OF LINE
mph

REAL TRAINS SPECIFIED
Yes
No
(if Yes attach details separately)

RA

Bridge Configuration
Concrete Format:

Plain

Reinforced

Span Data:

Number of Spans:
Support condition:

Track Data:

Post-tensioned

Simply Supported

Pre-tensioned

Continuous

Span Length(s)

metres

Skew angle(s)

degrees

Number of:
Track Support:

Longitudinal timbers

Sleeper Type:

Transverse sleepers
Timber

Ballasted Track:

Yes

Type of Rail:
3

Concrete

No

BH

Electrified:

Direct fixing

113A

rd

4

th

UIC60
OHLE

None

Minimum Ballast Depth from Underside of Sleepers

mm

Radius of Track Curvature

metres

Rail Joints on Bridge:

Yes

No

Nearest Rail Joints off Bridge from C/L Bearings:
Element Types:

Main Beams:

metres

Slab

Yes

No

insitu

precast

Beams

Yes

No

insitu

precast

Portal

Yes

No

insitu

precast

Culvert

Yes

No

insitu

precast

Arch

Yes

No

insitu

precast

Other

Yes

No

insitu

precast

Type

Inverted T

Depth

Constant

Box Girder
Variable

Table E4
Concrete Structure Summary Table

U

Rectangular

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 13 of 16

Assessment Information Summary Sheet
Concrete Structures cntd.
Bearings:

Type:

Spreader plates
Elastomeric

Articulation:
Materials

Rollers

None

Other:

Known
Assumption
(*1)

Concrete

Linear rocker
Pot

Unknown

f y or f pu (N/mm²)

Jacking or Pre-stress
Force (kN)

Insitu
Pre-cast

Steel

Reinforcement
Main tendons
Other Tendons

Assessed by:

*1

Date:

Abbreviations:
SS
=
Strength specified
M
=
Strength measured
ASG =
Strength Assumed from specified grade
A
=
Assumed
Table E4
Concrete Structure Summary Table, cntd.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 14 of 16

ASSESSMENT RESULTS SUMMARY
FOR CONCRETE STRUCTURES
BENDING
CAPACITY (kNm)
REF/
SECTION

DEAD
x

20 BSU
x

1+ϕ

1

2

3

γfLγf3

γfLγf3

γfLγf3 (1+ϕ)

20 BSU x

MU
CAPACITY

LIVE
CAPACITY

(2x3)

4

(4-1)

BSU

static

MAIN
BEAMS

SLAB

Longitudinal
Transverse

OTHER

SHEAR
CAPACITY (kN)
REF/
SECTION
MAIN
BEAMS
vertical

SLAB
OTHER

longt./
interface

Table E5
Concrete Structure Assessment Results
Summary Table

RA

+
impact

static

+
impact

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 15 of 16

Assessment Information Summary Sheet
Cast Iron
Line or Branch

Mileage

ELR
M

At or between

Bridge

Ch

Zone

Construction date

Spanning

Grid Ref

Capacity EUDL kN
Member
Loaded
Length

RA of Line

Static Capacity
kN
BSU
Units

RA

Dynamic Capacity
kN
BSU
RA
Units

Permanent Speed Restriction on:-

Dynamic Summary
Tens
Comp
Shear
kN
kN
kN

Line
Bridge

Stresses N/mm²
Member

Permissible
Stress
T

C

Fatigue
Member
Material
Live load stress
Allowable Stress Range

Assessed by:

S

Total Stress
for Assessment
Loading
T
C
S

Type
Speed
T

C

Total Stress under Real Vehicles
Type
Type
Speed
Speed
S
T
C
S
T

Based on Calculation

Based on Tests

Date

Table E6
Cast Iron Structure Summary Table

C

S

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix E – Model Bridge Assessment Report

Page 16 of 16

Assessment Information Summary Sheet
Timber Structures
Line or Branch

Mileage

ELR
M

At or between

Bridge

Ch

Zone

Construction date

Spanning

Grid Ref

Capacity EUDL kN
Member
Loaded
Length

RA of Line

Static Capacity
kN
BSU
Units

RA

Dynamic Capacity
kN
BSU
RA
Units

Permanent Speed Restriction on:-

Dynamic Summary
Tens
Comp
Shear
kN
kN
kN

Line
Bridge

Stresses N/mm²
Member

Permissible
Stress
T

Assessed by:

C

S

Total Stress
for Assessment
Loading
T
C
S

Type
Speed
T

C

Total Stress under Real Vehicles
Type
Type
Speed
Speed
S
T
C
S
T

Date

Table E7
Timber Structure Summary Table

C

S

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 1 of 87

Appendix F gives additional information about, and guidance on the use of, Issue 1 of
RT/CE/C/025. Information is given in Section order for ease of reference. Wherever
possible Clause numbers are the same as those used in the main text but with a suffix
letter F added.
WARNING: Appendix F is not intended to give comprehensive guidance. It should
not be assumed to indicate all aspects of the structure that should be checked in the
assessment of an underbridge.

CONTENTS
INFORMATIVE ANNEX
Section 1 - Introduction ............................................................................................................. 1
Section 2 - Assessment Philosophy.......................................................................................... 2
Section 3 - Inspection for Assessment .................................................................................... 4
Section 4 - Loading ...................................................................................................................... 6
Section 5 - Steel and Wrought Iron and Appendix A ........................................................ 12
Section 6 - Masonry Arches..................................................................................................... 52
Section 7 - Concrete Structures and Appendix B .............................................................. 65
Section 8 - Composite Structures and Appendix C ........................................................... 69
Section 9 - Cast Iron Structures............................................................................................. 73
Section 10 - Timber................................................................................................................... 77
Section 11 - Substructures....................................................................................................... 81
Section 12 - Bearings................................................................................................................. 82
Appendix D - Fatigue Assessment of Steel and Wrought Iron........................................ 83
SECTION 1 - INTRODUCTION
No further commentary required.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 2 of 87

SECTION 2 - ASSESSMENT PHILOSOPHY
2.3F Assessment Situations
In considering a particular structure, the assessing engineer should take account of all
loading scenarios that the structure is likely to be subjected to, and using his
judgement and knowledge determine those which will be critical to the capacity of
the bridge. The scenarios should include environmental effects such as due to wind
or temperature where relevant.
Whilst accidental situations, such as collision and derailment, or unusual loading
conditions which may occur during maintenance operations, such as the stock piling
of ballast on adjacent tracks, will not govern the load carrying capacity of the bridge it
may be necessary to consider these. When required, appropriate guidance will be
provided by the Railtrack Director’s Nominee.
2.4F Limit States
The governing condition for the assessment of most structures is the Ultimate Limit
State (ULS). For individual structural elements it may also be necessary to check that
Serviceability Limit State (SLS) criteria are also met, see Clauses 4.2.2A, 4.1.1B and
4.3.2C.
In checking for the ULS, it is necessary to make realistic assessment of the interaction
which can occur within the structure at ultimate load. In some cases load-induced
breakdown of interaction within the structure may reduce its ULS capacity. A prime
example is the case where composite behaviour between main girders and a
transverse deck structure is required to justify sufficient ULS longitudinal capacity of
the main girders. It may not be possible to determine by inspection that the
composite behaviour exists, but an SLS check on the interaction may indicate that
cracking/separation/loss of bond between main girders and transverse structure is
unlikely. This result would give some support to an assumption of composite action in
checking the ULS for the main girders. It would not however establish that full
composite action is present as main girders reach their ULS. In such cases, the
assumptions to be used should be defined by the Railtrack Director’s Nominee.
Some situations where Operational Safety Limit States (OSLS) may require to be
checked are given below:


Twist of the deck to minimise the risk of train derailment, especially on skew
bridges (refer to Clause 4.5.1);



Vertical and longitudinal deflection of the deck throughout each span to
ensure acceptable track geometry is maintained;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 3 of 87



Longitudinal, vertical and rotational displacements at deck ends which may
cause disturbance to ballast and adjacent track formation;



Run-on details for longitudinal timbers.

Where applicable criteria should be agreed in accordance with Railtrack’s Technical
Approval Procedures.
2.6F Load Combinations
Table 2.1 gives the various partial factors, γfL , to be applied to the different elements
of loading which when combined in the manner shown, constitute the assessment
loading for a structure. It may be noted that the combinations presented are similar
to those given in BD37/88: Loads for Highway Bridges.
For the vast majority of structures Load Combination 1 will govern. In certain
situations, however, such as for a deep plate girder where the consideration of wind
effects may be important, or for a continuous multi-span deck where temperature
causing restraint at bearings may have a significant influence, Combinations 2 and 3
should also be considered. The most onerous effect arising from the three
combinations should be adopted for the element being assessed. To determine wind
and temperature loads, reference should be made to BD37/88.
Partial factors shown in Table 2.1 are similar to those given in BD37/88. The figure of
γfL = 1.75 for ballast is considered to cover the situations where the ballast is
waterlogged and dirty, resulting in a unit weight greater than the nominal value of
1800 kg/m³, and where ballast depths may increase beyond previously measured
levels as a result of track lifting or maintenance operations. When control measures
are in place to maintain the ballast at a specific level, or the form of construction
dictates the main ballast depth, a reduced value of γfL may be adopted.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 4 of 87

SECTION 3 - INSPECTION FOR ASSESSMENT
3.4.1F General
Following or during the inspection, identification, sampling or testing of materials may
be required. A general description of procedures and test techniques relating to the
main structural materials may be found in:
Appraisal of Existing Structures. Institution of Structural Engineers, 1996, Sections 5 and
6 and Appendices 7 and 8.
For concrete structures the following may be found useful:
BS 6089: Guide to the Assessment of Concrete Strength in Existing Structures;
BS 1881: Testing Concrete.
Removal of samples should be undertaken in locations of low stress but in the vicinity
of the location at which material properties are required to be determined. Sampling
should not be carried out in locations where the removal of material would affect the
strength of the Bridge, for example at the inner support of a continuous bridge where
shear forces and hogging bending moments are at a maximum. The quantity of
material removed should be the minimum required to enable satisfactory testing to
be carried out. The location and extent of testing should be agreed with Railtrack
Director’s Nominee before any testing activities commence on site.
Some uses of testing are to:


confirm assumptions that have been made or need to be made in carrying out
the assessment of the Bridge;



determine the frequency and distribution of locally identified defects such as
reinforcement corrosion;



identify possible reasons for the deterioration and possible susceptibility to
future deterioration.

The location and number of tests/ samples to be undertaken should take account of
factors such as:


the likely variation in material properties within and between parts of the
Bridge;



the probable number of critical locations;



the possible errors that may occur during testing.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 5 of 87

3.4.5F Timber Bridges
Inspection for assessment will generally be as for other types of structure.
Peculiar to timber bridges is the necessity to identify the species and grading of the
timber. This determines density of material for assessment of dead loads, as well as
the strength of the timber. If drawings or other records do not confirm timber
species and grade, then if the initial assessment calculations, based on the weakest
likely timber, show that a lower strength timber is inadequate, or inspection reveals
decay that could affect strength, a sample should be obtained at time of inspection for
testing by a laboratory. Field grading of the visible parts of the timber may also be
required.
It is suggested that in the absence of other information the timber is initially taken as
Douglas Fir (Canada and USA), SS Grade.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 6 of 87

SECTION 4 - LOADING
GENERAL
The code gives information on dead loads, superimposed dead loads and live loads
including vertical loading with dynamic effects, nosing, centrifugal and longitudinal
loads. No specific information is given on other loads such as wind, temperature and
horizontal soil pressures. Where load combinations including these effects need to
be considered, BD37/88 should be used. In practice load combinations, including
wind, temperature or horizontal earth pressures are unlikely to govern assessment.
All loads are given as nominal (unfactored) values. For steel, wrought iron and
concrete and steel/concrete composite structures loads should be factored as given
in Section 2.
4.1F Dead Loads
Table 4.2 gives typical densities of materials. For riveted structures, it is important to
make due allowance for stiffeners and rivet heads. In the absence of specific
allowance the gross cross sectional area of the beam or girder may be multiplied by
1.12 to allow for stiffeners and rivet heads. Where dead load may be highly critical to
the assessment, for example for a long span bridge, the dead load should be calculated
more accurately.
4.2F Superimposed Dead Loads
Ballast loading should be based on actual measurement of depth and width using a
density of 1800 kg/m³. The γfL factors in Section 2 allow for the possibility of
waterlogged ballast and/or whether control measures are present to prevent overballasting. The weights of permanent way in Table 4.1 are an expansion of those
taken from the previous code, RT/CE/C/015. Loads from services and ducts should
be allowed for and may be significant, for example for parapet girders.
4.3F Live Load
4.3.1F Vertical Loading
This clause is based on the RA classification as used in the permissible stress
assessment code, RT/CE/C/015 Issue 1. 20 BSU’s should be applied (i.e. RA10) to the
member or structure and its capacity factor, C, determined as a proportion of 20
BSU’s with the RA number obtained from Table 4.3. EUDL and end shears for simply
supported spans are given by Table 4.4 and are the same as those used in
RT/CE/C/015. For continuous multi-span bridges, although Table 4.4 will give an
approximation of the loading within the “span” taken as the summated length of the
spans which are considered as loaded, axle loads should be applied as Figure 4.1.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 7 of 87

It should be noted that the values in Figure 4.1 and Table 4.4 have been derived by
conversion from the Imperial equivalents given in BS 153 and RT/CE/C/015 (Issue 1)
receptively. If the load train in Figure 4.1 is used to determine EUDL’s directly, minor
discrepancies between the calculated value and that shown in Table 4.4 may result.
Note that reduction to 75% of loading on second and subsequent tracks is only
allowed when specifically approved.
4.3.2F Dynamic Effects
The dynamic factor is as derived in UIC document 776-1(1) ,with the exception of
cross girders dynamic factor I4.
The dynamic factor depends on:
Train speed;
Natural frequency of the member;
Quality of track.
For members other than cross girders, values of the dynamic increment ϕ may be
derived using Clause 4.3.2.2 or conveniently from Figures 4.2 to 4.14, which give
values for different train speeds. The natural frequency n0 is calculated assuming the
member is simply supported with a span equal to the effective span L . Where n0 as
calculated to Clause 4.3.2.2 is within the range between 0.5 x low frequency and 2.0 x
high frequency, defined in Clause 4.3.2.3, ϕ should be calculated from the formulae in
Clause 4.3.2.2. Where no as calculated is outside the range between 0.5 x low
frequency and 2.0 x high frequency impact may need special consideration including
the monitoring or witnessing of behaviour under moving trains.
During the development of this code, a detailed investigation into the dynamic
behaviour of cross girders was undertaken(4). It showed that the track irregularity
component ( ϕ11 ) of the impact factor was the dominating effect for cross girders. It
was also found that, if the calculation procedure developed by the UIC for
longitudinally spanning members was also applied to cross girders, the dynamic
allowance would be significantly higher than in RT/CE/C/015 Issue 1. Until the
applicability of ϕ11 to transverse spanning members can be confirmed, the impact
factor for cross girders should be determined from Figure 4.15, which is identical to
Factor I4 presented in Issue 1 of RT/CE/C/015.
In general “normal” track quality should be assumed unless otherwise authorised.
Reduced values of ϕ should be used for any fatigue calculations. The vertical live
loading is multiplied by (1+ϕ) to give the live loading including dynamic effects.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 8 of 87

The dynamic factor does not allow for the effects of rail joints or points and crossings
which if they occur within the bridge itself should be taken into account.
4.3.2.1F Wheel Flats
Flats on wheels cause short-duration impact loads on rails of up to twice the static
wheel loads. This loading has been evident from analysis undertaken by AEA of
measurements obtained by means of wheel impact load detectors.
Only parts having a very high natural frequency and low inertia, such as rails, respond
to such impact. It will be seen from Figures 4.2 to 4.14 that dynamic increments for
short spans are of the order of unity for speeds of 100 mph.
It can be assumed that the dynamic factor ϕ takes account of the effects of wheel
flats.
Longer spans do not respond to wheel flat impulses, since their natural periods
considerably exceed the durations of the dynamic forces and their inertias are
relatively high. Furthermore, when the governing load effects are due to multiple
axles on a bogie or several bogies, the axle load impacts are unlikely to be correlated
in time or magnitude. For these reasons wheel flats (like rail irregularities) contribute
little to the dynamic increment when the natural frequency of the bridge is less than
about 5 Hz.
4.3.3F Dispersal Of Live Loading
Clause 4.3.3 gives rules for dispersal of EUDL and axle loads through the track onto
the structure, taken from RT/CE/C/015, but does not take into account any
distribution achieved within the structure itself. This distribution should be evaluated,
where it will benefit an otherwise unrestricted assessment, by a distribution analysis,
for example such as a two dimensional grid analysis of a continuous reinforced
concrete slab interconnecting cross girders.
The dispersal angle of 15° indicated in Figure 4.16 is in accordance with UIC774-2C(5).
4.3.4F Nosing and 4.3.5F Centrifugal Loads
Nosing and centrifugal loads should be considered. Since centrifugal loads tend to
suppress the lateral oscillatory motion of vehicles causing nosing, centrifugal and
nosing loads do not need to be considered in combination on the same track. The
vertical effects of nosing and centrifugal loads should be included in the assessment of
rail bearers, reduced to take account of cant, but generally only have a marginal effect
on transverse members and main girders. If bridge floors are capable of diaphragm
action, such as continuous concrete or metal plate decks, horizontal loads can be
disregarded except on bearings. Nosing forces have been derived from the results of

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 9 of 87

a parametric study for ERRI D181 which considered lateral forces on railway bridges
and showed a dependency on speed.
4.3.6F Longitudinal Loads
The allowance for longitudinal loading is based on BS 5400: Part 2 but is reduced pro
rata for loading less than 20 BSU’s. Longitudinal loading is required mainly for
checking of bearings or other fixity to substructures.
4.5.1F Track Twist
The criterion is based on UIC leaflet 776-3R(2). It is related to risk of derailment. It is
particularly likely to be relevant to the ends of skew bridges or where the two
railbearers beneath a track have different rigidity.
4.6.1F Collision Loads
For bridges over highways, assessment in relation to collisions by highway vehicles
should only carried out where required by Railtrack’s Assessment Engineer.
Reference should be made to BD 60/94(3). For bridges over railways collision loading
resulting from a derailment should be considered only where agreed with the
Railtrack Assessment Engineer. An example might be where supports are less than
4.5 metres from rail and pointwork or sharp track curvature exists beneath the
bridge.
FURTHER INFORMATION ON LIVE LOADING
Stress under “Real” Vehicles and Speeds
Where the assessed RA number is below the RA of the line, the effects under static
EUDL’s for the “real” (actual) permitted vehicles and combinations, together with
dynamic factors for their respective permitted speeds, may be considered acceptable.
This is provided the capacity of the structure is within the limits imposed by this
Code.
It should be noted that the RA effect of vehicles on a specific “span” (loaded length) is
often less than the RA classification for the vehicle since this classification has to allow
for a full range of bridge spans.
Explanatory Note re EUDLs
The capacity of a simply supported beam can be expressed as the Uniformly
Distributed Load (UDL) it can safely carry; this may be governed by bending or shear
at various cross sections. If the load coming onto the beam is already a UDL, it can
be compared directly to the UDL capacity, for example the load on a box culvert
with generous cover could be a single axle load applied at the rail and fully
dispersed/distributed onto the slab.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 10 of 87

In most practical cases, however, the axle loads are only partially spread and come
onto a longitudinal beam as a series of short UDLs or “patch” loads (overlapping as
appropriate). The calculation of these loads and their effects is laborious and it is
normal practice to proceed, conservatively, as if the axle loads were placed directly
on the beam. (Empirical reduction factors may be applied to the effects; for metal
structures these are given in Section 4.)
The maximum specified moment or shear force, at a location, of a train travelling
across a span, is calculated. The UDL required to produce the same effect is called
the Equivalent UDL (EUDL), and can be compared with the UDL capacity per track
mentioned above.
EUDL tables for selected real trains on a range of spans should be used for:1.

Obtaining the maximum bending moment in span, occurring under an axle at
or near mid span. The EUDL is frequently taken as (maximum bending
moment x 8/span), but should strictly be derived from the parabola enveloping
the real bending moment diagram. Bending moments at other points in the
span may be derived using tabulated information in Section 5, or by a grillage
or other suitable method of analysis.

2.

Obtaining the maximum end shear, occurring with an axle at the span end.
The EUDL is, by definition, twice the end shear force.

EUDLs should not be used for combined bending and shear calculation for continuous
beams.
The loading on main girders, transferred as point loads from cross girders, is
routinely taken as the EUDL for the train. This approximation is normally acceptable
but care should be taken in cases where it could be seriously unconservative (e.g. in
the extreme, a single cross girder at mid span).
For a transverse member, such as a cross girder, loaded by simply supported rail
bearers the appropriate loaded length for applying the EUDL is the influence line
length equal to 2 x L where L is the spacing of the transverse members.
British Standard Unit (BSU) Loading
The standard BSU loading, based on BS 153 and used for the RA classification, is
shown in Figure 4.1. One BSU is defined as one unit of Type RA1 loading. Figure 4.1
is shown in terms of 20 BSUs equivalent to RA10 static loading. This loading should
be used where it is inappropriate to apply EUDL’s, for example continuous spans.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 11 of 87

Type RA1 loading was defined by BS 153 in 1925 as the static unit loading for
standard (1432 mm) gauge. For main line railways 20 units of Type RA1 loading was
recommended with dynamic effect added. Dynamic effects were investigated for the
1928 report of the Bridge Stress Committee as relevant for steam locomotives and
added to Type RA1 loading to derive type RB loading which was used until the
introduction of RU loading in 1973 to suit modern traction stock. RU loading was
included in BS 5400: Part 2 in 1978.
REFERENCES
1.
2.
3.

UIC. UIC776-1R Loads to be considered in the Design of Railway Bridges.
International Union of Railways, 1990;
UIC. UIC776-3R Deformation of Bridges. International Union of Railways,
1989;
THE HIGHWAYS AGENCY. BD 60/94: The Design of Highway Bridges for
Vehicle Collision. The Stationery Office, 1994;

4.

CASS HAYWARD & PARTNERS. Investigation into the Dynamic Behaviour of
Cross Girders. Railtrack plc, June 2000;

5.

UIC. UIC774-2R Distribution of Axle-Loads on Ballasted Railway Bridges.
International Union of Railways, 1994.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 12 of 87

SECTION 5 - STEEL AND WROUGHT IRON
Clause numbers in the guidance notes following do not tie up with the Clause
numbers in Section 5. The Clauses giving guidance specific to individual Appendix A
Clauses have the same number as the Appendix A Clause but with the addition of the
suffix letter F.
5.1F General
This part of Appendix F contains general guidance on common structural forms of
steel and wrought iron rail underbridges. Information is given on the theoretical
approach for features of construction which are not covered by BS 5400: Part 3 such
as buckle plates. The steps in carrying out the assessment of different member types
are summarised with specific Clause references to BS 5400: Part 3 and supplemented
as necessary.
The majority of rail underbridges in steel and wrought iron are riveted structures of
half-through type with solid web main girders. Hog-backed girders are common.
Tracks are generally carried by cross girders with or without rail bearers and floors
of troughing, jack arches, metal plate or timber. Longitudinal timbers often support
the track by direct fixing without ballast. Floors are often supported directly by
abutments without trimmer beams, especially on skew bridges. Rigid ‘U’-frames or
bearings at supports are often absent. Joints between floors or cross girders and
main girders often do not meet BS 5400 requirements, and sometimes cross girders
are not coincident with vertical stiffeners. A 3 girder arrangement is often found on
two track lines. Some bridges have half-through or through trusses, often with
underslung cross girders. Others use trough main girders beneath each rail carrying
longitudinal timbers. Some more modern structures are of welded construction such
as Type ‘A’ standard bridges or trapezoidal box girders.
BS 5400: Part 3(1), as a modern limit state code, could be used to assess many aspects
of these structures. However, many Clauses are not directly applicable or the
structural details do not comply with BS 5400. Steel and wrought iron bridges should
be assessed, therefore, using Section 5 of this code, making use of Appendix A which
is written as a supplement to BS 5400: Part 3, and of Appendix D for fatigue
assessment. Appendix A refers to Standards BD 56/96(2) and BD 13/90(3) used by the
Highways Agency for road bridges. Wherever possible the requirements in BD 56/96
are adopted to assist uniformity of practice within the industry. However, some
Clauses relate to forms of construction not usual in railway bridges, for example, BD
56/96 Clause 9.17 for stiffened support diaphragms in box girders and Clauses for
longitudinally stiffened plates and a predominance in highway bridges of deck type
rather than half-through arrangements.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 13 of 87

5.1.1F Applicability
A limit state approach is used based upon BS 5400: Part 3. Appendix A is based on
BD 56/96, where appropriate, and currently proposed amendments to BS 5400:
Part 3. These amendments to BS 5400 are expected to be the next revision to
BS 5400: Part 3, and are particularly relevant for rail bridge assessments because they
incorporate a more rational approach for U-frame bridges.
5.3F Material Properties
Steels may be categorised as shown below, whereas the previous code differentiates
only between steels before 1905 and after.
• Pre 1905

No specified mechanical properties, values based on BRR Report
LR MF 115;

• BS 15 1906

UTS specified, but not yield stress;

• BS 15 1948

Yield stress first specified in addition to UTS;

• BS 4360 1962 Modern standard for weldable steels.
For wrought iron generally no mechanical properties were specified at the time of
manufacture. Production of wrought iron slowed after 1880 with the introduction of
steel and ceased by about 1900. The values shown in Table F5.1 for yield stress are
based upon the BRR Report LR MF 115(4) and are the mean value minus two standard
deviations of test data. The values given in BD 21/97(5) were not adopted as their
source is unknown.
For steel to BS 15 (1906), the BD 21/97 value of yield stress given in Table F5.1 is
considered reasonable in the absence of other information. The value is mid-way
between that for pre 1905 steels and 1948 steels when a specified yield stress was
first introduced.
Section 5 states that detailed theoretical assessment of steel and wrought iron is
carried out using Appendix A. The basis of Appendix A is BS 5400: Part 3. It is
written as a supplement in a similar way to BD 56/96, which (although containing
much which is appropriate) was written for highways structures and tends to give
emphasis towards continuous longitudinally stiffened beams and to large box girders.
BD 56/96 bases material properties on fairly extensive testing or on mill certificates
that may be available from the time of construction of the highway bridge, typically
the 1960’s and 70’s when many major viaducts were built. A different approach is
required for rail underbridges where the majority are small span riveted structures,
many being built before 1900. It is not practicable or justifiable generally to carry out
extensive testing on the material of such structures. It is necessary and appropriate
to rely on the past information on early steels and wrought iron, although test data in
any particular case would always be valuable. Hence information is given on available

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 14 of 87

early information, but with the option to carry out material testing. Such testing may
be appropriate for major structures, for example such as Forth, Tay, Runcorn and
Grosvenor bridges.
5.4F Partial Factors
Reliance on the previous permissible stress Code has not resulted in major failures in
service. An appraisal was therefore made of the implied safety factors in Issue 1 since
it is clearly desirable the present code gives compatible results. At the same time the
aim in preparing the present code was to achieve more consistent reliability and not
condemn structures which are in fact capable of safely carrying higher loads.
Material properties are given for early steel and wrought iron materials in Table F5.1.
MATERIAL

PERMISSIBLE
BENDING STRESS

YIELD STRESS

minimum

UTS

Yield Stress
Permissible
Stress

UTS
YIELD

in PREVIOUS assessment
code

Steel

Pre 1905
BS 15
1906
BS 15
1948
BS 4360
1962

(grade 43)

Wrought Iron

Present
Code
* 205
230

BD 21
& LUL
230
**230

*370
430

1.32
1.24

1.80
1.87

230

430

1.32

1.76

186 (144 in BS 153 1962)

245
(=19mm)
245
(=16mm)

240

430

1.32 (1.70)

1.76

130

*190

220

*285

1.46

1.50

155
186
186

* BRR Report LR MF 115 - mean minus two standard deviations
** Origin of BD 21/97 yield stress not known
Table F5.1
Bending Stresses Compared with Yield Stress - N/mm²
Yield Stress
in Table F5.1 representing the factor of safety in the
Permissible Stress
previous Code may be compared with the product of the partial factors in the
present Code (i.e. γfL x γf 3 x γm ). Parity between the present and previous codes
The values of

would be achieved if the partial load factor γfL is about 1.25, that is lower than the
value of 1.4 normally used for new design. Typically for beams in bending and shear,
the overall safety factor would be γfL x γf 3 x γm = 1.25 x 1.10 x 1.05 = 1.44. This
value compares closely with the total factor of safety in the previous Code for
wrought iron (1.46), but is somewhat higher than that for steel (1.32). Parity in steel
is, however, achieved if γf 3 is taken as 1.0, i.e. 1.25 x 1.00 x 1.05 = 1.31. This value of

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 15 of 87

γf 3 is permitted under Clause 4.3.3A of Appendix A for certain structures.
Therefore, use of the partial load factor γfL higher than 1.25 will often mean that a
lower assessment rating will be determined under the present code for wrought iron
bridges, and more especially steel bridges.
5.6F Structural Form
Some typical forms of steel and wrought iron bridges are shown in cross section in
Figures F5.1 and F5.4.
5.6.1F Half-through Riveted Bridges

Cross girder

Railbearer

Longitudinal timber

Non-structural floor

Transverse troughing

Timber floor

Cross girder

Non-structural floor

Underslung cross girders
(sling bolts or gussets)

Longitudinal troughing

Figure F5.1
Forms of Half-through Riveted Bridge
These bridges can be assessed at Level 1 by their components separately. Rail
bearers, cross girders, troughing, and main girders may all be assumed simply
supported unless continuity clearly exists, for example continuous rail bearers. Top
flange stability generally relies on U-frame behaviour. If this is weak as evidenced by
inadequate moment connections or significant inward bowing/tilting of the girders
stability may have to rely on end bearing stiffeners. Often floor ends are directly
supported on abutments (i.e. without trimmers). U-frame rigidity produced
unintentionally by concrete or other haunching of cross girder ends may be taken
into account.
It is important during inspection to record verticality of main girders to supports and
bow of top flanges. If any bow is other than a single wave form, its shape should be
recorded. U-frame bridges rely for overall stability on in-plane stability of the floor.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 16 of 87

If the floor is an open type, i.e. without continuous floor plating, slab, or troughing, inplane stability may not be properly achieved and U-frame behaviour should be
ignored unless there is stiff vierendeel behaviour of the members in plan.
Composite behaviour of the floor as part of the main girder section should not be
assumed unless there is a continuous plate, slab, or longitudinal trough floor
continuously connected to the main girders.
Clause 9.9.2.3A: Cross girders with sloped bottom flange may be critical for shear
capacity at the ends. A contribution to shear from the flange inclination may be
considered. For hog-backed main girders, a shear contribution from the sloping
flange may be taken into account.
For underslung cross girders, assessment of sling bolts or other tension elements that
transfer cross girder reaction plus U-frame forces should be included. The flexibility
of the connection should be modelled in a level 2 assessment by a separate study to
evaluate the ‘ f ’value.
5.6.2F Half-Through Welded Bridges (e.g. E & A Type)
Shear plate
connection

'E'
Type

'A'
Type

ZED
Type

Figure F5.2
Forms of Half-through Welded Bridge
These bridges generally have HSFG bolted end plate/shear plate connections to the
cross girders which coincide with any vertical web stiffeners so that U-frame
behaviour is achieved. The concrete slabs give improved distribution between cross
girders. Trimmers are usually present, but sometimes these are pinned and offer no
end U-frame rigidity, except that rigidity may be provided via linear rocker bearings
or cross girders near to the ends of the span. Where slabs are haunched at main

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 17 of 87

girders the joint rigidity is enhanced if positively connected to the web via shear studs
or welded reinforcement, but this connection may attract unwanted stresses.
Level 1 assessment should consider dispersal from track to cross girders, but static
distribution of loads otherwise. Level 2 assessment should include a 2-dimensional
grid analysis allowing for distribution between cross girders where concrete covers
the top of the cross girder. In Type A decks where concrete is infill only, any
longitudinal continuity should normally be ignored.
Where the top flange is asymmetric with respect to the web as in ‘Zed’ Type bridges
then an additional lateral force effect is applied to the U-frames which has to be
resisted by the connections.
5.6.3F Half-Through Riveted Box or Twin Plane Truss Bridges

T
2D
p

=

p
p

CROSS GIRDERS
DIAPHRAGM OR
CROSS GIRDERS
TOP CHORD
WEB MEMBERS

P
AT
BEARINGS

P
2

P
2

+

T
2B
+

T
T
2D
2D
SHEAR TORSION DISTORTION
PB
T= 2
WEB SHEAR
DIAPHRAGMS
d d d d d
INTERMEDIATE
CROSS GIRDERS

BTM CHORD
UNDERSLUNG CROSS GIRDERS

SECONDARY BENDING FROM 2P
AT INTERMEDIATE
CROSS GIRDERS
LONGITUDINAL BENDING

Figure F5.3
Forms of Half-through Riveted Box or
Twin plane Truss Bridges
Depending on the span length, proportions, and type of bearings, the main girders
may or may not rely on U-frame behaviour. If stability can be satisfied without Uframes, cross girder/main girder connections can be assumed as pinned only.
Provided adequate intermediate diaphragms or cross bracings exist, the full cross
section can be assumed to carry the floor and live loads. Stresses from restraint of

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 18 of 87

distortion or warping, and transverse distortional bending stresses should be checked
if diaphragms are at greater spacings than cross girders. Provided that intermediate
diaphragms are capable of transmitting 50% of the cross girder reactions to the outer
web, shear may be assessed to be equally resisted by both webs, plus torsional shear
flow. Overturning stability of the box at bearings should be checked taking account
of fixity reliably provided by end cross girders, bearings or surrounding brickwork.
If stability is not achieved at bearings, a Level 2 assessment should consider 3dimensional behaviour, for example using an upstand grillage with representation of
the joint rigidities at cross girder ends.
5.6.4F Half-Through Beam and Decking Type Bridges

DISPERSAL FROM SLEEPER

e.g. BUCKLE
CROSS GIRDER
OR TROUGHING
PLATES
CONTINUOUS FLOOR
DISCONTINUOUS FLOOR

GRID DISTRIBUTION

STATIC DISTRIBUTION

Figure F5.4
Half-through and Decking Type Bridges
Some bridges have deck-type girders together with edge or parapet girders which
contribute to live load capacity. For a Level 1 assessment, if the floor provides
transverse continuity then a distribution analysis, such as a 2-dimensional grid analysis,
should be carried out to determine the share of loading between the girders. If the
floor is discontinuous, such as buckle plates only or separate spans of troughing, static
distribution should be assumed. With ballasted track, the dispersal from sleepers
through ballast should be taken into account, see Section 4, Clause 4.3.3.3.
When checking the stability of edge girders, an assumption of U-frames may be
necessary. In this case the distance B (Appendix A Clause 9.6.5A) may be taken as
from the edge girder to the first internal girder.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 19 of 87

5.6.5F Deck Type Bridges
non-structural
parapet

tim b e r , tro u g h i n g
o r c r o s s g ird e r s

T W IN
P L A T E G IRD E R

M U LTIPLE
P L A T E G IRD E R

SINGLE BOX

M U LTIPLE BO X

Figure F5.5
Forms of Deck Type Bridges
Where supported by only two plate girders or a single box, static distribution of
loads may be applied. With more than two plate girders or a single box, a
distribution analysis such as a 2-dimensional grillage should be carried out assuming
the floor has transverse continuity and/or effective bracing exists between girders.
5.6.6F Trusses

BOWSTRING
HALF
THROUGH
MULTIPLE
INTERSECTION
WARREN
THROUGH

DECK
TYPE

MODIFIED
WARREN (1)
MODIFIED
WARREN (2)
PRATT or
"N"

Figure F5.6
Forms of Truss Bridges
Most trusses are half-through or through type and are simply supported. Cross
girders are often underslung. Composite behaviour of the floor as part of the chord
is usually not present and should not be assumed unless there is continuously

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 20 of 87

connected floor plating. Static distribution of floor and cross girder loads to trusses
should be assumed. For Level 1 assessment, a 2-dimensional analysis should normally
be carried out which may be by a simple manual calculation for trusses such as
Warren, modified Warren or N types. If cross girders do not occur at nodes, it is
essential to consider local bending of the chord due to the load transmitted by the
cross girder. A rigid jointed frame analysis is generally required. However, if there is
no ‘off-joint’ loading (other than self weight of members and minor items such as
walkways), bending effects within the plane of the trusses can be ignored where the
truss is fully triangulated, the centroids at joints all intersect, and the joints have
untensioned bolts or rivets. In these cases, the analysis may assume either pinned or
rigid joints and the moments can be ignored, see Clause 12.1A.
Half-through trusses generally rely on U-frame behaviour for stability. They can be
assessed similarly to half-through plate girders with diagonal members included as
applicable, see Clause 12.5.1A. The values of I1, I3, and I4 should take account of
whether paired members (as in a box form of truss) are effectively connected so as to
form a compound section or not. Often tension diagonals in a truss are not
connected between intersections, whereas compression members are battened or
laced together.
Members should only be included in the U-frame calculations where they are
adequately connected to the cross members either directly or via stiffening.
Judgement is required in some cases.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 21 of 87

5.6.7F Effective Span
The effective span of beams should be taken as given in Table F5.2.
Type of Support

Effective Span

Onto Outer Abutments and Piers
Bearings of all roller, rocker, elastomeric, pot
or other type with defined point of rotation

Centre to centre of bearings

Flat plate, mortar, lead or other bedding or
directly supported on masonry concrete or
brickwork

Centroid of pressure diagram, taken as
maximum at front edge giving effective bearing
area to zero at back of bearing area. Length of
bearing area should be taken as not greater
than depth of beam or troughing supported.

Onto Other Beams
Cross girders or troughing connected to beam
web

Centres of supporting webs of main beams

Cross girders, rail bearers or troughs bearing
onto flange or seating cleats without
connection to beam web other than nominal
top cleat

Centres of effective stiff bearing areas which
are capable of carrying the imposed load and
eccentricity to the main beams.

Longitudinal timbers or rail bearers supported
on top of cross girders

Centre to centre of cross girders

Onto Metal Columns

Centre to centre of effective support points to
beam on columns that are capable of carrying
the eccentric imposed load to the columns

Table F5.2
Effective Span of Beams
5.7F Assessment Process
5.7.1F General
The following is a description of the assessment steps in calculating the RA number
for rail bearers, cross girders and main girders. References are given to Clauses in
BS 5400: Part 3 as modified by Appendix A. It is expected that the calculations will
be carried out either manually or using spreadsheet computer processes.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 22 of 87

5.7.2F Railbearers
Railbearers are discrete members parallel to the track which support one rail only
and are carried by cross girders or at the ends of the bridge span by the abutments or
trimmers (Figure F5.7). They are usually discontinuous and supported by a web
and/or seating cleats onto the cross girders. In some cases they may be integral with
floor plates or slab.

1 .50m

C E N T R IFU G A L

N O SIN G

1.80m

S H A R E O F R E S IST A N C E
DEPENDS ON FIXING

c TRACKS

7

RAILBEARER

CANT 'C'

h

S

LOADS TO RAILBEARERS

Figure F5.7
Railbearers and load Application
For assessment railbearers may be assumed to be simply supported with effective
span from centre to centre of the cross girders if web cleated. For Level 2
assessment continuity can be assumed if integral floor plates are also interconnected
longitudinally over the cross girders but, if the connections fail assessment simple
supports may be assumed. Where top and bottom flange continuity is achieved by
the connections or by integral welded construction, continuity may be assumed.
Assume static distribution of dead and live loads unless the bearers are
interconnected by a slab with transverse continuity. Note γf 3 = 1.0 if rail bearers are
assumed simply supported.
Assessment Steps - Simply supported I-section rail bearer:
Step

Clause of
BS 5400:
Part 3 or
Appendix A

Clauses in
Code

1

Determine effective span

Table F5.2

2

Calculate dead loads - static distribution

Table 4.2

3

Calculate dead loads - moment (WL/8) and end shear
(W/2) apply γfL

Table 2.1

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 23 of 87

Assessment Steps - Simply supported I-section rail bearer:
Step

Clause of
BS 5400:
Part 3 or
Appendix A

Clauses in
Code

4

Determine live EUDL and end shear for 20 BSU on
effective span from one track

Table 4.4

5

Determine reduction factor for dispersal

Table 4.7

6

Determine static live loads moments and end shear for 20
BSU - static distribution of track load to railbearers

7

Calculate reductions of cross girder properties

9.4.2.4

7a

Determine K c factor if any integral plating to
compression flange

9.4.2.4

7b

Check compression flange outstand limit if no integral
plating and adjust σys if required (only likely to apply

9.3.1A

occasionally to railbearers)
8

Calculate plastic Mpe , elastic I , Z xc , Z xt at mid-span (no

9.7.1A

hole deductions)
9

To derive n0 calculate unfactored deflection under dead
 5WL 
load plus 20 kN EUDL δ0 = 

 384 EI 
17.75
then n0 =
δ0
3

10

Using train speed and n0 determine dynamic increment
ϕ for bending (for shear ϕ× 2 3 ).

Figs 4.2 to
4.14

11

Allow vertical effects of nosing/centrifugal to calculate
dynamic vertical live loads

4.3.4/4.3.5

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 24 of 87

Assessment Steps - Simply supported I-section rail bearer:
Step

Clause of
BS 5400:
Part 3 or
Appendix A

On straight track allow for lateral nosing
11a Calculate vertical load on railbearer:
 EUDL (1+ ϕbend ) +2 × nosing load× h  as a UDL


2
s

 end shear (1+ ϕ shear ) + nosing load× h  as a shear force


2
s

or
On curved track calculate vertical effect of centrifugal
11b
18 + h 
force Vn =
Fc 

 s 
h
(but not less than 2 x nosing load × )
s
Calculate vertical load on rail bearer:
 EUDL (1+ ϕ bend ) + V −1.2EUDL (1+ ϕ )× c  as a UDL


n
bend
2
s

 end shear (1+ ϕ shear ) + Vn −1.2end shear (1+ ϕ )× c  as a


shear
2
2
s

shear force

4.3.4
Fig F5.7

either

12

Calculate dynamic live loads moment and end shear for
20 BSU

13

If compression flange restrained by plating λ LT = 0,
otherwise calculate λ LT
Typically le = 0.85 x effective span, η= 1, i = 0.5
(symmetrical)

Clauses in
Code

4.3.5
Fig F5.7

9.7.2 A

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 25 of 87

Assessment Steps - Simply supported I-section rail bearer:
Step

14* Assume non-compact initially. Calculate Mult (elastic)
Calculate β using Equation A20
Determine MR Mult assuming curve for
k=0

Clause of
BS 5400:
Part 3 or
Appendix A
9.8A

Clauses in
Code

Fig. A10
or A11

9.3.7.2.1,
If RA number for bending is restricted try use of compact
9.3.7.2.3A,
section.
9.2.7.3.1
If compact β =
Figure A11) x Mpe

λ LT

σ yc
355

∴ MR = Figure A10 ratio (or

15

For compression flange MR = Figure A10 ratio (or Figure
A11) x Z xc × σ ys

9.8A

16

For tension flange allow any rivet/bolt holes to tension
flange (note: K 2 = 1.15 for wrought iron)
Ae
nett Z xt =
× Z xt , ∴ MR = nett Z xt × σ yt
A

11.3.2A
& 11.3.3

17

Mid-Span Bending
(MR − MDead × γ fL )
Static BSU =
× 20
Static Live Moment
(MR − MDead × γ fL )
Dynamic BSU =
× 20
Dynamic Live Moment

18

Static RA number = Static BSU - 10 = RA....
Dynamic RA number = Dynamic BSU - 10 = RA ....

Table 4.3

19

Fatigue
Consider Stage A assessment. Calculate live load stress
range under required RA in bottom flange at mid-span.
Use ϕ for fatigue. Ignore nosing. Where σ f max

Appendix D

exceeds σ 0 proceed to Stage B assessment.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 26 of 87

Assessment Steps - Simply supported I-section rail bearer:
Step

20

Shear Capacity
Calculate shear capacity of web VD =

21

 t w d w  σ yw


γ
γ
 m f3  3

Clause of
BS 5400:
Part 3 or
Appendix A
9.9.2.2A
4.3.3A

(VD − dead shear × γfL )

× 20
Static Live Load Shear
(VD − dead shear × γfL )
Dynamic BSU =
× 20
Dynamic Live Load Shear
Static BSU =

22

Static RA number = static BSU - 10 = RA....
Dynamic RA number = dynamic BSU - 10 = RA....

23

End connection vertical shear
Determine shear capacity of rivet/bolt

24

Calculate shear capacity of connection and determine
BSU and RA number as steps 20,21

25

End connection - horizontal shear on rivets
VAy
Calculate horizontal shear on rivet =
x spacing
I
and determine RA number as steps 21,22

26

If nosing/centrifugal resisted by rail bearers and not
restrained by plating etc, determine lateral force and
shear as steps 11a or 11b

Table 4.3
14.5.3.4A &
14.5.3.6A

4.3.5

Decide if resisted by one rail bearer or is shared.
27

Calculate moment My and end shear due to
nosing/centrifugal

28

Clauses in
Code

Calculate M

Dy

9.9.1.2A

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 27 of 87

Assessment Steps - Simply supported I-section rail bearer:
Step

29

Clause of
BS 5400:
Part 3 or
Appendix A
9.9.4.2

Using requirements for combined Mx and My


20 / BSU vertical

BSU combined = 
M
 20 / BSU vertical + y
MDy


Clauses in
Code




 BSU vertical



∴ RA = BSU combined - 10
5.7.3F Cross Girders
Cross girders are discrete members transverse to the track which carry the bridge
floor and span onto main girders. They are normally above flange level, but some
cross girders are underslung with hanger bolts. Others span on top of main girders
forming deck type bridges. At skew ends they may span from main girder to
abutment pier or trimmers. Some form of moment rigidity usually exists at end
connections. This rigidity is a necessity where main girders rely on U-frames for
stability. Cross girders may be integral with floor plates or slab.

B.M.

SHEAR
MAX.

6 FOOT GIRDER
JOINT WITH
MOMENT
CAPACITY

MAX.

BOX
B.M.

MAX.

SHEAR

MAX.

1 TRACK
LOAD
2 TRACK
LOAD

Figure F5.8
Typical Cross Girders

MAX.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 28 of 87

Cross girders can be assumed as simply supported with effective span between centre
to centre of main girders and centre to centre of inner webs if the supporting
members are box girders. For bridges with centre girders, continuity of
interconnecting cross girders can only be assumed if the connections are rigid and
able to resist the hogging moments when both tracks are loaded. Static distribution
between cross girders should be assumed unless interconnected by effectively
continuous rail bearers or slab floor. End connections should be checked for FR + FC
restraint forces in U-frame bridges. These effects are also carried at mid-span of the
cross girder, but FC will, at the heaviest loaded cross girder, produce a hogging
moment of opposite sign to the live load moment. For adjacent lighter loaded cross
girders, FC will produce a sag moment which is additive. Effects of centrifugal and
nosing forces are generally small on cross girders. γf 3 = 1.0 if cross girders are
assumed simply supported.
Assessment Steps: - I-Section Cross Girder:
Step

Clause of
BS 5400:
Part 3
Appendix A

Clauses in
Code

1

Determine effective span

Table F5.2

2

Calculate dead loads. Assumption of UDL is usually
acceptable

Table 4.2

3

Calculate dead loads - moment (WL/8) and end shear
(W/2) apply γfL

Table 2.1

4

Determine live loads from (EUDL for 2 x cross girder
spacing) ÷ 2, per track.
Apportion track load equally between the two rails.

Table 4.4

5

Determine reduction factor FA for dispersal

4.3.3.2

6

Determine static live load moment and end shear for
20 BSU

7

Calculate reduction of gross section properties.

7a

Determine K C factor for any integral plating to top flange 9.4.2.4

9.4.2.4

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 29 of 87

Assessment Steps: - I-Section Cross Girder:
Step

7b

Check compression flange outstand limit if no integral
plating and adjust σys if required (likely to apply

Clause of
BS 5400:
Part 3
Appendix A
9.3.1A

Clauses in
Code

occasionally to cross girders)
7c

Use effective width of slab if composite slab

9.15.2.1

8

Calculate plastic Mpe , elastic I , Z xc , Z xt at mid-span (no

9.7.1A

hole deductions)
9

Determine impact factor, I4.

Fig. 4.15

10

Allow vertical effects of nosing/centrifugal forces to give
maximum bending/end shear and add to dynamic live
loads. The vertical effects are generally small and can be
omitted for Level 1 assessment

4.3.4 & 4.3.5
4.3.6

11

Calculate dynamic live load moment and end shear for 20 9.12.2.2A
BSU. For U-frame bridges only, add sag moment due to
FR buckling force (may disregard for Level 1 assessment)

12

If compression flange fully restrained by plating or slab
λ LT = 0, otherwise calculate λ LT .

9.7.2A

13

Assume non-compact initially. Calculate Mult (elastic)
Calculate β using Equation A20
Determine MR Mult assuming curve for
k=0

9.8A

14

For compression flange MR = Figure A10 ratio (or
Figure A11) x Z xc × σ ys

Fig. A10 or
A11
9.8A

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 30 of 87

Assessment Steps: - I-Section Cross Girder:
Step

Clause of
BS 5400:
Part 3
Appendix A
11.3.2 &
11.3.3

Clauses in
Code

15

For tension flange allow any rivet/bolt holes to tension
flange (note: K 2 = 1.15 for wrought iron)
Ae
nett Z xt =
× Z xt , ∴ MR = nett Z xt × σ yt
A

16

Mid-Span Bending
(MR − MDead × γ fL )
Static BSU =
× 20
Static Live Moment
(MR − MDead × γ fL )
Dynamic BSU =
× 20
Dynamic Live Moment

17

Static RA number = static BSU-10 = RA....
Dynamic RA number = dynamic BSU-10 = RA.

Table 4.3

18

Fatigue
Consider Stage A assessment. Calculate live load stress
range under required RA in bottom flange at mid-span.
Use ϕ for fatigue. Ignore nosing. Where σ f max

Appendix D

exceeds σ 0 proceed to Stage B assessment.
19

Calculate shear capacity of web at ends
 t w d w  σ yw


VD =
 γm γ f 3  3

20

Shear

(VD − dead shear × γfL )

× 20
Static Live Load Shear
(VD − dead shear × γfL )
Dynamic BSU =
× 20
Dynamic Live Load Shear
Static BSU =

21

Static RA number = static BSU - 10 = RA....
Dynamic RA number = dynamic BSU - 10 = RA....

9.9.2.2A
4.3.3A

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 31 of 87

Assessment Steps: - I-Section Cross Girder:
Step

22

End Connection to Main Girder
Calculate FR buckling and Fc (live load) U-frame effects on
connections
Determine coexistent shear and bending moment on
connection

23

Clause of
BS 5400:
Part 3
Appendix A
9.12.2.2A &
9.12.2.3A

Clauses in
Code

Calculate shear capacity of connection using same ratio of
shear and bending moment
Static BSU =

(Shear capacity - dead loads shear)
x 20
static live shear

Dynamic BSU =

(Shear capcity - dead load shear ) x 20
dynamic live shear

24

Static RA number = Static BSU - 10 = RA....
Dynamic RA number = dynamic BSU - 10 = RA...

25

End of Cross Girder – Horizontal Shear
Calculate horizontal shear on rivet =

x spacing
I
then determine RA number as steps 21,22

Main girders are primary longitudinal members normally parallel to the track. They
may be through-truss, half-through or deck-type, plate girder, box girder or trusses.
Most
older examples of spans up to about 30 metres incorporate no specific bearings and
bedstones sometimes with flat bearing plate or
bedding medium.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 32 of 87

e
U-FRAME
WEB ONLY

RIVETED
OUTER

WELDED

A-TYPE

INNER

e

e
STIFF.

LOCAL DISTORTIONAL
BENDING
WARPING

º

e
e
STIFF. U-FRAME

INNER
OUTER

LONGITUDINAL
DISTORTION

Figure F5.9
Typical Main Girders
Static distribution of loads to main girders can normally be assumed unless the bridge
is a deck type with multiple girders and there is transverse continuity. In this case a
distribution analysis (say 2-dimensional grillage) is appropriate. Composite behaviour
with deck type flooring should be assumed if proper interconnection exists. Halfthrough main girders usually rely on U-frame behaviour. Joint rigidity and the
presence of stiffeners in line with cross girders are important parameters in deciding
whether proper U-frame rigidity exists. The presence or otherwise of end U-frames
and/or of bearings which give torsional restraint is important in determining the
effective length of the top flange. Figure F5.9 shows options for the value of δ e used
in Clause 9.6.5.2A of Appendix A. Box girder stability requires consideration of cross
cross girder connections, where stability is achieved by the end linear rocker bearings
centred towards the inner web.
γf3
≤ 25o and the girder is simply
supported, or if it is continuous with splices welded or HSFG bolted/riveted with
cover plates to both flanges.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 33 of 87

Assessment Steps I-Section Main Girder U-Frame Bridge:
Step

Clause in
BS 5400:
Part 3
Determine effective span

Clause in this
Code
Table F5.2

Calculate dead loads – assumption as UDL is usually
acceptable
3

Calculate dead loads - moment (WL/8) and end shear

Table 2.1

fL

Determine live loads on all tracks. Apply static
distribution for loading to main girders
5
6a

Determine static live load moment and end shear for
BSU. Determine moments at curtailment points.
C

factor for any integral plating to top flange

6b

Check compression
outstand limits if no integral
plating and adjust σys if required.

9.3.1A

6c

Check effective web thickness if slender.

9.4.2.5

7

To derive n0 calculate unfactored deflection under dead

4.3.2.2

 5WL 
load plus 20 kN EUDL δ o =

 384EI 
17.75
then no =
δ0
3

8

Calculate plastic Mpe , elastic I , Z xc , Z xt at mid-span and

9.7.1A

curtailment points
9

Using train speed and n0 determine dynamic increment
ϕ for bending (for shear ϕ× 2 3 ).

Figure 4.02
to 4.14

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 34 of 87

Assessment Steps I-Section Main Girder U-Frame Bridge:
Step

Clause in
BS 5400:
Part 3
Appendix A

4.3.4 &
4.3.5

10

Allow vertical effects of nosing/centrifugal forces to give
maximum bending/end shear and add to dynamic live
loads. These effects are generally small and could be
omitted for Level 1 assessment.

11

Calculate dynamic live load moments and end shear for
20 BSU

12

For intermediate U-frames calculate δ R .
For end U-frame or stiffener calculate δ e . ( δ e should be
taken as infinity, i.e. k5 = 3.6 if there is no end frame or
effective stiffener within le 3 or L 5 of support).
Calculate le

9.6.5.2A

13

Calculate λ LT

9.7.2A

14

Assume non-compact initially. Calculate Mult (elastic)
Calculate β using Equation A20

9.8A

Determine MR Mult assuming curve for k = 0
y
(∆F-0.001x) 2 = 0 or as measured bow.
ry
Main girders rarely satisfy compact criteria.
15

For compression flange MR = Figure A10 ratio (or
Figure A11) x Z xc × σ ys x 0.95
The 5% reduction allows for transverse bending, see
Appendix A.

16

For tension flange allow any rivet/bolt holes to tension
flange (note: K 2 = 1.15 for wrought iron)
Ae
nett Z xt =
× Z xt , ∴ MR = nett Z xt × σ yt
A

Clause in this
Code

11.3.2
11.3.3

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 35 of 87

Assessment Steps I-Section Main Girder U-Frame Bridge:
Step

17

Clause in
BS 5400:
Part 3
Appendix A

Clause in this
Code

Mid-Span Bending
− MDead × γ fL )
× 20
Static Live Moment
(MR − MDead × γ fL )
Dynamic BSU =
× 20
Dynamic Live Moment
Static BSU =

(M

R

18

Static RA number = Static BSU - 10 = RA....
Dynamic RA number = Dynamic BSU - 10 = RA.

19

Fatigue
Consider stage A assessment. Calculate live load stress
range under required RA in bottom flange at point of
maximum bending and curtailment points. Use ϕ for
fatigue. Ignore nosing. Where σ f max exceeds σ 0

Table 4.3
Appendix D

proceed to stage B assessment.
20

Calculate shear capacity at ends ( VD ).Use Figs. 11 etc for
web buckling.
For variable depth girders Clause strictly 9.11 should be
applied, but 9.9.2.2 is acceptable provided the maximum
bending is not greater than 0.5MR at positions of critical
shear, i.e. 9.9.2.2 is applicable to simply supported beams.

21

Shear

(VD − dead shear × γfL )

× 20
Static Live Load Shear
(VD − dead shear × γfL )
Dynamic BSU =
× 20
Dynamic Live Load Shear
Static BSU =

22

Static RA number = static BSU-10 = RA....
Dynamic RA number = dynamic BSU-10 = RA...

9.9.2.2

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 36 of 87

Assessment Steps I-Section Main Girder U-Frame Bridge:
Step

23

Clause in
BS 5400:
Part 3
Appendix A
End of Main Girder Horizontal Shear

VAy
x spacing
I
then determine RA number as steps 21,22
Calculate horizontal shear on rivet =

24

Intermediate Stiffeners at U-Frames
Check bending capacity.
Compare with moments due to FR + FC and determine RA
capacity.

25

Intermediate Stiffeners not located at U-Frames

9.12.2.2A
9.12.2.3A

9.13

Check capacity and determine RA number
26

Restraint at Supports
Calculate:
Fs =
Fs1 + Fs 2 + Fs 3 + Fs 4
Fs1 - buckling effect depending on flange bow
Fs 2 - buckling effect depending on web verticality
Fs 3 - out of verticality effect
Fs 4 - skew effect (skew bridges only)
Calculate FL – for U-frames
Total force = Fs + FL
Calculate moment due to Fs + FL on supports, i.e. bearing
stiffener or end U-frame (including, if required, all Uframes within le 3 or L 5 of support).

9.12.4.1A

9.12.4.2A

If capacity insufficient for RA number derived from
bending capacity of girder, recalculate MR assuming
δe = ∞
27

Assess bearing stiffeners based on combined effects of
restraint at supports and vertical bearing load.

9.12.4.1A
9.12.4.2A
9.14.4

Clause in this
Code

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 37 of 87

Assessment Steps I-Section Main Girder U-Frame Bridge:
Step

Clause in
BS 5400:
Part 3
Appendix A

28

Curtailment points
Check bending as steps 17 and 18 at flange curtailment
points.

29

Splices
Check splice capacities in bending and shear.

Clause in this
Code

14.4.1.1A

5.7.4.1F Skew Bridges
Skew tends to reduce the bending effects in main girders which span longitudinally. If
static distribution is assumed, Figure F5.10 may be used to derive bending moments in
main girders. Alternatively, a grid analysis may be used.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 38 of 87
L in e t h r o u g h p o i n t s
o f "effective" bearing

µs

L
s

= Lead on G irders

c TRACK

7

A lso a p p l i c a b l e
when abutm ents
a r e n o t p a r a l l e l.

Live load span m .

L ive load line
w /ft. run

= E .U . D . L . f o r Bm. M . o n s p a n m
A
L I V E L O A D B .M . O N G I R D E R A B =

SKEW FACTOR

0.10

0

0 .1

0 .2

0 .3

L² x SKEW F A C T O R FRO M BELO W
0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

1 .0
0.10

0.09

0.09

0.08

0.08

0.07

0.07

0.06

0.06

0.05

0.05

0.04

0.04

0.03

0.03

0.02

0.02

0.01

0.01

0

0

0 .1

0 .2

0 .3

D E A D L O A D B .M . FR O M F L O O R =

S
2

L in e t h r o u g h p o i n t s
o f "effective" bearing

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

0
1 .0

L² x SKEW F A C T O R FRO M BELO W

D ead Load on G irder
AB

S
2

W h e n A B is an Inner G irder
B . M . f r o m b o t h flo o r s t o b e
added.

SKEW FACTOR

B

L

L
= Lead on G irders

w /ft. run

A

B

L

0.125

0.125

0.100

0.100

0.075

0.075

0.050

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

0.050
1 .0

Figure F5.10
Bending Moments in Main Girders of Skew Bridges

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 39 of 87

5.7.5F Bridge Floors
5.7.5.1F General
Floors of pre-1948 riveted bridges are rarely composite with main girders whether
they comprise flat plates, buckle plates, troughing, old rails or jack arches. Most
modern deck-type bridges use floors of reinforced concrete or steel plate composite
with main girders.
5.7.5.2F Flat Plate Floors
Flat plate floors act structurally when supporting ballasted track. Where tracks are
carried by longitudinal timbers, the floor is generally non-structural except for any
composite action with rail bearers or cross girders if continuously connected.
Where the track is ballasted, floors are subjected to dispersed wheel loads from
sleepers. This can be reduced where rail bearers act as rigid supports (Section 4,
Clause 4.3.3.3). Support conditions should be considered, i.e. whether 2 or 4 sided,
and whether continuity is achieved at the supports. Where plates are in separate
panels connected by a single line of rivets to supporting rail bearers/cross girders,
continuity should not be assumed. Continuity may be assumed where plates are
continuous or connected by more than one line of rivets. The plastic modulus may
be assumed in all cases at ULS. The elastic modulus should be used for fatigue checks.
Plastic hinge or yield line global analysis may be used for steel plates. Yield line
analysis may use standard published expressions or be derived from first principles.
Elastic global analysis should be used for wrought iron. Elastic global analysis may use
published coefficients (e.g. Steel Designer’s Manual).
5.7.5.3F Buckle Plate Floors
Buckle plates consisting of vertically curved steel plates supporting ballast or nonstructural filling and spanning between supporting members may be assessed by
Clauses 5.5.3.1 or 5.5.3.2 as appropriate.
5.7.5.3.1F Spans of 1.2 metres or less
Where the clear span measured between edges of supporting members is 1.2 metres
or less and complies with both of the following:
(a)
(b)

rise between 1/23 and 1/18 of the clear span, and
plate thickness is 6 mm or more

The strength should be assessed assuming arch or catenary action and the horizontal
wL2
thrust is taken as
per unit width.
8r
where:

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 40 of 87

w

is the pressure on the plate surface due to dead loads and dispersed live loads.
Dispersal may be assumed at 1:1. The pressure calculated should be assumed
to occupy the full area of the plate;

L

is the span of the buckle plate between edges of supporting members;

r

is the rise of the buckle plate.

5.7.5.3.2F Spans Exceeding 1.2 metres
Domed (i.e. concave upward) buckle plates may be checked as straight compression
σy
l
members in accordance with Clause 10.6 of BS 5400: Part 3 taking e
as
r 355
(λ −15) when calculating the value of σ c and an applicable effective length of not less
than 0.25L where L is the curved length between supports. The strength of
suspended (i.e. concave downwards) buckle plates may be checked as tension
members under axial load in accordance with BS 5400: Part 3 Clause 5.5.3.1. The
fixings and surrounding construction should be capable of resisting the horizontal
thrust. Concentrated wheel loads over the plate may be dispersed at 1:1.
Alternatively, the buckle plate may be considered as an encastré flat plate in which
case the effects of horizontal thrust may be ignored.
5.7.5.4F Troughing Floors
Live loading to troughing should be dispersed as defined in Section 4. Troughing
generally satisfies compact section criteria (BS 5400: Part 3 Clause 9.3.7.2.3) allowing
use of the plastic modulus. The elastic modulus should be used for U-frame effective
length calculations where transverse troughing forms cross girders (Appendix A
Clause 9.6.5A or BS 5400: Part 3, Clause 9.6.6).
Composite action with concrete infill may be assumed in strength calculations and for
U-frame rigidity, where relevant, in accordance with Appendix A Clause 8.8.3A,
where:
(i)
(ii)

troughs are filled up to at least 75 mm above crests with concrete known to
be dense and without significant evidence of slip or separation, and
rivet heads or other positive shear connections occur along at least every
alternate crest.

The strength or rigidity determined should not exceed by more than 30% the
calculated strength or rigidity for the troughs unless demonstrated by appropriate
testing.
Plastic global analysis may be used for continuous steel troughing where the section is
compact.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 41 of 87

5.7.5.5F Old Rail Floors
Floors made up of old rails should be assessed assuming that the material has a yield
stress of 230 N/mm². Composite action with any infill should be ignored in
calculating bending resistance of the rails. Rails may be assumed to be compact
sections. In the absence of site measurements, the depth of rail should be assumed to
be 5 mm less than the original depth on the basis of the rails being worn and secondhand. Rails may be assumed to act as reinforcement bars within concrete provided
the load stresses permitted by Appendix B are not exceeded.
5.7.5.6F Transverse Stiffened Floor - Standard Box Girders
A longitudinal dispersal length of 3.6 metres for calculation of EUDL for Level 1
assessments may be assumed for transverse tee rib stiffened steel floors as used in
the Railtrack 1989 ‘standard’ box girders, or for similar floors of welded construction
with minimum plate thickness of 20 mm. For Level 2, if required, a local grid analysis
should be carried out which takes into account the flexibility of the track and ballast
as used for the design of the Railtrack 1989 ‘standard’ box girders.
For fatigue assessment using Appendix D, the floor plate fatigue classification should
be assumed as ‘E’.
5.7.5.7F Longitudinally Stiffened Floors - Part of Top Flange
Where floors are longitudinally stiffened and are assumed to form part of the top
flange of main girders, local and global stresses should be combined under a SLS check
(Appendix A Clause 4.2.2A ) since this is not required at ULS. The EUDL for global
bending is normally the full span length. RA axle loading should be considered for the
calculation of local stresses.
Load Case

Load One Bay Between Centre to
Centre Cross Frames

Load Two Adjacent Bays

Centre to centre cross frames

Up to 3 metres

> 3 metres

Up to 1.5 metres

> 1.5 metres

EUDL

EUDL x FEUDL
(3 metres)

EUDL for
centre to
centre

EUDL x FEUDL
(3 metres)

EUDL for
centre to
centre

Sketch

Table F5.3
Load Requirements for Longitudinally Stiffened Floor Plates

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 42 of 87

GUIDANCE NOTES TO APPENDIX A
4.2.3AF Fatigue
Appendix D, as required by Clause 4.2.3A, includes a method for fatigue assessment which is
subject to further development and trial. It is written as a supplement to BS 5400: Part 10(6).
The concept of ‘safe life’ and ‘damage tolerant’ elements is adopted. The former are primary
elements where ability to redistribute live load exists in the event of failure. The latter are
generally secondary elements, either with capacity for redistribution or the permanent way
provides a measure of spanning capability in the event of their failure. Fatigue endurance is
determined in stages from (A) through to (D). Where the stated criteria can be satisfied at
any stage, then subsequent stages representing more rigorous approaches need not be
evaluated. Stage A compares the calculated live load stress range with stated nonpropagating stress values similarly to the previous Code.
4.3.3AF Values of Partial Safety factors
The proposal for using γf 3 = 1.0 aims to give equivalent overall safety of the new code
compared with the existing code as far as possible.
6.1AF Performance
The strengths for rivets have been based on tests on full size steel riveted girders recovered
from the reconstruction of underbridge LEC1/248, London to Rugby Line, tested in 1998 and
reported by Cass Hayward and Partners in a Report to Railtrack dated August 1998. Test
results on wrought iron rivets from two other bridges were also studied. Yield and ultimate
tensile strengths in Table A2 are based on tests of the rivet material. The use of 0.9σ ult
instead of σ y for the value of σ q in Clause 14.5.3.4A for rivets subjected to shear is based
on tests of riveted web to flange joints taken from underbridge LEC/248. The value of γm
for rivet shear in Table A1 for ULS is taken as 1.33, instead of 1.10 as in BS 5400: Part 3, to
allow for the effects of rivet slip as measured during the tests on the girders. This is so that
under SLS loading rivets will be loaded to about 0.5 of their shear capacity, corresponding
with a slip of 0.5 mm so as to limit possible overstress in the connected elements through
loss of interlock between them. No SLS check is then required. In riveted girders there will
be a tendency for a redistribution of bending stress from the flanges to the web causing
overstress of the web through slip of the rivets in longitudinal shear. The load in slip
behaviour of rivets in shear was deemed to be similar to that of headed shear studs used in
composite construction. The effect upon assessment is that the capacity of rivets in shear
will be some 50% greater than in RT/CE/C/015 Issue 1. For checking the bearing capacity of
rivets in shear then the value of σ y is taken as the lesser of that of rivet or connected
material from Table A2 with γm taken as 1.05 in a similar way to BS 5400: Part 3. Generally
the capacity will be governed by rivet shear rather than bearing.
6.4AF Ductility
Potentially wrought iron would appear to have insufficient ductility for plastic analysis.
However, it is likely to have sufficient ultimate to yield strength for plastic stress analysis.
Note draft EC3: Part 2 requires 15% elongation for global plastic analysis.
6.5AF Notch Toughness
The recommendations of BS 5400: Part 3 do not solve the potential difficulty of assessing the
notch ductility of steel in existing bridges where there is no test data on the notch ductility of

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 43 of 87

the steel. Known failures are rare. Most structures will have shed their secondary stresses in
service. Ideally an insitu test method is needed for the future
6.6AF Properties of Steel and Wrought Iron
Properties for wrought iron are from BRR Report LR MF 115 (4).
7.1AF Global Analysis for Load Effects - General
Plastic global analysis is permitted by Clause 7.1A, except for wrought iron structures due to
the limited elongation of wrought iron. Plastic stress analysis is, however, considered
appropriate for wrought iron because the ultimate stress to yield stress ratio is considered
adequate. Clause 9.3.7.5A of Appendix A clarifies that the plastic modulus is permitted for
flat plates such as in floor plates.
8.5.1AF Imperfections Allowed For
Clause 8.5.1A caters for elements that have deviations or tolerances which vary from those
given for new construction to BS 5400: Part 6 and which are implicit within the strength
rules of BS 5400: Part 3. In particular, measured bow and verticality of girders should be
taken into account as necessary in the assessment where appropriate, for example for halfthrough bridges. The requirement to measure web panel imperfections is probably
unnecessary for most small underbridges because out-of-flatness does not affect strength
significantly unless the distortion is noticeable.
9.3.1AF Shape Limitations - General
Under Clause 9.3.1A, a “fictitious yield” approach, as in BD 56/96, is used for section
outstands. They often exceed the limits of BS 5400: Part 3.
9.6.1AF General
This provision is to cater for bridges where the support restraint is not over the support. Its
resistance could be added to the gravity cantilever resistance of a bearing stiffener if this is
limited by strength or by overturning stability on the bearing. The limits on le are based on
Table 8.
9.6.2AF Beams with intermediate lateral restraints
Clauses 9.6A and 9.12A provide for assessment of U-frame bridges, other than those with
rigid support restraint, as in the proposed amendments to BS 5400: Part 3. Account can be
taken of end restraints of any rigidity together with effects of skew. There is a penalty in that
the effective length has to be increased in compensation. It is somewhat unfortunate that the
proposed amendments are not already published. If they were the bulk of Appendix A would
be less.
9.6.3AF Beams (other than cantilevers) without Intermediate Lateral Restraints
This clause is similar to Clause 9.6.3 of BS 5400: Part 3 but the expressions for le enable the
flexibility of the support restraints to be taken into account.
9.6.5AF Beams with U-frame restraints
U-frames are covered by the following Clauses:
9.6.5A

Effective length – le

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 44 of 87

9.12.2.2A
9.12.2.3A

Intermediate U-frames – Force FR
Intermediate U-frames - live load on cross girders – Force FC

9.12.4.1A
9.12.4.2A

Support restraints – Force FS
Support restraint – Force FL

The Effective length for U-frames can only take account of the intermediate U-frame where
the lateral stiffeness of the decking system meets Clause 9.12.2.2A. If not the U-frames are
to be ignored
The selection of δ R (intermediate U-frames) and δ e (end supports) is illustrated in Figure
F5.11.

UNIT
FORCE
e
STIFF.

e
(END FRAME)
AT SUPPORTS OR WITHIN µe
3

R
INTERMEDIATE

SKEW

SKEW

<25°

<25°
e (STIFF.)

e (END FRAME)
µe

<25°

<25°

3



<

µe

e= R

3

<25°

>25°
e (STIFF.)

µe

3

>25°

<

µe

3

e=º

e= R
>25°
e = e (STIFF.)

Figure F5.11
U-Frame Stiffness
Values of ‘f’ are obtained from Figure 42A. Support restraint force (Fs + FL ) should be
applied to the end bearing stiffener, and U-frames if within

le
L
, but ( . If the RA number
3
5

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 45 of 87

is found to be restricted below that of the main girder in bending, le should be recalculated
with an appropriate increase in δ e in order to seek a more favourable RA number.

δ e first Assessment
δ e (stiff)

Amend δ e to:

δ e (U-frame)



δ e (stiff) + δ e (U-frame)

δ e (U-frame)

∞ or δ e (end U-frame) if trimmer
present

Table F5.4
Magnification of δ e
9.7.1AF General
It is considered rational to use the short term modular ratio only because the majority of
stresses in the composite condition will be from live loading in rail bridges.
9.7.2AF Uniform I, Channel, Tee or Angle Sections
For U-frame bridges the transverse section properties can take account of the lateral
stiffnesses of the decking system even if this does not meet the requirements of Clause
9.12.2.2A, or does not form part of the tension flanges of beams.
BS 5400: Part 3 uses the long term modulus but this is inconsistent with Clause 9.7.1. See
also Clause 9.7.1AF.
9.7.3.1AF Uniform Rectangular or Trapezoidal Box sections
See Clause 9.7.2AF.
9.9.2.1AF General
The only likely cases for rail underbridges where webs need to be checked to the interaction
criteria of BS 5400: Part 3 Clause 9.11 are beams with flanges curved in elevation which are
not simply supported.
9.9.2.3AF Shear Resistance of Simply Supported Hog back beams or sloped Bottom flange
Clause 9.9.2.3A includes a method for reducing the shear on webs of hog-back beams. The
limit imposed on web slenderness, λ , is to ensure that the compression diagonals of the
analogous truss do not buckle.
9.12.2.3AF U-Frames with Cross members subjected to Vertical Loading
In calculating θ , the differences in rotation between consecutive U-frames, then point loads
corresponding to the RA loads shown by Figure 4.1 should be used. The loading however
should be compatible with that assumed to derive force FR in Clause 9.12.2.2A.
The proposal for 5% reduction in MR to compensate for the effect of lateral flexure is
subject to further study.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 46 of 87

9.12.4.1AF Restraining Forces
∆ is D/300 in BS 5400: Part 6. This value has been enhanced by 1.2 in accordance with
Clause 8.5.1A. The minimum value of ∆ is 3 mm x 1.2.
9.12.4.2AF Additional U-frame Restraints
See 9.12.2.3AF. The loading used to derive θ should be compatible with that used to derive
force Fs under Clause 9.12.4.1A.
11.3.2AF Effective Area

k2 =

0.8σ ULT
285
which gives 1.20 for wrought iron (i.e., 0.8 ×
). However, a
σY
190

conservative value should be applied to wrought iron in the absence of testing, i.e.,

285 
1.0 + 0.5
 −1.2 = 1.15
 190 

For beams, the deduction for rivet holes in tension areas may ignore any rivet holes in the
portions of web not enclosed by flange angles with the reduced value of section modulus
taken as:

nett Z xt =

Ae
× Z xt
A

where:

Ae =
At =

K 2 At but ( A;
s2t
A − ∑ Ah + ∑ .
4g

In some cases the web plate may need to be ignored in bending calculations altogether (see
Clause 5.6.3.4F).

GROSS AREA OF FLANGE
=A

g2

3
g1 g 2

7
c R.S.A. LEG

Figure F5.12

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 47 of 87

Rivet Holes in Tension
12.1AF Trusses - General
For black-bolted or riveted triangulated trusses with concentric joint intersections,
secondary moments can be ignored under, Clause 12.1A. Although fixed-joint analysis is not
arduous, the excessive effort in interpreting and checking the moments to all the in-coming
members is time-consuming and is irrelevant if the joints have flexibility. Other trusses
would require secondary moments to be considered for SLS, ULS and fatigue as codified. A 3
dimensional analysis can be reserved for second stage assessment.
14.4AF Splices
14.4.1.1AF General
(a)

Girder Splices

In girder splices, bending moment may be assumed to be carried entirely by the flanges (plus
flange angles and web plate between) and the shear resisted by the web only. This
assumption is made because web splices often contain only a single row of rivets/bolts and
would otherwise lead to a reduced capacity due to overload of web rivets in beam bending
plus shear.
(b)

Web Joints

Web joints may be concealed at tee or similar vertical stiffeners having two rows of rivets.
Bending capacity of girders should be assessed therefore ignoring the web unless drawings or
other evidence shows that web joints are not located at the point considered.
(c)

Flange Joints

Joints may occur in multiple flanges. They should be detected during inspection and from
drawings. Unless reinforced by extra cover plates or the flanges are in compression only and
the joint is tight fitting, the jointed flange should be ignored in the calculation of bending
capacity, as shown in Figure F5.13.

COVER PLATE

JOINT
IGNORE FLANGE SPLICED

JOINT

JOINT

Figure F5.13
Flange Joints

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 48 of 87

Where flange plate joints are located close together, the capacity of the whole flange should
be assessed at each joint in the flange and the anchorage of each flange considered with
respect to the numbers of rivets connected.
(d)

Flange Plate Curtailment

In flange plate curtailment calculations, the point of theoretical curtailment may, unless
calculations are carried out to determine the anchorage length, be taken as at the third line of
rivets as shown in Figure F5.14.

ACTUAL CURTAILMENT

ASSUMED POINT
OF CURTAILMENT
Figure F5.14
Flange Curtailment
14.5.3.4AF Fasteners Subject to Shear Only
The value of γm for rivets is 1.33 The value has been increased compared to the previous
Code to ensure, in the absence of SLS checks, excessive slip causing redistribution of stresses
at working loads is avoided.. For other fasteners γm is 1.0 (see Table A1).
The diameter of rivets are taken as 1/16” (1.6 mm) greater than the nominal diameter, i.e. the
diameter of the hole. For example a 7/8” in diameter rivet is taken as 15/16” (23.8 mm) in
diameter for assessment calculations. Rivets are commonly 3/4” or 7/8” nominal diameter.
More than one size of rivet often occurs within a bridge.
Horizontal shear capacity of rivets should be checked at all span ends. Checks at
intermediate locations may be unnecessary where an appropriately uniform rivet spacing is
present, typically 4” (102 mm).
B3.4.2AF STRENGTH
Other amendments to BS 5400: Part 3 B.3.4.2 relating to braced diaphragms have been
omitted.
APPENDIX D
Clause 9.9.6 of BS 5400: Part 3 now refers to a modified Appendix D which corrects errors
in the existing in BS 5400: Part 3 clauses for patch loading. Patch loading is relevant for non-

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 49 of 87

compliant bearing stiffener areas. This Appendix does not cover the common arrangement
without bearings and with a stiffener at the bedstone edge and an end plate.
APPENDIX E
Further consideration was needed as to how Appendix E of BS 5400: Part 3 should be
applied or modified for assessment of transverse bending on U-frame bridges. Appendix E is
very conservative, and is based on theoretical work which assumed a simply supported Uframe bridge with the girder ends having infinitely rigid restraint against twist. For U-frame
bridges it is proposed that Appendix E is not used and that a blanket allowance of 5% be
added to vertical bending effects in compensation.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 50 of 87

REFERENCES - SECTION 5 AND APPENDIX A
1.

BSI, BS 5400: Part 3: 1982 - Steel, Concrete and Composite Bridges. Code of Practice for
Design of Steel Bridges (incorporating amendments No 1 & No 2), British Standards
Institution;

2.

The Highways Agency. BD 56/96 The Assessment of Steel Highway Bridges and
Structures (DMRB 3.4.12). The Stationery Office, 1997;

3.

The Highways Agency. BD 13/90 Design of Steel Bridges. Use of BS 5400: Part 3:1982
(DMRB 1.3). The Stationery Office, 1990;

4.

BEAGLES, M. Static and Fatigue Properties of Wrought Iron and Early Steels. British Rail
Research Report LR MF 115, November 1993;

5.

The Highways Agency. BD 21/97 The Assessment of Highway Bridges and Structures
(DMRB 3.4.3). The Stationery Office, 1997;

6.

BSI, BS 5400: Part 10: 1980 - Steel, Concrete and Composite Bridges. Code of Practice
for Fatigue. British Standards Institution;

Other Sources of reference used in the development of Section 5 and Appendix A included:
7.

BS 7668 Specification for Weldable Structural Steels. Hot Finished Hollow Sections in
Weather Resistant Steels. British Standards Institution;

8.

BS EN 10025 Hot Rolled Products of Non-alloy Structural Steels. Technical Delivery
Conditions. British Standards Institution;

9.

BS EN 10029 Specification for Tolerances on Dimensions, Shape and Mass for Hot Rolled
Steel Plates 3 mm Thick and above. British Standards Institution;

10.

BS EN 10034 Structural Steel I and H Sections - Tolerances on Shape and Dimensions.
British Standards Institution;

11.

BS EN 10056 Structural Steel Equal and Unequal Leg Angles - Part 2 Tolerances on Shape
and Dimensions. British Standards Institution;

12.

BS EN 10113 Hot Rolled Products in Weldable Fine Grain Structural Steels. British
Standards Institution;

13.

BS 5135 Specification for Arc Welding of Carbon and Carbon Manganese Steels.. British
Standards Institution;

14.

BS EN 10137 Plates and Wide Flats Made of High Yield Strength Structural Steels in the
Quenched and Tempered or Precipitation Hardened Conditions. British Standards
Institution;

15.

BS EN 10155 Structural Steels with Improved Atmospheric Corrosion Resistance.
Technical Delivery Conditions. British Standards Institution;

16.

BS EN 10210 Hot Finished Structural Hollow Sections of Non-alloy and Fine Grain
Structural Steels. British Standards Institution;

17.

DD ENV 1993-1-1 Eurocode3: Design of Steel Structures. Part 1.1 General Rules and
Rules for Buildings. British Standards Institution;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 51 of 87

18.

DD ENV 1994-1-1 Eurocode 4: Design of Composite Steel and Concrete Structures.
Part 1.1 General Rules and Rules for Buildings. British Standards Institution;

19.

ENV 1991-3 Eurocode 1: Basis of Design and Actions on Structures. Part 3 - Traffic
Loads on Bridges (CEN 1994). British Standards Institution;

20.

BSI 96/103556 National Application Document for use in the UK. British Standards
Institution;

21.

ENV 1993:3:1994 (EC1: 3) Traffic Loads on Bridges. British Standards Institution;

22.

BSI. Proposed Amendments to BS 5400: Part 3 dated 09/02/00. British Standards
Institution;

23.

EC3: Part 2 “Eurocode for Steel Bridges” (Draft). British Standards Institution;

24.

National Application Document for EC3: Part 2 (Draft). British Standards Institution;

25.

UIC. UIC776-1R Loads to be Considered in the Design of Railway Bridges. International
Union of Railways, 1990;

26

LUL. Standard for Assessment of Railway Underbridges: CED-ST-3114-A1. London
Underground Ltd.;

27.

Railtrack. RT/CE/C/015 Railtrack Code of Practice - The Assessment of Underbridge
Capacity” Issue 1. Director Civil engineering;

28.

The Highways Agency. BD 37/88 Composite Version of BS 5400: Part 2 for the
Specification of Loads Used for the Design of Department of Transport Highway Bridges
and Associated Structures. The Stationery Office, 1988.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 52 of 87

SECTION 6 - MASONRY ARCHES
6.1.1F Applicability
Clause 6.1.1 identifies the various elements of arch structures which are covered by
this Section of the code. It should be noted that it is not applicable to structures
which have been modified by the replacement of sections of the arch barrel with steel
or concrete elements.
6.1.2F Levels of Analysis
Refers back to Clause 1.6 where different levels of assessment are defined. It is
intended that techniques presented in Section 6 are related to Clause 1.6 as follows:
Single span arches and multi span arches with stocky piers:
Level 1
Level 2
Level 3

MEXE analysis
2-D analysis using elastic or mechanism method
3-D finite element

Multi-span arches with slender piers:
Level 1

2-D analysis using elastic or mechanism method

6.1.3F Assumptions
All the features from which an arch derives its strength are not directly measurable
during the site survey and inspection. Nor may all the information be readily available
from existing record drawings. Inevitably certain assumptions have, therefore, to be
made during the course of the assessment. The assumptions could include ring
thickness and backing levels, as well as the level of passive pressure to be permitted
during the analysis. Since some of these assumptions are likely to be judgmental, it is
important that the assessor explores the sensitivity of his analysis to changes in
assumed parameters before reaching a conclusion on the capacity of the arch.
6.1.4F Loading and Load Distribution
A longitudinal dispersal angle of 1 horizontally to 2 vertically from the underside of
sleepers is recommended. In applying this assumed dispersal to a single axle, account
may be taken of the fact that the load may also be shared by adjacent sleepers as
indicated in Clause 4.3.3.3. If this dispersal model is applied due account should be
taken of the load concentration beneath the middle sleeper. It is recognised that
certain proprietary analysis packages incorporate other load dispersal models (for
example, ARCHIE uses a sinusoidal dispersal and MAFEA a Boussinesq model (1)) such
dispersal methods are acceptable.
Transversely, load is assumed to be dispersed at an angle of 45o. The extent of
transverse distribution defines the effective width to be considered for assessment.
The point where the dispersal lines of loads from adjacent tracks intersect defines the

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 53 of 87

limit of the effective width for the situation where adjacent tracks are loaded. The
effective width should be limited in circumstances where the arch barrel is cracked,
or contains discontinuities, for example, if it has been extended and it is believed
prudent to assume that the fill is also cracked and unable to distribute the load.
It is recognised that both longitudinal and transverse angles of dispersal are greater
than the 15° spread specified in Figure 4.16. This is in recognition of the fact that a
significant proportion of the material between the underside of the sleeper and the
arch ring will be well compacted fill, rather than ballast. The increased dispersal in
the transverse direction also makes some allowance for load distribution by the arch
barrel.
Where the track over the structure is curved, centrifugal effects may be taken into
account by assessing the effective width associated with the more heavily loaded rail.
Where the dispersal lines overlap, again the limit of the effective width should be
taken at the point of intersection.
6.1.6F Skewed Arches
A 35o limit has been set for permitting skewed arches to be assessed as two
dimensional structures based on the skew length of their spans. This limit has been
based on judgement, and may be subject to review when further research on this
subject has been undertaken.
Torsion in the piers of skewed multi-span structures due to non-uniform thrust at the
support positions is essentially a three-dimensional phenomenon which cannot be
readily dealt with by the two-dimensional methods of analysis most commonly used.
Since three-dimensional techniques for the assessment of multi-span structures are
unlikely to be commonly available for some time, torsional effects on piers may
remain a matter for qualitative assessment (i.e. do the piers exhibit any unusual
defects such as horizontal or inclined cracks which may be attributable to torsion).
6.1.7F Permissible Capacity
The permissible capacity for arch structures is dependent on single or multiple axle
loading which may be specified for particular trains in the brief.
The overall factor of safety on the Ultimate Capacity is based on the value of 3.4 given
in BD 21/97. In BD 21, this factor is derived by combining the live load partial factor,
γfL, with an impact factor of 1.8, whilst γf3 is assumed to be 1.0. Within this code, the
components of the overall factor have been separated and γfL set at 1.4 to maintain
consistency with other sections of the code. The maximum value of impact of 1.8
proposed in BD 21/97 has also been considered appropriate for railway structures,
and so to achieve the overall level of safety required γf3 has been increased to 1.35.
There is scope within this clause to adopt a lower value of impact factor which may
be appropriate in certain circumstances, such as when there is a large amount of fill
above the arch, or when trains cross the structure at low speeds.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 54 of 87

Permanent loads may produce a beneficial effect on the structure in some situations.
Two sets of analysis are therefore required, one with the partial factors on
permanent loads set to their maximum value and one using the minimum values of
the same parameters.
6.2F SINGLE SPAN STRUCTURES
6.2.1F The MEXE Method of Assessment
6.2.2.1F General
The MEXE method contained in this code has been adopted from the previous
version with some minor variations. The curves for determining Provisional Axle
Capacity are now presented in a metric format. The basis for these is given below. In
certain circumstances it may be more convenient to use the formulae presented to
calculate values of Qp directly rather by interpolation of Figures 6.7 to 6.12.
It is important to note that the MEXE method for railway structures has been
developed differently to that for highway structures as defined in BD 21/97.
Consequently, if the two methods were used to assess the same structure, different
answers would be obtained. To maintain the distinction between the two methods,
the nomenclature has not been harmonised with BD 21/97.
6.2.2.2F Provisional Axle Capacity (Figures 6.7 to 6.12)
General
The curves were prepared generally by reference to the methods proposed in “A
study of the MEXE approach to Masonry Arch Assessment” by A Kennedy and K A
Jenkins (2). Unfortunately the text of that report contains a number of errors,
although the curves produced in the report appear to be correct. Reference has also
been made to the 1976 Underbridge Assessment Code (3).
The new curves have been prepared in metric units resulting in several of the
equations being amended accordingly.
The steps and mathematical expressions used to prepare the curves are described
below.
Available Live Load Stress
The correct equation for available live load stress in imperial units is:
Pa =
where:
Pa

13.0 −

L2
L (h + d )
L3

+
672d
32d
3584d 2

(ton/ft²)

is the available live load stress (ton/ft²);

Equation F6.1

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex
L
d
h

RT/CE/C/025
Issue: 1
Date: February 2001
Page 55 of 87

is the span (ft);
is the thickness of arch ring at crown (ft);
is the depth of fill beneath sleeper soffit and arch ring at crown (ft).

This equation is derived from the original Pippard formulae (1 and 2). The equation’s
derivation is correctly described in Appendix G6 of the 1976 assessment code. This
equation converts into the following metric expression:
Pa =

1400 −

11L2 11L(h + d ) 11L3

+
21d
d
112d 2

(kN/m²)

Equation F6.2

where:
Pa
is in kN;
L ,h ,d are in metres.
Dead Load Stress
The above expression for available live load stress can produce values far in excess of
1400 kN/m2 when the span is large compared to the thickness at the crown.
Therefore, a check needs to be carried out for dead load overstress. The dead load
stress is calculated by extracting the relevant terms for dead load stress from the
equation for available live load stress:
3
11L  L
 + 11L
+
h
+
d


d  21
 112d 2
--------axial-------- -bending-

Pd =

Equation F6.3

When the dead load stress exceeds 1400 kN/m² the curves are terminated.
Longitudinal Loaded Length
The longitudinal loaded length is the equivalent loaded length on the arch due to the
spread of the load through the fill. The imperial expressions previously used are:
ls =

6h + 2.5
(ft)
3

Equation F6.4

lb =

6h + 25
(ft)
3

Equation F6.5

where:
ls
lb

is the loaded length due to axle load (ft);
is the loaded length due to bogie load (ft).

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 56 of 87

These equations convert into the following metric expressions:
ls =

2h + 0.25 (m)

Equation F6.6

lb =

2h + 2.5 (m)

Equation F6.7

Ratio of Loaded Length and Span
The following ratios are defined to simplify the expressions relating to the influence
lines for bending moment and horizontal thrust.
xs =

L − ls
2L

Equation F6.8

xb =

L − lb
2L

Equation F6.9

where:
xs ,xb

are the distances from the abutment to the edge of the loaded area
expressed as a fraction of the span.

These expressions are non-dimensional and therefore do not need converting for
metric units.
Influence Line for Bending Moment
An influence line curve for (bending moment/span) at the crown due to a unit point
load at a distance xL from the abutment is shown in the 1989(2) report. A similar
curve is shown in the 1976 assessment code.
A very close fit to this curve was obtained from the following expression for values of
x between 0 and 0.5 (illustrated in Figure F6.1):
F=

0.7 x 3 + 0.14 x 2 − 0.135 x (kN-m/kN-m)

Equation F6.10

Integrating this equation to obtain the area under the curve over the loaded length
produces the following expression after minor rationalisation:
A=

0.135 x 2 − 0.094 x 3 − 0.35 x 4 (kN-m*m/kN-m)

This expression is calculated for the axle and bogie loads as As and Ab .

Equation F6.10

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 57 of 87

0.055

F

0.0

0.5

1.0

-0.02
x

Figure F6.1
Bending Moment Influence Line
Influence Line for Horizontal Thrust
The 1989 report and 1976 assessment code show a curve for thrust at the arch
crown due to a unit point load at a distance xL from the abutment.
An approximation to the curve has previously been shown in those documents to be
a parabola with the following equation (illustrated in Figure F6.2):
T=

0.78( x − x 2 ) (kN/kN)

Equation F6.11

This expression is non-dimensional since it relates to the thrust at the crown
produced by a unit point load on the arch. It is a rearrangement of that shown in the
1989 report so that the thrust is always positive. This expression gives 0.25 times the
actual thrust for an arch with a span/rise ratio of 4. The equation has been left in this
format so that it is compatible with the previous reports and the Pippard equations.
The value of T is multiplied by 4 in the expressions for PUH in the later paragraph
entitled “Unit Live Load Horizontal Thrust Stress”.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 58 of 87

0.195

T

0
0

0.5

1

x

Figure F6.2
Horizontal Thrust Influence Line
Section Properties
Section properties for area and section modulus are shown in the 1989 report based
on a transverse load spread of 4 h + 4 (ft) for values of h≤1.5 (ft) and 2h + 7 (ft) for
values of 1.5 ≤ h ≤ 3.0 (ft). It is assumed that the depth of fill (h) does not exceed 3ft.
h ≤1.5 (ft)

B=
M=

1 . 5 < h ≤ 3 . 0 (ft)

B=
M=

( 4h + 4 )d (ft²)
d2
( 4h + 4 )
(ft³)
6

Equation F6.12

( 2h + 7 )d (ft²)
d2
( 2h + 7 )
(ft³)
6

Equation F6.14

Equation F6.13

Equation F6.15

where:
B
M

is the cross sectional area of loaded arch ring (ft²)
is the Section modulus of loaded arch ring (ft³)

These equations convert into the following metric expressions:
h ≤ 0.45 (m)

B=

( 4h +1.2 )d (m²)

M=

( 4h +1.2 )

d2
(m³)
6

Equation F6.16
Equation F6.17

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex
0.45 < h ≤ 0.9 (m)

B=

( 2h + 2.1)d (m²)

M=

( 2h + 2.1)

RT/CE/C/025
Issue: 1
Date: February 2001

d2
(m³)
6

Page 59 of 87
Equation F6.18
Equation F6.19

It is assumed that the depth of fill (h) does not exceed 0.9 metres.
Unit Live Load Bending Stress
The unit live load bending stress is calculated from the area under the bending
moment influence line as follows:
PUB =

AL2
(kN/m2/kN)
lM

Equation F6.20

These expressions are calculated for axle load and bogie load as PUBs and PUBb.
Unit Live Load Horizontal Thrust Stress
The unit live load horizontal thrust stress due to the effect of the load on the loaded
area of the influence line is calculated as follows. It relates to the properties of the
area under a parabola:
PUH =

4( 0.39 + T )
(kN/m2/kN)
3B

Equation F6.21

This expression is calculated for the axle and bogie loads as PUHs and PUHb.
Provisional Axle Capacity
The provisional axle capacity is obtained by dividing the available live load stress by
the sum of the unit live load bending and horizontal thrust stresses:
Cs =

Pa
(kN)
PUB s + PUH s

Equation F6.22

Cb =

0.5Pa
(kN)
PUBb + PUH b

Equation F6.23

The provisional axle capacity Qp is taken as the minimum of Cs or Cb and is shown on
the curves expressed in tonnes.
Profile Factor (Figure 6.13)
A curve for the profile factor Kp is shown in the 1989 report and the 1986 assessment
code. A close fit to these curves was found to be obtained from the following
expression which has been used for drawing the new curve Figure 6.13.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

Kp =

 L 
2.64 
 Rc 

RT/CE/C/025
Issue: 1
Date: February 2001
Page 60 of 87

−0.7

Equation F6.24

6.2.2.3F Modifying Factors for Provisional Axle Capacity
Three changes have been made to the modifying factors used in the MEXE method.
The first of these is the inclusion of a Material Factor (KM) for mass concrete. The
value of 1.2 adopted is consistent with that given in UIC 778-3R (4).
Secondly, further definition has been provided on the Crack Factor (KC) for lateral
and diagonal cracks. These definitions have been obtained from a report prepared by
the Working Party : Assessment of Underbridges (Strength of Masonry Arch Bridges
(5)
) dated 1971. This document contains a version of the MEXE method which was
rationalised prior to it being included in the 1976 issue of British Rail’s Assessment
Code.
The final modification is the removal of the span factor, which previously allowed
MEXE to be used for the assessment of multi-span arches on slender piers.
Hughes (6)and Melbourne (7) have both demonstrated that this approach could be nonconservative. It should be noted that MEXE may still be used for multi-span
structures where the piers can be shown to be ‘stocky’ and the criteria in Clause
6.2.1 are satisfied.
6.2.3F Other Methods of Analysis
6.2.3.1 General
i.
Mechanism analysis, although possible by hand, is generally carried out using a
proprietary software package. Packages in most common use at present are
ARCHIE, ARCH and ASSARC. In carrying out a mechanism analysis there
should be no restriction on the position of the line of thrust, provided it lies
within the arch ring (that is, the Geometric Factor of Safety should be set at
1.0).
ii.

Elastic analysis may be carried out by two methods:
(a)

Elastic analysis based on Castigliano’s Theorems, using computer
programme, CTAP.

(b)

Finite element method. The most appropriate computer program
using this method for arch assessment is MAFEA-ID.

These methods can be particularly useful since they give additional information on the
behaviour of the arch as it is loaded (including stresses and deflections).

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 61 of 87

6.2.3.2F Limitations
MAFEA-ID can take account of geometric non-linearity and hence is suitable for the
analysis of arches which may be susceptible to ‘snap through’ failure.
6.2.4F Advanced Analysis Methods
i.

The MAFEA program also has a 2-D plane strain version which is suitable for
investigating the effects of ring separation. This method has not been
approved by Railtrack and requires permission of the Professional Head of
Structures Engineering before its use. Other finite element packages which
have not been developed exclusively for arch assessment may also be suitable.

ii.

It is envisaged that a full 3-D modelling is rarely required. When it is required
it will be necessary first to establish firm guidelines for the analytical process
with Railtrack’s Assessment Engineer.

6.3F MULTI SPAN STRUCTURES
6.3.1F Modes of Failure
The limiting pier thickness of h/2 has been adopted based on the work carried out
hitherto for the Bridgeguard 3 assessment programme.
An alternative expression for limiting pier thickness based on research carried out by
Hughes is as follows:
220d 1.792 h 0.632 (f + d ) (w + s )
r 0.185 s 5.937
0.735

t lim =

3.963

Equation F6.25

where:
d
h
f
s
r
w

is the arch ring thickness;
is the height of the pier;
is the depth of fill from underside of sleeper to arch crown;
is the clear span length between the faces of successive supports;
is the arch rise at mid span;
is the loaded length at the underside of sleeper level.

The above expression has been developed assuming constant values of all parameters
throughout adjacent spans. Where these vary, the largest values of d , h , f , w and r and
the smallest value of s should be adopted.
Use of the above expression may be considered but requires permission of the
Professional Head of Structures Engineering.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

w

Page 62 of 87

Level at underside
of sleepers

p

f

r

d
t
h

s

Figure F6.3
Definition of Parameters for Calculations of tlim
6.3.3F Assessment for Failure Mode 2
Sub paragraph (i) describes computer analysis program MULTI, which is part of the
ARCHIE suite of programs. This analysis involves an interactive, iterative process
whereby the position of the thrust line within each of the unloaded spans is adjusted
in an effort to ensure that it does not fall outside a pre-determined zone within the
arch ring and piers. Details of this method are outlined in the paper by Harvey and
Smith (8).
A mechanism analysis of the type described in sub paragraph (ii) can be carried out by
spreadsheet. The calculation process is described by Hughes(6).
6.3.5F Advanced Assessment Methods
Multi-span structures cannot be modelled satisfactorily three-dimensionally at present
due to limitations of current computing power. At present, the only viable option is
to use two-dimensional plane strain finite element modelling.
6.4F SPANDREL WALLS
Although there has been some recent research on the assessment of spandrel walls
for lateral loading, as well as on their interaction with the arch barrel, appropriate
assessment techniques have yet to be established. Spandrel walls should, therefore,
continue to be assessed on a qualitative basis.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 63 of 87

REFERENCES
1.

DUBOST A. Masonry Structures project: Pressure Distribution in the Arch Fill
Material. British Rail research Report IM CES 055, 1990;

2.

KENNEDY, A and JENKINS, KA. A Study of the MEXE Approach to Masonry
Arch Assessment.. British Rail research Report TM/CES/118, 1989;

3.

BRB. Assessment of Live Load Carrying capcity of Underbridges. British Railway
Board, 1976;

4.

INTERNATIONAL UNION of RAILWAYS. Recommendations for the
Assessment of the Live Load Carrying Capacity of existing Masonry and Mass
Concrete Arches. UIC 778-3R (1st Edition), 1995;

5.

BRITISH RAIL LONDON MIDLAND REGION. Working Party: Assessment of
Underbridges (Strength of Masonry Arch underbridges), Final Report on Suggested
Method of Analysis. 1971;

6.

HUGHES, TG. Analysis of Twin Masonry Arch Bridges. Proceedings of The
Institution of Civil Engineers, Structures and Buildings, November 1995;

7.

MELBOURNE, C, GILBERT, M and WAGSTAFF, M. The Collapse Behaviour of
Multispan Brickwork Arch Bridges. The Structural Engineer, 2 September 1997;

8.

HARVEY, WJ and SMITH, FW. The Behaviour and Assessment of Multispan
Arches. The Structural Engineer, 17 December 1991.

Other documents which have been referred to in the development of Section 6 Masonry Arches are:
9.

HEYMAN, J. The Masonry Arch. Ellis Horwood Ltd., 1982, ISBN 0853125015;

10.

PAGE, J. Masonry Arch Bridges - State of the Art Review. TRL, 1993,
ISBN 0115511903;

11.

HARVEY, WJ. Application of the Mechanism Analysis to Masonry Arches. The
Structural Engineer, 1 March 1988;

12.

SMITH, FW, HARVEY, WJ and VARDY, AE. Three hinge Analysis of Masonry
Arches. The Structural Engineer, 5 June 1990;

13.

GILBERT, M and MELBOURNE, C. Rigid-block Analysis of Masonry Structures.
The Structural Engineer, 1 November 1994;

14.

MELBOURNE, C and GILBERT, M. The Behaviour of Multiring brickwork Arch
Bridges. The Structural Engineer, 7 February 1995;

15.

HARVEY, WJ. The Origin and treatment of Longitudinal Cracks in Masonry
Arches. The Structural Engineer, 5 December 1995;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 64 of 87

16.

HUGHES, TG and BLACKLER, MJ. A Review of UK Masonry Arch Assessment
Methods. Proceedings of The Institution of Civil Engineers, Structures and
Buildings, August 1997;

17.

NG, KH, FAIRFIELD, CA and SIBBALD, A. Finite Element Analysis of Masonry
arch Bridges. Proceedings of The Institution of Civil Engineers, Structures and
Buildings, May 1999;

18.

HENDRY, AW. Masonry Properties for Assessing Arch Bridges. TRRL
Contractor Report 244, 1990;

19.

BROWNHEAD, SF. Review of Arch Assessment Methods. British Rail Research
Report RR-CES-007, 1991;

20.

CLARK, GW. Serviceavility of Brick Masonry. British Rail Research Report LRCES-151, 1994;

21.

THOMPSON, DR. Spandrel Wall Failures. British Rail Research Report
RR-ICE-009, 1995;

22.

DEAN, PA. The Deflection of Arches. British Rail Research Report
ICE-095, 1997.

RR-

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 65 of 87

SECTION 7 AND APPENDIX B - CONCRETE STRUCTURES
7.2.1F Introduction
The concrete section of this underbridge Code is based largely on BD 44/95: The
Assessment of Concrete Highway Bridges and Structures. BD 44/95 is currently being
revised. Some of the changes to BD 44/95 are anticipated in this Code but other
changes have been proposed since this Code was prepared and are therefore not
included. It was considered desirable to incorporate the known changes included in
this Code to reduce unnecessary differences between the documents. In addition
the changes have technical merit, to and may help to avoid unnecessary strengthening
work and/or reduce requirements for Departures from Standard. Some of the
background and justification for the more significant changes is given here. For
further general guidance refer to BA 44/96.
7.2.2.3F Material Partial Factor for Steel
BS 5400: Part 4, CP 110: 1972 and BS 8110:1985 all used a value for partial factor γm
for reinforcement and prestressing steel of 1.15. This value compares with the factor
of 1.05 used in BS 5400: Part 3 and BS 5950 for structural steel. This difference is
difficult to justify. Following extensive statistical investigation by Beeby(1), it was
concluded that the lower factor could be justified for reinforcement and prestressing
steel as well. The change has therefore been incorporated into the amendment
AMD9882 of BS 8110.
During the review of BD 44/95 it was decided that some account of this change
should be made. However a rigorous statistical investigation of all the steel sources
that could be found in assessment of existing highway structures could not be
undertaken. The slightly more cautious factor of 1.1, which is consistent with
Eurocode 2 (EC2)(2), has therefore been adopted. Theoretically, since modern
practice is to specify “characteristic” yield strength whereas past practice used
“guaranteed minimum” values, the factor could justifiably be smaller than in design.
GUIDANCE ON APPENDIX B
4.3.3BF Values of γm
Refer to Clause 7.2.2.3F above.
5.3.3.3BF Short Anchorage Lengths
2.5 diameters is the limit of test data. The rule may be safe with even shorter
anchorages but it has not been tested. A version of this rule was originally proposed
by Gifford and Partners based on work done on ASR affected specimens. The
present version is a modification following more recent work by Clark et al (3) and
Cullington et al (4).

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 66 of 87

5.3.3.5BF Varying Angle Truss Approach
Current codes use the addition principle to design links or to assess the shear
strength of members with links. The contribution of the links to shear strength is
calculated using a 45° truss analogy and it is added to the calculated strength of the
section without links. Historically, this approach was used because the 45° truss
analogy had been found to give conservative results. It had no theoretical
justification. Indeed, the mechanism of transmitting shear in sections with significant
links is fundamentally different from that in sections without. One consequence of
the differences is that the addition approach over-estimates the strength of sections
with small areas of links. Such links actually have very little effect. Codes for design,
however, can overcome this problem by specifying minimum link areas below which
links are assumed ineffective.
An alternative is to assume all the shear is taken on the truss mechanism but allow
the angle of the truss to be optimised for maximum strength. This approach has the
great advantage that, unlike other methods, it does have a theoretical justification
which is explained by Nielsen (5). However, it is based on plastic theory. It is
debatable whether the assumptions of this approach (principally ductility) are justified.
Calibration against tests has been done. The results showed a need to apply an
“effectiveness factor” and to limit the angle of the truss. With these limitations the
approach was specified in the 1990 CEB/FIP code (6) and is given as an option in EC2.
It can give significantly greater shear strengths than the addition approach particularly
for sections with large areas of links. The approach included in this Code is based on
EC2.
The approach does has two implications. Firstly, the use of flatter angles to increase
the shear strength for a given link area implies a greater force in the main flexural
reinforcement. This limits the value of the approach in members with curtailments.
The second implication is that, rather than having an essentially empirical “web
crushing limit”, it should theoretically be possible to check the crushing strength of
the inclined concrete struts in the same way as the compressive strength of any other
concrete compression member is checked. However, tests show that this approach
over-estimates strength, hence the need for the “effectiveness factor”.
Statistical analysis suggests that the varying angle truss approach gives much better
correlation with test results than other methods of predicting shear strength.
5.4.1B Moments and Shear Forces in Slabs
References for the particular elastic analysis and yield line methods are listed below:
PUCHER A., Influence Surfaces of Elastic Plates. Springer Verlag, Wien and New York
1964, ISBN 3211811389;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 67 of 87

WESTERGAARD, HM. Computation of Stresses in Bridge Deck Slabs due to Wheel
Loads. Public Roads, Volume 2, Number 1, March 1930;
JOHANSEN, KW. Yield Line Theory. CACA London. 1967.
6.3.4.2BF Equation B53
This equation is based on calculating the maximum principal tensile stress at the
centroid in a rectangular section using normal elastic beam theory. The maximum
shear stress for a rectangular section is 1.5 times the average shear stress (V/bh),
hence the 0.67 in the equation.
For flanged sections this code takes “b” to be the web width but still includes the 0.67
factor. With very heavily flanged sections, the theoretical maximum shear stress can
be as little as 1.05B/bh. Hence the equation is conservative. However, if the section
is not under a uniform longitudinal stress (that is, if it is subjected to bending) the
theoretical maximum principal tensile stress does not necessarily arise at the neutral
axis and can be greater than predicted by the equation. In this situation, the only
justification for the equation is calibration against tests. The implementation
therefore used either the actual calculated maximum principal tensile stress (justified
by theory) or that calculated by Equation 28 (justified by tests). There is evidence
that the approach is still conservative for flanged sections. Using the maximum
principal tensile stress at the neutral axis may be safe for flanged sections too but this
has not been proved.
6.3.4.5BF Maximum Shear Force
BD 44/95 relates the web crushing limit to the square root of concrete compressive
strength. This appears illogical. The limit is based on a crushing failure. It might
therefore be expected to be related directly to crushing strength. This relationship is
implied by the work on varying truss analogy mentioned above. Some research (7) has
indicated that the effectiveness factor reduces with increasing concrete strength but
the overall sensitivity to remaining concrete strength is invariably greater than the
square root relationship.
Batchelor, George and Campbell (7) undertook a major statistical study of all the
available web crushing research on slender webs, and found that concrete strength
was not a factor in the effectiveness factor. The crushing strength is therefore
directly proportional to concrete strength. For 50N concrete and typical web
thickness to depth ratios, their work suggests that the crushing strength could be as
much as 70% greater than calculation to BS 5400: Part 4.
6.7.2BF Loss of Prestress other than Friction Loses
Specialist literature that can be referred to is contained in references 2 and 8.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 68 of 87

REFERENCES
1.

BEEBY, AW. Partial Safety Factors for Reinforcement. The Structural Engineer,
Volume 72 No. 20.18, October 1994;

2.

ENV 1992 -1 -1 Eurocode 2: Design of Concrete Structures;

3.

CLARKE, LA, BALDWIN, MI and GUI, M. Assessment of Concrete Bridges with
Inadequately Anchored Reinforcement. Bridge Management 3. London, E&FN
Spon. 1996. pp 667-674. pp. 225-232;

4.

CULLINGTON, DW, DALY, AF and HILL, ME. Assessment of Reinforced
Concrete Bridges: Collapse Tests on Thurloxton Underpass. Bridge Management 3.
London, E&FN Spon. 1996. pp. 667-674;

5.

NIELSEN, MP. Limit Analysis and Concrete Plasticity, Prentice Hall, 1984,
ISBN 0849391261;

6.

CEB/FIP: Model Code for Concrete Strucutres 1993, ref MC90;

7.

BATCHELOR, BdeV, GEORGE, HK and CAMPBELL, TE. Effectiveness Factor
for Shear in Concrete Beams. Journal of the Structural Engineering Division,
ASCE Vol. 112, No 6. June 1986. pp. 1464-1477;

8.

NEVILLE, AM. Creep of Concrete Plain Reinforced and Prestressed. Amsterdam
North Holland Publishing Company, 1970.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 69 of 87

SECTION 8 - COMPOSITE STRUCTURES
8.1F Introduction
The number of modern composite underbridges owned by Railtrack are small
compared with other metal bridges. The first was bridge no 59, Bushey Arches West
Coast Main Line built in 1963. Most are simply-supported deck-type plate or box
girders with insitu or precast floor. After 1967 these bridges are likely to have been
designed to CP 117: Part 2, and should be amenable to assessment using a BS 5400:
Part 5 approach since that code followed CP 117.
Modern half-through bridges built since the early 1950’s, especially ‘E’ type, have filler
beam cross girders, which were usually not designed for composite behaviour.
However, their construction often included reinforcement passing through web holes
thus providing some composite resistance. A significant number of underbridges are
of cased or filler beam construction. This is catered for in BS 5400: Part 5.
8.2F Limit state approach
Appendix C is based on BS 5400: Part 5, and provides a limit state approach to
assessment. It is written as a supplementary code to BS 5400: Part 5 in a similar way
to Section 5 for steel and wrought iron. For the steel (or wrought iron) and concrete
elements, Sections 5 and 7 respectively are referred to in a similar way to BS 5400.
Appendix C adopts some amendments due to BD 16/82: Design of Composite Highway
Bridges. Use of BS 5400: Part 5. Reference has also been made to BD 61/96: The
Assessment of Composite Highway Bridges. BD 61/96 is however considered to be too
unwieldy and over-mathematical for assessment of most rail underbridges. It uses the
serviceability limit state extensively, whereas it is considered that for assessment of
most rail underbridges, only the ultimate limit state (ULS) should be checked. The
serviceability limit state (SLS) should be checked only where it is clearly necessary, for
example for track twist and for fatigue. Some information relating to different forms
of shear connection has been adopted from BD 61/96.
The SLS requirements in BS 5400: Part 5 for checking concrete crack widths,
deflections and strength of the shear connection are not included in Appendix C.
Only in unusual cases, for example when the spacing of shear connectors is excessive,
would it be necessary to check serviceability. To compensate for the removal of the
SLS check on the shear connection, ULS checks have been introduced which have the
same basis as for SLS in BS 5400: Part 5, i.e. elastic distribution of horizontal shear,
uncracked modulus for deriving horizontal shear and similar global analysis.
Composite behaviour may be assumed in cased and filler beams provided the
specified bond stress is not exceeded. The resistance to horizontal shear can be
enhanced where transverse bars are continuous through holes in the web as found in

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 70 of 87

many ‘E’ type bridges. The SLS bending stress given in BS 5400: Part 5 is increased to
a value which is applicable to the ULS.
Some of the additions and amendments in Appendix C to BS 5400: Part 5 are based
on BD 16/82 and BD 61/96. Acknowledgement is made to the Highways Agency for
allowing these standards to be referred to so as to form a common basis between the
assessment of rail underbridges and highway.
GUIDANCE NOTES TO APPENDIX C
4.3.2CF Serviceability Limit State
Checks for serviceability are compatible with Sections 5 and 7. It is desirable to
eliminate them as far as possible. The recommendations of Clause 8.2F virtually
eliminate their need in practice.
5.2.6CF Control of Cracking in Concrete
Crack control checks should not be necessary for assessment where inspection is
carried out followed by maintenance as necessary.
6.1.2CF Deck Slabs Forming the Flanges of Composite Beams
This accords with BS 5400: Part 4 and Eurocode 4(1). The 75% criterion is based on
judgement.
6.3.3.3CF Interaction between Longitudinal Shear & Transverse Bending
This Clause was misprinted in BS 5400: Part 5.
6.3.4CF Shear Connectors
The static strength values are as BS 5400: Part 5. In practice assuming γ FL = 1.4 at
ULS and γ fL = 1.1 at SLS for live load, the overall safety factor compares thus:
At ULS

(Q × γ

fL

At SLS

(Q × γ

fL

1. 1 
 γ 
× γ f 3 ) m  = (1.4 ×1.1)
=
 0. 8 
 0.8Pu 

1.925

1.0 
 γ 
× γ f 3 ) m  = (1.1×1.0 ) 
=
 0.55 
 0.55Pu 

2..00

This is a conservative approach because no advantage has been taken of the
redistribution between connectors, that occurs at ULS, resulting in a tendency
towards a uniform rather than triangular distribution of horizontal shear between the
point of zero shear and the end of the beam.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 71 of 87

It appears inappropriate to consider composite behaviour for type ‘A’ decks, (see
Clause 5.2.2F), due to the discontinuity of concrete which tends to be subject to
global tension from the main girder bottom flange. These decks generally have
200 mm deep beams. For beams deeper than say 300 mm, it is probably reasonable
to assume some composite behaviour.
8.5.2CF Cased Beams
The values of local bond stress for SLS given in BS 5400: Part 5 have been increased
by approximately:
0.8Pu 1.1
=
0.55Pu

1.32

The increase is proportional to the relative ULS/SLS capacities of shear connectors.
8.5.3CF Filler beams
For filler beams the shear capacity includes the transverse bars through the beam
web in addition to bond as in Expressions C9 and C10. Expression C9 allows for
shear strength of the reinforcement taken as 1.4 Aw f ry . This takes the double shear
capacity of the reinforcement 0.7 Ae f ry as in BS 5400: Part 5 Clause 6.3.3.2 and which
approximates to

f ry

2 ×1.05
bolt to BS 5400: Part 3.

which is similar to a calculation for the shear strength of a

Expression C10 can be compared with the bearing capacity of a bolt to BS 5400: Part
3 Clause 14.5.3.6, which is
0.85 × 2.5×1.2 ×1.0σ y
γ m =1.05

= 2.43σy

but 2.0σy is used conservatively.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 72 of 87

REFERENCES
1.

ENV 1994 -1 -1 Eurocode 4: Part 2 Design of Composite Steel and Concrete
Structures;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 73 of 87

SECTION 9 - CAST IRON STRUCTURES
9.1F General
The previous version of this code gave little guidance on the assessment of cast iron
structures, merely providing stress limits for tension and compression in bending and
shear. Section 9 of this code is based on the Department of Transport Standard BD
21/97 which gives more specific guidance. In addition Section 9 contains provisions
for the assessment of members subject to combined axial load and bending, such as
arch ribs. There are known to be a small number of such members in service within
the existing Railtrack bridge stock.
Since the failure of cast iron members in tension is always brittle and sudden,
Section 9 gives provisions for assessment based on working loads and permissible
stresses.
In determining the dynamic factor for the rib of an arch structure in accordance with
Clause 4.3.2.2, it should be noted that the fundamental mode of vibration generally
involves anti-symmetric bending of the arch rib and any associated stiffening members
within the spandrels. The simplified expression for natural frequency given in Clause
4.3.2.2 is based on whole span bending which represents a higher mode of vibration
for an arch structure, and will result in a higher value of dynamic factor. For such
arch structures, therefore, a more rigorous approach will be required to determine
the lowest natural frequency.
9.2F Material Properties
9.2.1F Elastic Modulus
Cast iron is a highly variable material whose properties are primarily influenced by
the mix proportions and the rate of cooling during manufacture. In addition, it does
not behave in a linear elastic manner under load. The elastic modulus, therefore,
typically lies anywhere within the range 90,000 N/mm2 to 138,000 N/mm2. Provided
that stresses are kept within the limits outlined in Clauses 9.3.1, the degree of nonlinearity is not significant, and a mean value of E may be adopted for global analysis.
9.3F Strength
9.3.2F Beams Continuously Restrained by Surrounding Fill
This clause takes account of the fact that composite action may occur between a
girder and surrounding fill in which it is firmly embedded. It is considered unlikely
that this clause will be required for railway bridges. It is understood that there are no
longer any cast iron girder bridges remaining within the Railtrack bridge stock.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 74 of 87

9.3.3F Beams with Intermediate Lateral Restraints
Clause 9.3.3 relates to members which are susceptible to lateral torsional buckling
and has been included primarily to cater for cast iron arch ribs which are subject to
combined bending and axial load. Since no recognised method is available for
determining the limiting compressive strength, an appropriate method should be
established during the Technical Approval process. One possible method, which is
considered to be conservative, may involve the adoption of the curve presented in
Figure 9.1F. This is based on the Gordon-Rankine equation, which is generally used
for the assessment of cast iron struts, with a maximum value of 154N/mm2 at
λ LT = 30 . The value of λ LT was chosen to be compatible with Figure A10 for steel
and wrought iron elements. It should be noted that Figure 9.1F has not been
validated by tests.
If the procedure outlined in the preceding paragraph is adopted, a less conservative
value of the slenderness parameter, λ LT , may be obtained by determining the buckling
parameter (k4) using Appendix B2.5 of BS 5950: Part 1, where it is defined as u. This
is likely to be beneficial as cast iron members tend to have relatively stocky
proportions, with the web and flanges often being of similar thicknesses.
9.3.4F Compression Members
The Gordon-Rankine equation, which is considered to be the strut formula most
applicable to cast iron, is presented. It is an empirical formula which was developed
in the second half of the 19th century. It was also applicable to wrought iron and
steel, but was found to be most appropriate for cast iron. The current formula
includes a factor of safety of 5, which recent research has indicated may be
conservative.
9.3.5F Members Subject to Bending and Axial Compression
A rule for combining axial and bending stresses in compression is given together with
a re-arranged version of the Rankine-Gordon equation for calculating permissible
stresses in axial compression.
9.3.6F Restraints to Elements in Compression
Refers to Section 5 to determine forces to be used in assessing the adequacy of
bracing members.
9.4F Fatigue
The equations placing additional limits on the permissible stresses given in Clause 9.4
are identical to those given in BD 21/97. Although their exact derivation is unknown,
sufficient evidence exists to suggest that these equations ensure the live load stresses
are always below the non-propagating stress. In determining the live load effects
appropriate to this Clause, the dynamic factor for fatigue, as defined in Table 4.5,
should be adopted.

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 75 of 87

160

150

140

130

120

110

100

90

pbc
(N/mm2)

80

70

60

50

40

30

20

10

0
0

50

100

150

200

250

λLT

Figure 9.1F
Permissible Compressive Stress due to Bending
of Beams with Intermediate Lateral Restraints

300

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 76 of 87

REFERENCES
1.

THE HIGHWAYS AGENCY. BD 21/97: The Assessment of Highway Bridges
and Structures. The Stationery Office, 1997, ISBN 0115519076;

2.

CHETTOE, CJ, DAVEY, N and MITCHELL, GR. The Strength of Cast Iron
Girder Bridges. Journal of The Institution of Civil Engineers No. 8, October
1944;

3.

BUSSELL, M. Appraisal of Existing Iron and Steel Structures. Steel Construction
Institute Publication 138, ISBN 1859420095;

4.

AITKEN, WK. East Coast Main Line, Bridge No. 184, River Nene Cast Iron
Viaduct, Peterborough. British Railways Board Research and Development
Division, Track Group, Technical Memorandum TM TS 55;

5.

AITKEN, WK. Cast Iron Railway Bridges - Properties of Cast Iron. British
Railways Board Research and Development Division, Track and Structures
Section, Technical Memorandum TM TS 19, December 1972;

6.

AITKEN, WK. Cast Iron Railway Bridges - Properties of Cast Iron. British
Railways Board Research and Development Division, Track and Structures
Section, Technical Memorandum TS 20, September 1973;

7.

SWAILES, T. 19th Cast Iron Beams: their Design, Maintenance and Reliability.
The Proceedings of The Institution of Civil Engineers, Civil Engineering,
February 1996;

8.

SWAILES, T and MARSH ,J. Structural Appraisal of Iron Framed Textile Mills.
ICE Design and Practice Guides - the Institution of Civil Engineers, 1998, ISBN
0727727133.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 77 of 87

SECTION 10 - TIMBER
10.1F Introduction
The majority of timber rail underbridges to be assessed take the form of longitudinal
main beams supporting transverse timber decking with track on longitudinal sleepers.
Timber decking is also used to support ballasted track on timber and metal bridges.
Substructures including abutments for these bridges are also likely to be of timber
comprising a timber crosshead carried on a number of braced pile bents.
10.2F Permissible Stress Approach
British Standard BS 5268: Part 2: 1996 “Structural Use of Timber, Part 2, Code of
Practice for Permissible Stress Design, Materials and Workmanship”(1) is written
primarily for timber applications within buildings and associated structures; in the
absence of bridge specific codes BS 5268 should be used for guidance.
The permissible stress principles for the design of timber structures is the approach
familiar to Engineers. North American and Canadian practice is similar.
10.4.5F Impact and Load Duration Factors
The allowable load for timber varies with the length of time the load is applied. The
shorter the duration of load, the higher the allowable load that can be sustained by
the structure. The timber grade stresses and joint strengths given in BS 5268: Part 2:
1996 are applicable to long-term loading and are increased for shorter-term loading
by use of modification factors. However, it is the cumulative effect of the individual
loading occurrences that determines the allowable load; this is not stated in BS 5268.
It is necessary, therefore, to arrive at the most appropriate duration modification
factor.
North American and Canadian practice is similar to that in the UK. In AASHTO
“Standard Specification for Highway Bridges”(2) stresses are based on a normal load
duration which contemplates that the member is stressed to the maximum stress
level, either continuously or cumulatively, for a period of approximately 10 years,
and/or stressed to 90% of the maximum design level continuously. Modification
factors are provided for either cumulative or continuous periods other than 10 years.
These factors are reproduced in Table 10F.1

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex

Page 78 of 87

Load Duration

CD

Permanent
10 years
2 months (vehicle live load)
7 days
1 day
5 minutes (railings only)

0.9
1.0
1.15
1.25
1.33
1.65

Table 10F.1
Load Duration Factor, DD
American railway engineering practice(3) adopts a different approach which is related
to consideration of impact due to live load:
“The dynamic increment of load due to the effects of speed, roll and track
irregularities is not well established for timber structures. Its total effect is
estimated to be less than the increased strength of timber for the short
cumulative duration of loading to which railroad bridges are subject in service
and is taken into consideration in the derivation of allowable working stresses
for design”.
Stresses given in the AREMA manual equate to 90% the value of stress quoted in
AASHTO for highway bridge design. The railway value of stress includes a 0.9 form
factor. In AASHTO impact effects are not added to the static loads for timber
structures.
It can be argued from the American approach that increases in working stress due to
short duration loading is at least equivalent to the magnitude of impact that could
have been applied. However, it is advantageous in assessment to include impact in
the loading effects so that variabilities due to speed can be included. There may be a
case for placing a limit on values for impact, especially for high speed lines where the
value of assessment impact can be higher than UIC impact(4) for a new bridge.
Similarly for decking, where timbers are likely to be short spans and continuous over
a number of bays, the American approach could be adopted.
Values of impact in accordance with different practices are as follows:

RAILTRACK LINE CODE OF PRACTICE

RT/CE/C/025
Issue: 1
Date: February 2001

The Structural Assessment of Underbridges
Appendix F - Informative Annex
U.K.

Page 79 of 87
U.S.A.

SPEED
(MPH)

UIC CODE
FOR
NEW
BRIDGES

776-1R FOR
ASSESSMENT

DIESEL
LOCOS

STEAM
LOCOS

10

1.69

1.06

1.28

1.32

20

1.69

1.13

1.39

1.49

30

1.69

1.22

1.48

1.62

40

1.69

1.30

1.54

1.71

50

1.69

1.38

1.58

1.77

60

1.69

1.45

1.59

1.79

Table 10F.1
Values of Imapct for Various Practices
A speed-related impact allows live capacity to be maintained or improved at reduced
speed if necessary. A duration factor of 1.5 is proposed.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 80 of 87

REFERENCES
1.

BSI, BS 5268: Part 2 “Structural Use of Timber, Part 2, Code of Practice for
Permissible Stress Design, Materials and Workmanship”, British Standards
Institution, 1996;

2.

AASHTO. Standard Specification for Highway Bridges. American Association of
State Highway and Transportation Officials, 16th Edition, 1996,
ISBN 1560510404;

3.

AREMA. AREMA Manual for Railway Engineering. American Railway Engineering
and Maintenance of Way Association, CD Rom, 2000;

4.

UIC. UIC776-1R Loads to be considered in the Design of Railway Bridges.
International Union of Railways, 1990;

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 81 of 87

SECTION 11 - SUBSTRUCTURES
11.2F Assessment
To allow the calculation of lateral pressures on earth retaining structures due to
varying levels of live loading, Table 11.1 containing live load surcharge loading has
been included. The values in this table are based on the total axle loads from the
locomotive component of the Type RA1 load train (four axles, each of 200 kN) acting
over an area defined by it’s length (7.85 metres) and the effective width of a sleeper
(2.6 metres).

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex
SECTION 12 - BEARINGS
No additional commentary required.

RT/CE/C/025
Issue: 1
Date: February 2001
Page 82 of 87

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 83 of 87

APPENDIX D - FATIGUE ASSESSMENT OF STEEL AND WROUGHT
IRON
1.1DF. GENERAL
General philosophy for fatigue assessment follows the Highways Agency draft
standard for steel bridge fatigue life assessment (1), which has a similar to the
AASHTO guide specifications for fatigue evaluation of existing steel bridges (2).
Appendix F comprises a series of amendments to BS 5400: Part 10 (3) as being in a
form with which designers will be familiar in a similar way that Section 5 is an
adaptation of BS 5400: Part 3.
4.1DF Definitions
The concept of ‘damage tolerant’ and ‘safe-life’ elements is adopted. ‘Damage
tolerant’ elements are those which have capacity for redistribution or the permanent
way is capable of in effect spanning the gap left by a failure of the element, whereas
‘safe life’ elements have no ability to redistribute load in the event of failure. For rail
underbridges longitudinal members spanning less than 3 metres or cross girders
carrying only a single track can normally be taken as ‘damage-tolerant’. This will
effectively enhance the fatigue capacity of short span members such as railbearers and
single track cross girders through reduction of the calculated fatigue stresses by
application of the factors γ1 and γ 2 as used by the Highways Agency draft. Factor γ1
allows a reduction in applied fatigue stresses (i.e. γ1 < 1.0) where an advanced analysis
is used such as 3D finite element analysis or where strains are measured directly such
that secondary effects are taken into account. Factor γ 2 takes account of whether
the element is ‘damage tolerant’ or ‘safe life’ and whether access is feasible for
inspection against fatigue defects. In effect γ 2 modifies the σ - N (stress against
numbers of cycles) capacity by adjusting the probability factor from the minus 2
standard deviations below mean of fatigue results used in BS 5400: Part 10. The
value of γ 2 is approximately equivalent to failure probability as below using the
relationships given by BS 5400: Part 10, Appendix A.
Approximate
value of γ 2

Probability
of failure

0.75
0.80
0.86
1.0
1.25

50%
31%
16%
2.3%
0.14%

Number of
deviations below
the mean, d
0
0.5
1.0
2.0
3.0

Table D1F
Failure Probability

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 84 of 87

For ‘safe life’ elements which are accessible the BS 5400: Part 10, BS 7608: 1993 (4)
and ECCS recommendations (5) on new design is used, i.e. the minus 2 standard
deviations corresponding to a 2.3% probability of failure, i.e. γ 2 = 1.0
4.4DF Method of Assessment
Fatigue assessment is carried out in stages varying from elementary calculations
comparing the live load stress range with a cut off stress, through simplified
calculations to a full design spectrum and consideration of measured stresses. If at
any of these stages fatigue life is found to be satisfactory then further fatigue
consideration is not required.
Stages are:
A - identify fatigue criticality by inspection and cut-off stress - The live load stress
range multiplied by γ1 γ2 is compared with σco the cut-off stress, below which no
fatigue damage should occur such that no further calculation is necessary. At this
stage any particular fatigue defects such as cracks must be noted and acted upon.
It is important to appreciate that any calculations using Appendix D may be invalid
in the presence of significant fatigue cracking;
B - damage calculation to particular spectrum - Stage B involves use of the standard
load spectra of BS 5400: Part 10, Table 2 which is based on the heavy, medium or
light traffic types with standard trains as in BS 5400: Part 10, Appendix E (BS
5400: Part 10, Clause 9.2 is not applicable because it does not cater for applied
stresses below σco ). It should be noted that Table 2 is strictly only applicable to
simply supported spans. In critical cases then a Stage C assessment should be
carried out in verification. Use of Table 2 involves a modification to the load
proportions Kw in that RA unit not RU loading is applied, and that Section 4
applies a minimum length of 4 metres for the calculation of the dynamic factor
(1+ϕ), whereas the values of Kw in Table 2 imply higher impact factors for lengths
less than 4 metres;
C - damage calculation to particular spectrum (similar to BS 5400: 10 Clause 9.3) Stage C may be used to evaluate the residual fatigue life for typical trains such as
figure 19 of BS 5400: Part 10 with numbers of trains as in Table 15. It represents
a more accurate assessment by use of the rainflow method compared with Stage
B. Stage C may also be used to evaluate fatigue under real trains representing
past, present and future traffic. If 3D finite element analysis is carried out which
properly represents the stresses itemised in BS 5400: Part 10, Clause 6.1.5 (but

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 85 of 87

excluding residual stress and stress concentrations as itemised in BS 5400: Part
10, Clause 6.1.6) then γ1 may be assumed as 0.95;
D - assessment using measured strains - Strain measurements in the vicinities of
potentially fatigue prone details may be used to verify theoretical calculations for
an assessment carried out under Stage C. The strains should be recorded during
the slow passages or under stationary vehicles of known geometries and axle
loads and compared with those calculated by elastic methods. Adjustments
should be made to account for any discrepancies detected. Where the stresses
adjacent to the relevant detail are recorded at a distance from the detail such as
to avoid any local stress concentration, any magnification due to local
discontinuities should be subsequently allowed for in accordance with BS 5400:
Part 10. In appropriate cases the strains may be recorded in the locations of peak
strains and the corresponding stresses used directly in the assessment.
Strains at locations in which uniaxial stresses occur may be recorded by portable
extensometers when records are to be taken at increments of movement of a
test vehicle. These have the advantage of being mountable without removal of
protective coatings, but are not suitable for recording spectra under moving
traffic. However, mechanical strain range counting instruments are available
which may be used for deriving spectra. Where principal stresses may not be
derived from uniaxial strains electrical resistance strain gauge rosettes may be
used to derive principal stress ranges. Measurement of local strains in the
vicinities of stress concentrations requires the use of gauges of very short gauge
lengths and electrical resistance gauges are most appropriate for such purposes.
These require removal of any protective coatings and careful preparation of
surfaces before bonding and subsequent weather protection of the gauges and
their connections to the electrical circuits. They have the advantage that once
installed recording and processing of measurements may be undertaken remotely
from the gauge positions. Any gauges and their associated equipment should be
calibrated before use.
11.3.DF σ-N Relationship
σ-N relationships are assumed as in BS 5400: Part 10 for steelwork, but modified by
introduction of the cut-off stress σco as noted above. For wrought iron, the results
reviewed by Beagles (6) have been used to determine a σ-N relationship for plain and
riveted wrought iron. The proposed σ-N curve for riveted wrought iron is:
Log10 σ R = 3.393 – ¼ Log10 N
The value of σo is retained at 1 x 107 cycles as in BS 5400: Part 10 but with a slope
m=4 to fit the Beagles data with a cut-off stress σco corresponding to 108 cycles. The

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 86 of 87

σ-N relationship for riveted wrought iron is close to the ECCS-71 curve proposed by
Netherlands Railways (7) for low stress ranges where most fatigue damage will be
apparent. However, there are no test results at low stress levels reported by Beagles
so this is an area where further research is needed. The σ-N relationships for plain
wrought iron is based upon a cut-off stress of 65N/mm² equivalent to 0.29 x UTS as
put forward by Beagles (8), but is otherwise parallel to that for riveted curve. The
flatter σ-N relationships for wrought iron, i.e. m=4 compare with the slope of m=3
for steel. There is support for a flatter slope in wrought iron in that Beagles
determined (8) that for fatigue crack growth wrought iron showed slower growth at
lower ∆k values whereas at high ∆k values fatigue cracks grow marginally slower in
steel.

RAILTRACK LINE CODE OF PRACTICE
The Structural Assessment of Underbridges
Appendix F - Informative Annex

RT/CE/C/025
Issue: 1
Date: February 2001
Page 87 of 87

REFERENCES
1.

HIGHWAYS AGENCY. Draft of Standard for Steel Bridge Life Assessment. Draft
dated 1 December 1998;

2.

AASHTO. Guide Specifications for Fatigue Evaluation of Existing Steel Bridges.
American Association of State Highway and Transportation Officials, 1990;

3.

BS 5400: Part 10 Code of Practice for Fatigue, incorporating amendment no 1,
1999;

4.

BS 7608: Fatigue Design and Assessment of Steel Structures, 1993;

5.

ECCS Recommendations for the Fatigue Design of Steel Structures, No 43, ECCS,
1985;

6.

ARMITAGE, JD and BEAGLES, M. Fatigue Strength of Riveted Connections.
British Rail Research, September 1993;

7.

MAARSCHALKERWAART, H. Evaluation of Existing Structures;

8.

BEAGLES, M. Static and Fatigue Properties of Wrought Iron and Early Steels.
British Rail Research Report LR MF 115, November 1993.

RAILTRACK
BRIEFING NOTE

RT/CE/C/025 (Issue 1)

RAILTRACK LINE CODE OF PRACTICE:
The Structural Assessment of Underbridges

This Code of Practice gives recommendations for the appropriate standards and analytical
methods which should be used to determine the load carrying capacity of existing Railtrack
underbridges, in terms of British Standard Units of Type RA1 loading. The load carrying
capacity is determined in the context of the performance requirements of an underbridge of
meeting safety and serviceability criteria whilst regularly carrying rail traffic up to a level of
traffic load and speed in accordance with operational system requirements.
This Code of Practice may be used for the assessment of all Railtrack owned underbridges
and is applicable for line speeds up to a maximum of 125 mph.
Recommendations are provided for the assessment of underbridges constructed from steel,
wrought iron, cast iron, concrete, timber, or composite steel/concrete construction, and for
masonry arches, substructures and bearings.
The document is divided into 12 Sections with 6 accompanying appendices as follows:
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Section 8
Section 9
Section 10
Section 11
Section 12
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F

Introduction
Assessment Philosophy
Inspection For Assessment
Loading For Assessment
Steel And Wrought Iron Structures
Masonry Arches
Concrete Structures
Composite Structures
Cast Iron Structures
Timber Structures
Substructures
Bearings
Assessment of Steel and Wrought Iron
Assessment Version of BS 5400: Part 4
Amendments To BS 5400: Part 5
Fatigue Assessment of and Wrought Iron
Model Bridge Assessment Report
Informative Annex

Limit state principles are used in Sections 5, 7 and 8 and the corresponding appendices A, B
and C for the assessment of underbridges of steel, wrought iron, concrete and composite
construction respectively. Permissible stress or capacity principles are used in Sections 6, 9,
10, 11 and 12 for assessment of underbridges of other materials and forms of construction.

RT/CE/C/025

Where appropriate, guidance on the use of simple and more rigorous methods of analysis is
given. Unusual forms of construction such as cable stayed, moveable or combined road/rail
bridges are not specifically covered, but the principles outlined may be applied in checking the
elements of such structures.
Requirements for the assessment of superstructures and supports under accidental loading
conditions are excluded from RT/CE/C/025.
In comparison with RT/CE/C/015 (which has been retained for use for existing assessments
and record purposes) , the principle changes are:









clarification of the principles of assessment and description of the three levels of
assessment;
use of limit state principles based on the relevant section of BS 5400 for the sections
on steel and wrought iron;
inclusion of sections for concrete, composite construction, cast iron, timber,
substructures and bearings;
inclusion of revised summary report forms for steel and wrought iron bridges and
new summary report forms for other materials and forms of construction;
extensive revision of recommendations for inspection, and inclusion of detailed
recommendations for inspection of fatigue;
revised limits for capacities of rivets based on research carried out during the
development of the document;
inclusion of criteria for assessment of U frames;
revised criteria for determination of effective length of transverse members.

RT/CE/C/025 has been extensively trialed in comparison with the permissible code. Overall
the capacity of underbridges assessed using the limit state code should be not less than that
obtained using RT/CE/C/015. It is expected that significant benefits in terms of assessed
capacity will accrue in respect of riveted bridges and masonry arches assessed using the
MEXE method. It is possible that the assessed capacity of some bridges will be lower.
A series of appreciation courses are being arranged to give Railtrack staff and Railtrack’s
assessment consultants an understanding of the document content and details of the
significant changes. Such training will be provided in April and May 2001.
It is also proposed that technical support will be provided for 24 months in order that
clarification on interpretation of recommendations and advice on validity of any apparent
errors may be given by the drafters of the document.
In conjunction with the implementation of RT/CE/C/025, RT/CE/P/016: The Assessment of
Bridge Capacity is currently being revised.

Keith Ross
Senior Asset Manager
Structures Asset Management Group

November 2000

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close