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Concept design
Contents
1 Multi-storey buildings
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1.1 Hierarchy of design decisions 1.2 Anatomy of building design 1.3 Floor grids 1.4 Dimensional coordination
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1.4.1 Influence of building height 1.4.2 Horizontal coordination 1.4.3 Vertical coordination 1.5.1 Rigid frames 1.5.2 Braced frames 1.5.3 Concrete or steel cores
1.5 Structural options for stability
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1.6 Columns 1.7 Structural options for floor systems 1.8 Estimating steel quantities 1.9 Factors influencing structural arrangements
1.9.1 Site conditions 1.9.2 Cranes
1.10 Service integration
2 Single storey buildings 2.1 Hierarchy of design decisions 2.2 Architectural design
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2.2.1 Building form 2.2.2 Simple roof beam, supported on columns. 2.2.3 Portal frame 2.2.4 Trusses 2.2.5 Other forms of construction 2.3.1 Cladding types
2.3 Choice of building type
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2.3.1.1 Roofing 2.3.1.2 Walls
2.4 Concept design of portal frames
2.4.1 Frame stability 2.4.2 Member stability 2.5.1 Eaves connection 2.5.2 Apex connection
2.5 Connections
3 Resources 4 See also
This article presents information necessary to assist in the choice and use of steel structures at the concept design stage for modern multi-storey buildings and single storey buildings . The information is presented in terms of the design strategy, anatomy of building design and structural systems. For multi-storey buildings the primary sector of interest is commercial buildings, but the same information may also be used in other sectors. For single storey buildings the primary sector of interest is industrial buildings, but the same information can also be used in other sectors, such as commercial, retail and leisure .
Multi-storey buildings
In multi-storey buildings , the design of the primary structure is strongly influenced by many issues, as defined below:
The need to provide clear floor spans for more usable space The choice of cladding system Planning requirements, which may limit the building height and the maximum floor-tofloor zone
The services strategy and effective integration of building services Site conditions, which dictate the foundation system and location of foundations Craneage limitations and storage space for materials and components Speed of construction, which may influence the number of components that are used and the installation process.
Studies show that the cost of the building structure is generally only 10% of the total building cost - and the influence of the choice of structure on the foundations, services and cladding are often more significant. In reality, building design is a synthesis of architectural, structural, services, logistics and buildability issues. Steel frames are ideally suited for modern multi-storey commercial buildings.
Hierarchy of design decisions
The development of any proposal for a construction project requires a complex series of design decisions that are inter-related. The process should begin with a clear understanding of the client requirements and of local conditions or regulations. Despite the complexity, it is possible to identify a hierarchy of design decisions. Firstly, planning requirements are likely to define the overall building form, which will also include aspects such as natural lighting, ventilation and services. The principal design choices that need to be made in close consultation with the client are:
The depth of the floor zone and the overall structure/service interaction strategy The need for special structural arrangements in public spaces or circulation areas The provision of some tolerance between structure and services, to permit future adaptability The benefit of using longer spans, at negligible extra cost, in order to enhance flexibility of layout.
Based on the design brief, a concept design can then be prepared which is reviewed by the design team and client. It is at this early interactive stage where the important decisions are made that influence the cost and value of the final project. Close involvement with the client is essential.
[top]Anatomy
of building design
The building design is dependent on various parameters; these include:
Floor grid Building height Circulation and access space Services requirements and service integration.
[top]Floor
grids
Floor grids define the spacing of the columns in orthogonal directions, which are influenced by:
The planning grid (normally based on units of 300 mm but more typically multiples of 0.6, 1.2 or 1.5 m) The column spacing along the façades, depending on the façades material (typically 5.4 to 7.5 m) The use of the internal space, i.e. for offices or open plan space The requirements for building service distribution (from the building core).
Along the façade line, column spacings are normally defined by the need to provide support to the cladding system for example, a maximum column spacing of 6 m is normally required for brickwork. This influences the column spacing internally, unless additional columns are used along the façade line. The span of the beams across the building normally conforms to one of the following column grid arrangement:
Single internal line of columns, placed offset to the line of a central corridor. This is shown in the figure below Pairs of column lines on either side of a corridor Column-free internal spans with columns located along the façade line.
For naturally ventilated offices, a building width of 12 m to 15 m is typically used, which can be achieved by two spans of 6 to 7.5 m. A single span can also be provided with deep (400 mm or more) precast concrete hollow core units spanning the full width of the building. Natural lighting also plays a role in choice of the width of floor plate. In modern buildings, a long span solution provides a considerable enhancement in flexibility of layout. For air-conditioned offices, a clear span of 15 m to 18 m is often used. An example of the column grid for a long span option in a building with a large atrium is shown in the figure below.
Dimensional coordination
The choice of the basic building shape is usually the architect's responsibility, constrained by such issues as the site plan, access, building orientation, parking, landscaping and local planning requirements. The following general guidance influences the choice of structure.
Between sources of natural light there should be 13.5 m and 20 m intervals Naturally lit and ventilated zones extend a distance of twice the floor-to-ceiling height from the outer walls - artificial light and ventilation is required in other zones Atria improve the efficient use of the building, and reduce the running costs.
[top]Influence of building height
The building height has a strong influence on the:
Structural system that is adopted Foundation system Fire resistance requirements and means of escape Access (by lifts) and circulation space Choice of cladding system Speed of construction and site productivity.
For taller buildings, strategically placed concrete or braced steel cores are usually adopted. Ultra tall buildings are influenced strongly by the stabilising system, but are not covered here. Sizes of lifts and their speed of movement also become important considerations for tall buildings. Depending on the Regulations for fire safety, the use of sprinklers may be required for buildings of more than eight storeys (or approximately 30 m high).
[top]Horizontal coordination
Horizontal coordination is dominated by the need on plan for defined zones for vertical access, safe evacuation in fire, and vertical service distribution. Positioning of service and access cores is influenced by:
Horizontal distribution systems for mechanical services Fire resistance requirements, which may control evacuation routes and compartment sizes The need to distribute the stabilizing systems (bracing and cores) effectively throughout the building plan.
The two planning grids shown above present typical arrangements that satisfy these criteria. An atrium may be incorporated to increase lighting to the occupied space and to provide high value circulation areas at ground and intermediate levels. The design requirements for atria are:
Support to the long span roof of the atrium Access routes for general circulation Fire safety measures by smoke extraction and safe evacuation routes Light levels and servicing to internal offices.
[top]Vertical coordination
The target floor-to-floor height is based on a floor-to-ceiling height of 2.5 to 2.7 m for speculative offices, or 3 m for more prestige applications, plus the floor depth including services. The following target floor-to-floor depths should be considered at the concept design stage: Prestige office 4.0 - 4.2 m
Speculative office
3.6 - 4.0 m
Renovation project
3.5 - 3.9 m
These targets permit a range of structural solutions. If, for planning reasons, it is required to limit the overall building height, this can be achieved by use of shallow floor or integrated beam systems. Integrated beam systems are often used in renovation projects where the floor-to-floor height is limited by compatibility with the existing building or façades. For concept design of orthodox commercial multi-storey steel structures , the following 'target' floor depths may be used. Flooring system Target floor depth (mm)
Composite beam construction
800 – 1,200
Cellular beams (with service integration)
800 – 1,100
Downstand beams with precast concrete floor slabs
1,200 – 1,450
Shallow floor or integrated beams
600 – 800
Typical floor depths for multi storey buildings
[top]Structural
options for stability
The structural system required for stability is primarily influenced by the building height. For buildings up to eight storeys height, the steel structure may be designed to provide stability, but for taller buildings, concrete or braced steel cores are more efficient structurally. The following structural systems may be considered for stability.
[top]Rigid frames
For buildings up to four storeys high, rigid frames may be used in which the multiple beam to column connections provide bending resistance and stiffness to resist horizontal loads. This is generally only possible where the beams are relatively deep (400 mm to 500 mm) and where the column size is increased to resist the applied moments. Full depthend plate connections generally provide the necessary rigidity.
[top]Braced frames
For buildings up to 12 storeys high, braced steel frames are commonly used in which cross, K or V bracing is used in the walls, generally within a cavity in the façades, or around stairs or other serviced zones. Cross bracing is designed in tension only (the other member being redundant). Cross bracing is often simple flat steel plate , but angle and channel sections may also be used. When bracing is designed to work in compression, hollow sections are often used, although angle and channel sections may also be used. A steel braced frame has the two key advantages:
Responsibility for temporary stability lies with one organisation As soon as the steel bracing is connected (bolted), the structure is stable.
[top]Concrete or steel cores
Concrete cores are the most practical system for buildings of up to 40 storeys high, but the concrete core is generally constructed in advance of the steel framework. In this form of construction, the beams often span directly between the columns on the perimeter of the building and the concrete core. Special structural design considerations are required for:
The beam connections to the concrete core The design of the heavier primary beams at the corner of core Fire safety and robustness of the long span construction.
A typical layout of beams around a concrete core is shown in the figure below, with the use of heavier beams at the corner of the core. A double beam may be required to minimise the structural depth at the corner of the cores.
Typical beam layout around a concrete core
A braced steel core
Braced steel cores may be used as an economic alternative where speed of construction is critical. Such cores are installed with the rest of the steelwork package. An example of a braced steel core is shown in the figure above right.
[top]Columns
Columns in multi-storey steel frames are generally H sections , predominantly carrying axial load. When the stability of the structure is provided by cores, or discreet vertical bracing, the beams are generally designed as simply supported. The generally accepted design model is that nominally pinned connections produce nominal moments in the column, calculated by assuming that the beam reaction is 100 mm from the face of the column. If the reactions on the opposite side of the column are equal, there is no net moment. Columns on the perimeter of the structure will have an applied moment, due to the connection being on one side only. Typical column sizes are given in the table below. Number of floors supported by column section Universal Column (UKC) serial size
1
152
2-4
203
3-8
254
5 - 12
305
10 - 40
356
Typical column sizes for small and medium span composite floors
Although small column sections may be preferred for architectural reasons, the practical issues of connections to the floor beams should be considered. It can be difficult and costly to provide connection into the minor axis of a very small column section.
[top]Structural
options for floor systems
A wide range of floor system solutions is available for which typical solutions are given in the table below. Form of construction Typical solution
Downstand beams Low rise, modest spans, no restriction on construction depth precast units or composite floors Downstand beams in the façade precast concrete units (15 m), composite floors with secondary Low rise, long span e.g. 15m steel beams spanning 15 m
Medium and high rise, modest spans, no restriction on construction depth Downstand beams, composite construction
Medium and high rise, long spans (to 18 m) restricted construction depth Composite floors with cellular long span secondary steel beams
Typical floor solutions
Although steel solutions are appropriate for short spans (typically 6 to 9 m), steel has an important advantage over other materials in that long span solutions (between 12 and 18 m) can be easily provided. This has the key advantage of column-free space, allowing future adaptability, and fewer foundations.
Floors spanning onto the steel beams will normally be either precast concrete units , or composite floors. The supporting beams may be below the floor, with the floor bearing on the top flange (often known as 'downstand' beams ), or the beams may share the same zone with the floor construction, to reduce the overall depth of the zone. The available construction zone is often the determining factor when choosing a floor solution. Beams within the floor zone are known as slim floor, or integrated beams. Beams may be non composite, or composite. In composite construction shear connectors are welded to the top flange of the beam, transferring load to the concrete floor. Precast concrete units may be used for low rise frames, but composite floors are common in both low rise and high rise structures. The span range of various structural options in both steel and concrete are illustrated in the table below. Long span steel options generally provide for service integration for spans of over 12m. Cellular beams and composite trusses are more efficient for long span secondary beams, whereas fabricated beams are often used for long span primary beams.
Span range of various structural options
[top]Estimating
steel quantities
For estimating purposes in the design of office buildings, representative weights of steel may be used for buildings of rectangular plan form. These quantities will increase significantly for non rectangular or tall buildings or for buildings with atria or complex façades. The approximate quantities are presented in the table below, and are expressed in terms of the total floor area of the building. They do not include the steelwork used in the façades, atrium or roof. Approximate steel quantities
(kg/m² floor area) Form of Building Beams Columns Bracing Total
3 or 4 storey building of rectangular form
25–30
8–10
2–3
35–40
6–8 storey building of rectangular form
25–30
12–15
3–5
40–50
8–10 storey building with long spans
35–40
12–15
3–5
50–60
20 storey building with long spans and a concrete core
40–50
10–13
1–2
50–65
Approximate steel quantities
Further guidance on estimating steel quantities and cost is available.
[top]Factors
influencing structural arrangements
The construction programme will be a key concern in any project, and should be considered at the same time as considering the cost of structure, the services, cladding and finishes. The structural scheme has a key influence on programme and cost, and structural solutions which can be erected safely, quickly to allow early access for the following trades.
[top]Site conditions
Increasingly, structures are constructed on 'brownfield' sites, where earlier construction has left a permanent legacy. In city centres, a solution involving fewer, albeit more heavily loaded foundations are often preferred, which lead to longer spans for the superstructure.
[top]Cranes
The number of cranes on a project will be dominated by the site footprint, the size of the project and the use of additional mobile cranes. Multi-storey structures are generally erected using a tower crane, which may be supplemented by mobile cranes for specific heavy lifting operations. In city centre projects, tower cranes are often located in a lift shaft or atrium.
Service integration
Most large office-type structures require air conditioning or 'comfort cooling', which will necessitate both horizontal and vertical distribution systems. The provision for such systems is of critical importance for the superstructure layout, affecting the layout and type of members chosen. See SCI P166 and P273. The basic decision either to integrate the ductwork within the structural depth or to simply suspend the ductwork at a lower level affects the choice of structural member, the fire protection system, the cladding (cost and programme) and overall building height. Other systems provide conditioned air from a raised floor.
Single storey buildings
Single storey buildings use steel framed structures and metallic cladding of all types. Large open spaces can be created, which are efficient, easy to maintain and are adaptable as demand changes. Single storey buildings are a 'core' market for steel in the UK. Single storey buildings tend to be large enclosures, but may require space for other uses, such as offices, handling and transportation, overhead cranes, etc. Therefore, many factors have to be addressed in their design. Increasingly, architectural issues and visual impact have to be addressed and many leading architects are involved in the design of modern single storey buildings .
Hierarchy of design decisions
Important design factors for single storey buildings
The development of a design solution for a single storey building, such as a large enclosure or industrial facility is more dependent on the activity being performed and future requirements for the space than other building types, such as commercial and residential buildings. Although these building types are primarily functional, they are commonly designed with strong architectural involvement dictated by planning requirements and client 'branding'. The following overall design requirements should be considered in the concept design stage ofindustrial buildings and large enclosures, depending on the building form and use:
Space use, for example, specific requirements for handling of materials or components in a production facility Flexibility of space in current and future use Speed of construction Environmental performance, including services requirements and thermal performance Aesthetics and visual impact Acoustic isolation, particularly in production facilities Access and security Sustainability considerations Design life and maintenance requirements, including end of life issues.
To enable the concept design to be developed, it is necessary to review these considerations based on the type of single storey building. For example, the requirements for a distribution centre will be different to a manufacturing facility. A review of the importance of various design issues is presented in the table on the right for common building types.
[top]Architectural
design
Modern single storey buildings using steel are both functional in use and are designed to be architecturally attractive. Various examples are presented below together with a brief description of the design concept.
[top]Building form
The basic structural form of a single storey building may be of various generic types, as shown in the figure below. The figure shows a conceptual cross-section through each type of building, with notes on the structural concept, and typical forces and moments due to gravity loads.
Structural concepts
The basic design concepts for each structural type are described below:
[top]Simple roof beam, supported on columns.
The span will generally be modest, up to approximately 20 m. The roof beam may be pre-cambered. Bracing will be required in the roof and all elevations, to provide in-plane and longitudinal stability.
[top]Portal frame
A portal frame is a continuous frame with moment resisting connections to provide stability in-plane. A portal frame may be single bay or multi bay. The members are generally plain rolled sections, with the resistance of the rafter enhanced locally with a haunch. In many cases, the frame will have pinned bases. Stability in the
longitudinal direction is provided by a combination of bracing in the roof, across one or both end bays, and vertical bracing in the elevations. If vertical bracing cannot be provided in the elevations (due to industrial doors, for example) stability is often provided by a rigid frame within the elevation.
[top]Trusses
Truss buildings generally have roof bracing and vertical bracing in each elevation to provide stability in both orthogonal directions. The trusses may take a variety of forms, with shallow or steep external roof slopes. A truss building may also be designed as rigid in-plane, although it is more common to provide bracing to stabilise the frame.
[top]Other forms of construction
Built-up columns (two plain beams, connected to form a compound column) are often used to support heavy loads, such as cranes. These may be used in portalised structures, but are often used with rigid bases, and with bracing to provide in-plane stability. External or suspended support structures may be used, but are relatively uncommon.
[top]Choice
of building type
Portal frames are considered to be a highly cost-effective way to provide a single storey enclosure. Their efficiency depends on the method of analysis, and the assumptions that are made regarding the restraint to the structural members, as shown in the table below. Most efficient Less efficient
Analysis using elastic-plastic software
Elastic analysis
Cladding considered to restrain the flange of the purlins and side rails
Purlins and side rails unrestrained
Purlins and side rails used to restrain both flanges of the hot-rolled steelwork The inside flange of the hot rolled steelwork is unrestrained
Nominal base stiffness utilised
Nominal base stiffness ignored
Efficient portal frame design
The reasons for choosing simple beam structures, portal frames or trusses are shown in the table below. Simple beam Portal frame Truss
Advantages
Simple design
Long span
Very long spans possible
Designed to be stable in plane
Heavy loads may be carried
Member sizes and haunches may be optimised for efficiency
Modest deflection
Disadvantages
Relatively short span
Software required for efficient design
Generally more expensive fabrication
Bracing needed for in-plane stability
Limited to relatively light vertical loading, and modest cranes to avoid excessive deflections
Generally bracing is used for in-plane stability
No economy due to continuity
Efficient portal frame design
[top]Cladding types
The main types of roofing and wall cladding used in single storey buildings are described as follows:
[top]Roofing
'Built-up' or double layer roofing spanning between secondary members such as purlins. Composite panels (also known as sandwich panels) spanning between purlins. Deep decking spanning between main frames, supporting insulation, with an external metal sheet or waterproof membrane.
[top]Walls
Sheeting, orientated vertically and supported on side rails. Sheeting or structural liner trays spanning horizontally between columns. Composite or sandwich panels spanning horizontally between columns, eliminating side rails. Metallic cassette panels supported by side rails.
Different forms of cladding (including vertically and horizontally orientated sheets) may be used together for visual effect in the same façades. Brickwork is often used as a 'dado' or 'dwarf' wall below the level of the windows for impact resistance.
[top]Concept
design of portal frames
Steel portal frames are widely used because they combine structural efficiency with functional form. A single-span symmetrical portal frame (as illustrated in the figure below) is typically of the following proportions:
A span between 15 m and 50 m (25 m to 35 m is the most efficient) An eaves height (base to rafter centreline) of between 5 and 15 m (7.5 m or more is commonly adopted). The eaves height is determined by the specified clear height between the top of the floor and the underside of the haunch.
A roof pitch between 5° and 10° (6° is commonly adopted) A frame spacing between 5 m and 8 m (the greater frame spacings being used in longer span portal frames) Members are I sections rather than H sections, because they must carry significant bending moments and provide in-plane stiffness. Sections are generally S275 or S355. As deflections may be critical, the use of higher strength steel is rarely justified. Haunches are provided in the rafters at the eaves to enhance the bending resistance of the rafter and to facilitate a bolted connection to the column. Small haunches are provided at the apex, to facilitate the bolted connection.
Single span symmetric portal frame
The eaves haunch is typically cut from the same size Standard open sections|rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the span. The length of the haunch means that the hogging bending moment at the 'sharp' end of the haunch is approximately the same as the maximum sagging bending moment towards the apex, as shown in the figure below.
No higher resolution available.
The end frames of a portal frame are generally called gable frames. Gable frames may be identical to the internal frames, even though they experience lighter loads. If future extension to the building is envisaged, portal frames are commonly used as the gable frames, to reduce the impact of the structural works. A typical gable frame is shown in the figure below.
Typical details of an end gable of a portal frame building
Frame stability
In-plane stability is provided by frame continuity. In the longitudinal direction, stability is provided by vertical bracing in the elevations. The vertical bracing may be at both ends of the building, or in one bay only. Each frame is connected to the vertical bracing by a hot-rolled member at eaves level. A typical bracing arrangement is shown in the figure below.
Typical bracing in a portal frame
The gable columns span between the base and the rafter, where the reaction is carried by bracing in the plane of the roof, back to the eaves level, and to the foundations by the vertical bracing. If diagonal bracing in the elevations cannot be accommodated, longitudinal stability can be provided by a rigid frame on the elevation.
[top]Member stability
Restraint locations
For economic design , restraints to the rafter and column must be considered. The purlins and side rails are considered adequate to restrain the flange that they are attached to, but unless special measures are taken, the purlins and side rails do not restrain the inside flange. Restraint to the inside flange is commonly provided by bracing from the purlins and side rails, as shown in the figure below. The bracing is usually formed of thin metal straps, designed to act in tension, or from angles designed in compression if bracing is only possible from one side. The arrangement of restraints to the inside flange is generally similar to that shown in the figure below and in all cases, the junction of the inside face of the column and the underside of the haunch must be restrained.
Restraint bracing to inside flange
[top]Connections [top]Eaves connection
Typical eaves connection
A typical eaves connection is shown in in the figure below. In almost all cases a compression stiffener in the column (as shown, at the bottom of the haunch) will be required. Other stiffeners may be required to increase the bending resistance of the column flange, adjacent to the tension bolts, and to increase the shear resistance of the column web panel. The haunch is generally fabricated from a similar size beam to the rafter (or larger), or fabricated from equivalent plate. Typically, the bolts may be M24 8.8 and the end plate 25 mm thick S275.
[top]Apex connection
A typical apex connection is shown in the figure below. The apex connection primarily serves to increase the depth of the member to make a satisfactory bolted connection. The apex haunch is usually fabricated from the same member as the rafter, or from equivalent plate. Typically, the bolts may be M24 8.8 and the end plate 25 mm thick S275.
Typical apex connection
[top]Resources
Steel Buildings in Europe - Multi-storey buildings: Part 1 Architect’s guide Steel Buildings in Europe - Multi-storey buildings: Part 2 Concept design Steel Buildings in Europe - Single storey buildings: Part 1 Architect’s guide Steel Buildings in Europe - Single storey buildings: Part 2 Concept design SCI P166 Interfaces: Design of Steel Framed Buildings for Service Integration, 1997
SCI P273 Service Integration in Slimdek, 2000 Steel Buildings, 2003, (Publication No 35/03), BCSA
[top]See
Chapter 3 - Single Storey Buildings Chapter 4 - Multi-Storey Buildings
SCI P167 Architectural Teaching Resource Studio Guide, 2000
also
Multi-storey buildings Single storey buildings Composite construction Floor systems Long span beams Portal frames Moment resisting connections Service integration Building envelopes Cost of structural steelwork Cost comparison study Retail buildings Leisure buildings Structural fire resistance requirements Continuous frames Braced frames Steel construction products Structural robustness Trusses Fire and steel construction Design software and tools
Portal frames
Contents
[hide]
1 Anatomy of a typical portal frame 2 Types of portal frames 3 Design considerations
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3.1 Choice of material and section 3.2 Frame dimensions
3.2.1 Clear span and height 3.2.2 Main frame 3.2.3 Haunch dimensions 3.2.4 Positions of restraints
4 Actions
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4.1 Permanent actions 4.1.1 Service loads 4.2.1 Imposed roof loads 4.2.2 Snow loads 4.2.3 Wind actions 4.2.4 Crane actions 4.2.5 Accidental actions 4.2.6 Robustness 4.2.7 Fire 4.2 Variable actions
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7 Design
4.3 Combinations of actions
5 Frame analysis at ULS 5.1 Plastic analysis 5.2 Elastic analysis
6 In-plane frame stability 6.1 Second order effects 6.2 First-order and second-order analysis 6.3 Calculation of αcr 6.4 Sensitivity to effects of the deformed geometry
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7.1 Cross-section resistance 7.2 Member stability 7.3 Rafter design and stability
o
7.3.1 Out-of-plane stability 7.3.2 Gravity combination of actions 7.3.3 The uplift condition 7.3.4 In plane stability 7.4.1 Out-of-plane stability 7.4.2 In plane stability
7.4 Column design and stability
8 Bracing
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8.1 Vertical bracing 8.1.1 Portalised bays 8.1.2 Bracing to restrain longitudinal loads from cranes 8.2.1 Restraint to inner flanges
8.2 Plan bracing
9 Connections
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9.1 Column bases
10 References 11 Further reading 12 Resources 13 See also 14 External links 15 CPD
Portal frames are generally low-rise structures, comprising columns and horizontal or pitched rafters, connected by moment-resisting connections. Resistance to lateral and vertical actions is provided by the rigidity of the connections and the bending stiffness of the members, which is increased by a suitable haunch or deepening of the rafter sections. This form of continuous frame structure is stable in its plane and provides a clear span that is unobstructed by bracing. Portal frames are very common, in fact 50% of constructional steel used in the UK is in portal frame construction. They are very efficient for enclosing large volumes, therefore they are often used for industrial, storage, retail and commercial applications as well as for agricultural purposes. This article describes the anatomy and various types of portal frame and key design considerations.
Multi-bay portal frame during construction Anatomy of a typical portal frame
Principal components of a portal framed building
A portal frame building comprises a series of transverse frames braced longitudinally. The primary steelwork consists of columns and rafters, which form portal frames, and bracing. The end frame (gable frame) can be either a portal frame or a braced arrangement of columns and rafters.
The light gauge secondary steelwork consists of side rails for walls and purlins for the roof. The secondary steelwork supports the building envelope, but also plays an important role in restraining the primary steelwork. The roof and wall cladding separate the enclosed space from the external environment as well as providingthermal and acoustic insulation. The structural role of the cladding is to transfer loads to secondary steelwork and also to restrain the flange of the purlin or rail to which it is attached.
Cross-section showing a portal frame and its restraints
Portal framed structures - overview
[top]Types
of portal frames
Many different forms of portal frames may be constructed. Frame types described below give an overview of types of portal construction with typical features illustrated. This information only provides typical details and is not meant to dictate any limits on the use of any particular structural form. Pitched roof symmetric portal frame
Generally fabricated from UKB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. 25 to 35 m are the most efficient spans. Pitched roof symmetric portal frame Lancashire Waste Development
Portal frame with internal mezzanine floor
Office accommodation is often provided within a portal frame structure using a partial width mezzanine floor. The assessment of frame stability must include the effect of the mezzanine; guidance is given in SCI P292. Crane portal frame with column brackets Portal frame with internal mezzanine floor Waters Meeting Health Centre, Bolton (Image courtesy BD Structures Ltd. and ASD Westok Ltd.)
Where a travelling crane of relatively low capacity (up to say 20 tonnes) is required, brackets can be fixed to the columns to support the crane rails. Use of a tie member or rigid column bases may be necessary to reduce the eaves deflection.
The spread of the frame at crane rail level may be of critical importance to the functioning of the crane; requirements should be agreed with the client and with the crane manufacturer. Tied portal frame
In a tied portal frame the horizontal movement of the eaves and the bending moments in the columns and rafters are reduced. A tie may be useful to limit spread in a crane-supporting structure. The high axial forces introduced in the frame when a tie is used necessitate the use of second-order software when analysing this
form of frame.
Mono-pitch portal frame
A mono pitch portal frame is usually chosen for small spans or because of its proximity to other buildings. It is a simple variation of the pitched roof portal frame, and tends to be used for smaller buildings (up to 15 m span). Propped portal frame
Where the span of a portal frame is large and there is no requirement to provide a clear span, a propped portal frame can be used to reduce the rafter size and also the horizontal Propped portal frame shear at the foundations. Rebottling Plant, Hemswell (Image courtesy of Metsec plc)
Mansard portal frame
A mansard portal frame may be used where a large clear height at mid-span is required but the eaves height of the building has to be minimised. Curved rafter portal frame
Portal frames may be constructed using curved rafters, mainly for architectural reasons. Because of transport limitations rafters longer than 20 m may require splices, which should be carefully detailed for architectural reasons. The curved member is often modelled for analysis as a series of straight elements. Guidance on the
stability of curved rafters in portal frames is given in SCI P281. Alternatively, the rafter can be fabricated as a series of straight elements. It will be necessary to provide purlin cleats of varying height to achieve the curved external profile. Cellular beam portal frame
Rafters may be fabricated from cellular beams for aesthetic reasons or when providing long spans. Where transport limitations impose requirement for splices, they should be carefully detailed, to preserve the architectural features. The sections used Cellular beam portal frame Hayes garden centre (Image courtesy of ASD Westok Ltd.)
cannot develop plastic hinges at a cross-section, so only elastic design is used.
[top]Design
considerations
In the design and construction of any structure, a large number of inter-related design requirements should be considered at each stage in the design process. The following discussion of the design process and its constituent parts is intended to give the designer an understanding of the inter-relationship of the various elements of the structure with its final construction, so that the decisions required at each stage can be made with an understanding of their implications.
[top]Choice
of material and section
Steel sections used in portal frame structures are usually specified in grade S275 or S355 steel. In plastically designed portal frames, Class 1 plastic sections must be used at hinge positions that rotate, Class 2 compact sections can be used elsewhere
[top]Frame
dimensions
Dimensions used for analysis and clear internal dimensions
A critical decision at the conceptual design stage is the overall height and width of the frame, to give adequate clear internal dimensions and adequate clearance for the internal functions of the building.
[top]Clear span and height
The clear span and height required by the client are key to determining the dimensions to be used in the design, and should be established early in the design process. The client requirement is likely to be the clear distance between the flanges of the two columns – the span will therefore be larger, by the section depth. Any requirement for brickwork or blockwork around the columns should be established as this may affect the design span. Where a clear internal height is specified, this will usually be measured from the finished floor level to the underside of the haunch or suspended ceiling if present.
[top]Main frame
The main (portal) frames are generally fabricated from UKB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. A typical frame is characterised by:
A span between 15 and 50 m An clear height (from the top of the floor to the underside of the haunch) between 5 and 12 m A roof pitch between 5° and 10° (6° is commonly adopted) A frame spacing between 6 and 8 m Haunches in the rafters at the eaves and apex A stiffness ratio between the column and rafter section of approximately 1.5 Light gauge purlins and side rails Light gauge diagonal ties from some purlins and side rails to restrain the inside flange of the frame at certain locations.
[top]Haunch dimensions
Typical haunch with restraints
The use of a haunch at the eaves reduces the required depth of rafter by increasing the moment resistance of the member where the applied moments are highest. The haunch also adds stiffness to the frame, reducing deflections, and facilitates an efficient bolted moment connection. The eaves haunch is typically cut from the same size rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the frame span. The haunch length generally means that the hogging moment at the end of the haunch is approximately equal to the largest sagging moment close to the apex. The depth from the rafter axis to the underside of the haunch is approximately 2% of the span. The apex haunch may be cut from a rolled section – often from the same size as the rafter, or fabricated from plate. The apex haunch is not usually modelled in the frame analysis and is only used to facilitate a bolted connection.
[top]Positions of restraints
General arrangement of restraints to the inside flange
During initial design the rafter members are normally selected according to their cross sectional resistance to bending moment and axial force. In later design stages stability against buckling needs to be verified and restraints positioned judiciously. The buckling resistance is likely to be more significant in the selection of a column size, as there is usually less freedom to positing rails to suit the design requirements; rails position maybe dictated by doors or windows in the elevation. If introducing intermediate lateral restraints to the column is not possible, the buckling resistance will determine the initial section size selection. It is therefore essential to recognise at this early stage if the side rails may be used to provide restraint to the columns. Only continuous side rails are effective in providing restraint. Side rails interrupted by (for example) roller shutter doors, cannot be relied on as providing adequate restraint. Where the compression flange of the rafter or column is not restrained by purlins and side rails, restraint can be provided at specified locations by column and rafter stays.
[top]Actions
Advice on actions can be found in BS EN 1991[1], and on the combinations of actions in BS EN 1990[2]. It is important to refer to the UK National Annex for the relevant Eurocode part for the structures to be constructed in the UK.
[top]Permanent
actions
Permanent actions are the self weight of the structure, secondary steelwork and cladding. Where possible, unit weights of materials should be obtained from manufacturers‟ data. Where information is not available, these may be determined from the data in BS EN 1991-1-1[3].
[top]Service loads
Service loads will vary greatly depending on the use of the building. In portal frames heavy point loads may occur from suspended walkways, air handling units etc. It is necessary to consider carefully where additional provision is needed, as particular items of plant must be treated individually. Depending on the use of the building and whether sprinklers are required, it is normal to assume a service loading of 0.1–0.25 kN/m on plan over the whole roof area.
2
[top]Variable
actions
Roof slope, α qk (kN/m²)
[top]Imposed roof loads
α < 30°
0.6
30° < α < 60° 0.6[60 - α)/30]
Imposed loads on roofs are given in the UK NA to BS EN 1991-1-1[4], and depend on the roof slope. A point load, Qk is given, which is used for local checking of roof materials and fixings, and a uniformly distributed load, qk, to be applied vertically. The loading for roofs not accessible except for normal maintenance and repair is given in the table on the right. It should be noted that imposed loads on roofs should not be combined with either snow or wind.
α > 60°
0
Imposed loads on roofs
[top]Snow loads
Snow loads may sometimes be the dominant gravity loading. Their value should be determined from BS EN 1991-1-3[5] and its UK National Annex[6] – the determination of snow loads is described in Chapter 3 of the Steel Designers’ Manual. Any drift condition must be allowed for not only in the design of the frame itself, but also in the design of the purlins that support the roof cladding. The intensity of loading at the position of maximum drift often exceeds the basic minimum uniform snow load. The calculation of drift loading and associated purlin design has been made easier by the major purlin manufacturers, most of whom offer free software to facilitate rapid design.
[top]Wind actions
Wind actions in the UK should be determined using BS EN 1991-1-4[7] and its UK National Annex[8]. This Eurocode gives much scope for national adjustment and therefore its annex is a substantial document. Wind actions are inherently complex and likely to influence the final design of most buildings. The designer needs to make a careful choice between a fully rigorous, complex assessment of wind actions and the use of simplifications which ease the design process but make the loads more conservative. Free software for establishing wind pressures is available from manufacturers. For more advice refer to Chapter 3 of the Steel Designers’ Manual and SCI P394. Wind loading calculator
[top]Crane actions
Gantry girders carrying an overhead travelling crane
The most common form of craneage is the overhead type running on beams supported by the columns. The beams are carried on cantilever brackets or, in heavier cases, by providing dual columns. In addition to the self weight of the cranes and their loads, the effects of acceleration and deceleration have to be considered. For simple cranes, this is by a quasistatic approach with amplified loads For heavy, high-speed or multiple cranes the allowances should be specially calculated with reference to the manufacturer.
[top]Accidental actions
The common design situations which are treated as accidental design situations are:
Drifted snow, determined using Annex B of BS EN 1991-1-3[5] The opening of a dominant opening which was assumed to be shut at ULS
Each project should be individually assessed whether any other accidental actions are likely to act on the structure.
[top]Robustness
Robustness requirements are designed to ensure that any structural collapse is not disproportionate to the cause. BS EN 1990[2] sets the requirement to design and construct robust buildings in order to avoid disproportionate collapse under accidental design situations. BS EN 1991-1-7[9] gives details of how this requirement should be met. For many portal frame structures no special provisions are needed to satisfy robustness requirements set by the Eurocode. For more information on robustness refer to SCI P391.
[top]Fire
Collapse mechanism of a portal with a lean-to under fire, boundary condition on gridlines 2 and 3.
In the United Kingdom, structural steel in single storey buildings does not normally require fire resistance. The most common situation in which it is required to fire protect the structural steelwork is where prevention of fire spread to adjacent buildings, a boundary condition, occurs. There are a small number of other, rare, instances, for example when demanded by an insurance provider, where structural fire protection may be required. When a portal frame is close to the boundary, there are several requirements aimed at stopping fire spread by keeping the boundary intact:
The use of fire resistant cladding Application of fire protection of the steel up to the underside of the haunch The provision of a moment resisting base (as it is assumed that in the fire condition rafters go into catenary)
Comprehensive advice is available in SCI P313.
[top]Combinations
of actions
BS EN 1990[2] gives rules for establishing combinations of actions, with the values of relevant factors given in the UK National Annex[10]. BS EN 1990[2] covers both ultimate limit state (ULS) and serviceability limit state (SLS), although for the SLS, onward reference is made to the material codes (for example BS EN 19931-1[11] for steelwork) to identify which expression should be used and what SLS limits should be observed.
All combinations of actions that can occur together should be considered, however if certain actions cannot be applied simultaneously, they should not be combined. Guidance on the application of Eurocode rules on combinations of actions can be found in SCI P362 and, specifically for portal frames, in SCI P400.
[top]Frame
analysis at ULS
At the ultimate limit state (ULS), the methods of frame analysis fall broadly into two types: elastic analysis and plastic analysis.
[top]Plastic analysis
Bending moment diagram resulting from the plastic analysis of a symmetrical portal frame under symmetrical loading
The term plastic analysis is used to cover both rigid-plastic and elastic-plastic analysis. Plastic analysiscommonly results in a more economical frame because it allows relatively large redistribution of bending moments throughout the frame, due to plastic hinge rotations. These plastic hinge rotations occur at sections where the bending moment reaches the plastic moment or resistance of the cross-section at loads below the full ULS loading. The rotations are normally considered to be localised at “plastic hinges” and allow the capacity of under utilised parts of the frame to be mobilised. For this reason members, where possible plastic hinges may occur need to be Class 1 sections, which are capable of accommodating rotations. The figure shows typical positions, where plastic hinges form in a portal frame. Two hinges lead to a collapse, but in the illustrated example, due to symmetry, designers need to consider all possible hinge locations.
[top]Elastic analysis
A typical bending moment diagram resulting from an elastic analysis of a frame with pinned bases is shown the figure below. In this case, the maximum moment (at the eaves) is higher than that calculated from a plastic analysis. Both the column and haunch have to be designed for these large bending moments. Where deflections (SLS) govern design, there may be no advantage in using plastic analysis for the ULS. If stiffer sections are selected in order to control deflections, it is quite possible that no plastic hinges form and the frame remains elastic at ULS.
Bending moment diagram resulting from the elastic analysis of a symmetrical portal frame under symmetrical loading
Portal frame analysis software (Fastrak model courtesy of CSC)
[top]In-plane
frame stability
When any frame is loaded, it deflects and its shape under load is different from the un-deformed shape. The deflection has a number of effects:
The vertical loads are eccentric to the bases, which leads to further deflection The apex drops, reducing the arching action Applied moments curve members; Axial compression in curved members causes increased curvature (which may be perceived as a reduced stiffness.)
Taken together, these effects mean that a frame is less stable (nearer collapse) than a first-order analysis suggests. The objective of assessing frame stability is to determine if the difference is significant.
[top]Second order effects
P-δ and P-Δ effects in a portal frame
The geometrical effects described above are second-order effects and should not be confused with non linear behaviour of materials. As shown in the figure below there are two categories of second-order effects:
Effects of displacements of the intersections of members, usually called P-Δ effects. BS EN 1993-1-1[11]describes this as the effect of deformed geometry. Effects of deflections within the length of members, usually called P-δ effects.
Second-order analysis is the term used to describe analysis methods in which the effects of increasing deflection under increasing load is considered explicitly in the solution, so that the results include the P-δ and P-Δ effects.
[top]First-order and second-order analysis
For either plastic analysis of frames, or elastic analysis of frames, the choice of first-order analysis or second-order analysis depends on the in plane flexibility of the frame, characterised by the calculation of the αcr factor.
[top]Calculation of αcr
The effects of the deformed geometry (P-Δ effects) are assessed in BS EN 1993–1–1[11] by calculating the factor αcr, defined as:
where: Fcr is the elastic critical buckling load for global instability mode, based on initial elastic stiffnesses FEd is the design load on the structure. αcr may be found using software or using an approximation (expression 5.2 from BS EN 1993-1-1[11]) as long as the frame meets certain geometric limits and the axial force in the rafter is not „significant‟. Rules are given in the Eurocode to identify when the axial force is significant. When the frame falls outside the specified limits, as is the case for very many orthodox frames, the simplified expression cannot be used. In these circumstances, an alternative expression may be used to calculate an approximate value of αcr, referred to asαcr,est. Further details are given in SCI P397.
[top]Sensitivity to effects of the deformed geometry
The limitations to the use of first-order analysis are defined in BS EN 1993–1–1[11], Section 5.2.1 (3) and the UK National Annex[12] Section NA.2.9 as: For elastic analysis: αcr ≥ 10 For plastic analysis:
αcr ≥ 5 for combinations with gravity loading with frame imperfections,
provided that: a) the span, L, does not exceed 5 times the mean height of the columns b) hr satisfies the criterion: (hr/ sa) + (hr/ sb) ≤ 0.5 in which sa and sb are the horizontal distances from the apex to the columns. For a symmetrical frame this expression simplifies to hr ≤ 0.25L.
2 2
αcr ≥ 10 for combinations with gravity loading with frame imperfections for clad structures provided that the stiffening effects of masonry infill wall panels or diaphragms of profiled steel sheeting are not taken into account
[top]Design
Once the analysis has been completed, allowing for second-order effects if necessary, the frame members must be verified.
Both the cross-sectional resistance and the buckling resistance of the members must be verified. In-plane buckling of members (using expression 6.61 of BS EN 1993-1-1[11]) need not be verified as the global analysis is considered to account for all significant in-plane effects. SCI P400 identifies the likely critical zones for member verification. SCI P397contains numerical examples of member verifications.
[top]Cross-section resistance
Member bending, axial and shear resistances must be verified. If the shear or axial force is high, the bending resistance is reduced so combined shear force and bending and axial force and bending resistances need to be verified. In typical portal frames neither the shear force nor the axial load is sufficiently high to reduce the bending resistance. When the portal frame forms the chord of the bracing system, the axial load in the rafter may be significant, and this combination of actions should be verified. Although all cross-sections need to be verified, the likely key points are at the positions of maximum bending moment:
In the column at the underside of the haunch In the rafter at the sharp end of the haunch In the rafter at the maximum sagging location adjacent to the apex.
[top]Member stability
Diagrammatic representation of a portal frame rafter
The figure on the right shows a diagrammatic representation of the issues that need to be addressed when considering the stability of a member within a portal frame, in this example a rafter between the eaves and apex. The following points should be noted:
There are no intermediate points of restraint for in plane flexural buckling Purlins provide intermediate lateral restraint to one flange. Depending on the bending moment diagram this may be either the tension or compression flange Restraints to the inside flange can be provided at purlin positions, producing a torsional restraint at that location.
In-plane, no member buckling checks are required, as the global analysis has accounted for all significant in-plane effects. The analysis has accounted for any significant second-order effects, and frame imperfections are usually accounted for by including the equivalent horizontal force in the analysis. The effects of in-plane member imperfections are small enough to be ignored.
Because there are no minor axis moments in a portal frame rafter, Expression 6.62 simplifies to:
[top]Rafter
design and stability
In the plane of the frame rafters are subject to high bending moments, which vary from a maximum „hogging‟ moment at the junc tion with the column to a minimum sagging moment close to the apex. Compression is introduced in the rafters due to actions applied to the frame. The rafters are not subject to any minor axis moments. Optimum design of portal frame rafters is generally achieved by use of:
A cross section with a high ratio of Iyy to Izz that complies with the requirements of Class 1 or 2 under combined major axis bending and axial compression. A haunch that extends from the column for approximately 10% of the frame span. This will generally mean that the maximum hogging and sagging moments in the plain rafter length are of similar magnitude.
[top]Out-of-plane stability
Purlins attached to the top flange of the rafter provide stability to the member in a number of ways:
Direct lateral restraint, when the outer flange is in compression Intermediate lateral restraint to the tension flange between torsional restraints, when the outer flange is in tension Torsional and lateral restraint to the rafter when the purlin is attached to the tension flange and used in conjunction with rafter stays to the compression flange.
Initially, the out-of-plane checks are completed to ensure that the restraints are located at appropriate positions and spacing.
[top]Gravity combination of actions
Typical purlin and rafter stay arrangement for the gravity combination of actions
The figure on the right shows a typical moment distribution for the gravity combination of actions, typical purlin and restraint positions as well as stability zones, which are referred to further. Purlins are generally placed at up to 1.8 m spacing but this spacing may need to be reduced in the high moment regions near the eaves. In Zone A, the bottom flange of the haunch is in compression. The stability checks are complicated by the variation in geometry along the haunch. The bottom flange is partially or wholly in compression over the length of Zone B. In Zone C, the purlins provide lateral restraint to the top (compression) flange. The selection of the appropriate check depends on the presence of a plastic hinge, the shape of the bending moment diagram and the geometry of the section (three flanges or two flanges). The objective of the checks is to provide sufficient restraints to ensure the rafter is stable out-of-plane. Guidance on details of the out-of plane stability verification can be found in SCI P397.
[top]The uplift condition
Typical purlin and rafter stay arrangement for the uplift condition
In the uplift condition the top flange of the haunch will be in compression and will be restrained by the purlins. The moments and axial forces are smaller than those in the gravity load combination. As the haunch is stable in the gravity combination of actions, it will certainly be so in the uplift condition, being restrained at least as well, and under reduced loads In Zone F, the purlins will not restrain the bottom flange, which is in compression. The rafter must be verified between torsional restraints. A torsional restraint will generally be provided adjacent to the apex. The rafter may be stable between this point and the virtual restraint at the point of contraflexure, as the moments are generally modest in the uplift combination. If the rafter is not stable over this length, additional torsional restraints should be introduced, and each length of the rafter verified.
[top]In plane stability
No in-plane checks of rafters are required, as all significant in-plane effects have been accounted for in the global analysis.
[top]Column
design and stability
Typical portal frame column with plastic hinge at underside of haunch
The most heavily loaded region of the rafter is reinforced by the haunch. By contrast, the column is subject to a similar bending moment at the underside of the haunch, but without any additional strengthening. The optimum design for most columns is usually achieved by the use of:
A cross section with a high ratio of Iyy to Izz that complies with Class 1 or Class 2 under combined major axis bending and axial compression A plastic section modulus that is approximately 50% greater than that of the rafter.
The column size will generally be determined at the preliminary design stage on the basis of the requiredbending and compression resistances. Whether the frame is designed plastically or elastically, a torsional restraint should always be provided at the underside of the haunch. This may be from a side rail positioned at that level, or by some other means. Additional torsional restraints may be required between the underside of the haunch and the column base because the side rails are attached to the (outer) tension flange; unless restraints are provided the inner compression flange is unrestrained. A side rail that is not continuous (for example, interrupted by industrial doors) cannot be relied upon to provide adequate restraint. The column section may need to be increased if intermediate restraints to the compression flange cannot be provided. The presence of a plastic hinge will depend on loading, geometry and choice of column and rafter sections. In a similar way to the rafter, both out-of-plane and inplane stability must be verified.
[top]Out-of-plane stability
If there is a plastic hinge at the underside of the haunch, the distance to the adjacent torsional restraint must be less than the limiting distance Lm as given by BS EN 1993-1-1[11]Clause BB.3.1.1. It may be possible to demonstrate that a torsional restraint is not required at the side rail immediately adjacent to the hinge, but may be provided at some greater distance. In this case there will be intermediate lateral restraints between the torsional restraints
If the stability between torsional restraints cannot be verified, it may be necessary to introduce additional torsional restraints. If it is not possible to provide additional intermediate restraints, the size of the member must be increased. In all cases, a lateral restraint must be provided within Lm of a plastic hinge. When the frame is subject to uplift, the column moment will reverse. The bending moments will generally be significantly smaller than those under gravity loading combinations, and the column is likely to remain elastic
[top]In plane stability
No in-plane checks of columns are required, as all significant in-plane effects have been accounted for in the global analysis.
[top]Bracing
Bracing in a portal frame (Image courtesy of William Haley Engineering Ltd.)
Bracing is required to resist longitudinal actions due to wind and cranes, and to provide restraint to members. It is common to use hollow sections as bracing members. Bracing arrangement in a typical portal frame
[top]Vertical
bracing
Common bracing systems
The primary functions of vertical bracing in the side walls of the frame are:
To transmit the horizontal loads to the ground. The horizontal forces include forces from wind and cranes To provide a rigid framework to which side rails and cladding may be attached so that the rails can in turn provide stability to the columns To provide temporary stability during erection.
The bracing may be located:
At one or both ends of the building Within the length of the building
In each portion between expansion joints (where these occur).
Where the side wall bracing is not in the same bay as the plan bracing in the roof, an eaves strut is essential to transmit the forces from the roof bracing into the wall bracing. An eaves strut is also required:
To ensure the tops of the columns are adequately restrained in position To assist in during the construction of the structure To stabilise the tops of the columns if a fire boundary condition exists
[top]Portalised bays
Longitudinal stability using portalised bays
Where it is difficult or impossible to brace the frame vertically by conventional bracing, it is necessary to introduce moment-resisting frames in the elevations in one or more bays. In addition to the general serviceability limit on deflection of h/300, where h is the height of the portalised bay it is suggested that:
The bending resistance of the portalised bay (not the main portal frame) is checked using an elastic frame analysis Deflection under the equivalent horizontal forces is restricted to h/1000, where theequivalent horizontal forces are calculated based on the whole of the roof area.
[top]Bracing to restrain longitudinal loads from cranes
Additional bracing in the plane of the crane girder
If a crane is directly supported by the frame, the longitudinal surge force will be eccentric to the column and will tend to cause the column to twist, unless additional restraint is provided. A horizontal truss at the level of the crane girder top flange or, for lighter cranes, a horizontal member on the inside face of the column flange tied into the vertical bracing may be adequate to provide the necessary restraint. For large horizontal forces, additional bracing should be provided in the plane of the crane girder.
[top]Plan
bracing
Plan view showing both end bays braced
Plan bracing is located in the plane of the roof. The primary functions of the plan bracing are:
To transmit wind forces from the gable posts to the vertical bracing in the walls To transmit any frictional drag forces from wind on the roof to the vertical bracing To provide stability during erection To provide a stiff anchorage for the purlins which are used to restrain the rafters.
In order to transmit the wind forces efficiently, the plan bracing should connect to the top of the gable posts.
[top]Restraint to inner flanges
Restraint to the inner flanges of rafters or columns is often most conveniently formed by diagonal struts from the purlins or sheeting rails to small plates welded to the inner flange and web. Pressed steel flat ties are commonly used. Where restraint is only possible from one side, the restraint must be able to carry compression. In these locations angle sections of minimum size 40 × 40 mm must be used. The stay and its connections should be designed to resist a force equal to 2.5% of the maximum force in the column or rafter compression flange between adjacent restraints.
[top]Connections
The major connections in a portal frame are the eaves and apex connections, which are both moment-resisting. The eaves connection in particular must generally carry a very large bending moment. Both the eaves and apex connections are likely to experience reversal in certain combinations of actions and this can be an important design case. For economy, connections should be arranged to minimise any requirement for additional reinforcement (commonly called stiffeners). This is generally achieved by:
Making the haunch deeper (increasing the lever arms) Extending the eaves connection above the top flange of the rafter (an additional bolt row) Adding bolt rows Selecting a stronger column section.
The design of moment resisting connections is covered in detail in SCI P398.
Apex connection
Eaves connection
Typical portal frame connections
Haunched connections
[top]Column
bases
Typical nominally pinned base
In the majority of cases, a nominally pinned base is provided, because of the difficulty and expense of providing a rigid base. A rigid base will involve a more expensive base detail, but more significantly, the foundation must also resist the moment, which increases costs significantly compared to a nominally pinned base. If a column base is nominally pinned, it is recommended that the base be modelled as perfectly pinned when using elastic global analysis to calculate the moments and forces in the frame under ULS loading. The stiffness of the base may be assumed to be equal to the following proportion of the column stiffness:
10% when assessing frame stability 20% when calculating deflections under serviceability loads.
[top]References
1. 2. 3. 4. 5. 6. ^ BS EN 1991, Eurocode 1: Actions on structures, BSI ^ 2.0 2.1 2.2 2.3 BS EN 1990: 2002, Eurocode - Basis of structural design, BSI ^ BS EN 1991-1-1: 2002 Eurocode 1: Actions on structures. General actions. Densities, self-weight, imposed loads for buildings , BSI ^ NA to BS EN 1991-1-1: 2002, UK National Annex to Eurocode 1. Actions on structures. General actions. Densities, self-weight, imposed loads for buildings, BSI ^ 5.0 5.1 BS EN 1991-1-3: 2003 Eurocode 1. Actions on structures. General actions. Snow loads, BSI ^ NA to BS EN 1991-1-3: 2003, UK National Annex to Eurocode 1. Actions on structures. General actions. Snow loads, BSI
7. 8. 9. 10. 11. 12.
^ BS EN 1991-1-4: 2005 +A1: 2010 Eurocode 1. Actions on structures. General actions. Wind actions, BSI ^ NA to BS EN 1991-1-4: 2005 +A1: 2010 UK National Annex to Eurocode 1. Actions on structures. General actions. Wind actions, BSI ^ BS EN 1991-1-7: 2006 Eurocode 1. Actions on structures. General actions. Accidental actions, BSI ^ NA to BS EN 1990: 2002 +A1: 2005 UK National Annex for Eurocode. Basis of structural design, BSI ^ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 BS EN 1993-1-1: 2005, Eurocode 3: Design of steel structures. General rules and rules for buildings, BSI ^ NA to BS EN 1993-1-1: 2005, UK National Annex to Eurocode 3: Design of steel structures. General rules and rules for buildings, BSI
[top]Further
reading
Steel Designers' Manual 7th Edition. Editors B Davison & G W Owens. The Steel Construction Institute 2012, Chapters 3 and 4
[top]Resources
SCI P292 In-plane Stability of Portal Frames to BS 5950-1:2000, 2001 SCI P281 Design of Curved Steel, 2001 SCI P391 Structural Robustness of Steel Framed Buildings, SCI, 2001 SCI P362 Steel Building Design: Concise Eurocodes, 2009 SCI P394 Wind Actions to BS EN 1991-1-4, SCI, 2013 SCI P397 Elastic Design of Single-span Steel Portal Frame Buildings to Eurocode 3, 2013 SCI P398 Joints in Steel Construction: Moment-resisting Joints to Eurocode 3, 2013 SCI P313 Single Storey Steel Framed Buildings in Fire Boundary Conditions, 2002 SCI P400 Interim report: Design of portal frames to Eurocode 3: An overview for UK designers, 2013
[top]See
also
Thermal performance Introduction to acoustics Steelwork specification Steel construction products Design codes and standards Member design Concept design Fabrication Braced frames Allowing for the effects of deformed frame geometry Modelling and analysis Structural robustness Structural fire resistance requirements Single storey buildings in fire boundary conditions Moment resisting connections Continuous frames Single storey industrial buildings
Retail buildings Building envelopes Design software and tools
Continuing Professional Development
Contents
[hide]
1 In-house technical CPD seminars
o o o o o o o o o o o o o
1.1 Sustainability and steel construction 1.2 Introduction to EC3 1.3 Worked examples to EC3 1.4 Design for fire 1.5 Portal frames 1.6 Steel floor construction 1.7 Steel grades and specifications 1.8 Corrosion protection 1.9 Weathering steel bridges 1.10 Design of floors for vibration 1.11 Acoustics 1.12 Steel the safe solution 1.13 Request Form
2 Fire Engineering Seminars 2013
In-house technical CPD seminars
Primarily aimed at engineers and architects, these free in-house technical CPD seminars last around 50 minutes, ending with a question & answer session, and are designed to be presented around the lunchtime period. They may either be delivered online, or by Tata Steel's experienced team of Regional Technical Managers.
[top]Sustainability
and steel construction
Vulcan House in Sheffield Steel framed: BREEAM Excellent
This presentation sets out the principal sustainable construction drivers in the UK and identifies key things that structural engineers can do to deliver sustainable buildings. It covers operational carbon emissions, building assessment , waste, materials and planning. Summary results from the Target Zero programme are presented and steel construction sustainability credentials demonstrated.
Request this in-house technical CPD seminar or go to online version
[top]Introduction
to EC3
Buckling curves for members in compression
This presentation highlights the more significant technical features of EC3, highlighting the major changes in presentation compared to BS 5950. The presentation covers the Eurocode system of determining of ultimate loads and then introduces the Eurocode approach to the assessment of frame stability and choice of steel sub-grade. An introduction to the calculation of member resistances is given, including the calculation of flexural buckling resistance(members in compression) and lateral-torsional buckling resistance (members in bending). A number of useful support resources are highlighted. This presentation is complemented by the presentation of worked examples to EC3. Request this in-house technical CPD seminar or go to online version
[top]Worked
examples to EC3
Erecting precast floor planks Image courtesy of Atlas Ward Structures Ltd.
A series of numerical worked examples are presented, demonstrating the application or EC3 to common design situations. The use of the expressions given in the Standard is demonstrated, but also the use of look-up tables and other support resources. Each worked example is complemented by using the resistance tables in the ‘Blue Book’. The examples cover the design of struts, restrained beams and unrestrained beams, and members subject to both compression and bending. The examples incorporate the influence of the UK National Annex.
Request this in-house technical CPD seminar or go to online version
[top]Design
for fire
Large compartment fire test
This presentation examines the regulatory background to fire precautions in buildings in the UK and the most common methods of meeting the demands of these regulations. It describes the most common forms of structural fire protection and explains the role of fire testing. It also describes the special case of fire precautions in single storey buildings. Finally, the role of fire safety engineering is explained and its role in providing more economical solutions for fire safety in buildings than is explored. Request this in-house technical CPD seminar or go to online version
[top]Portal
frames
Cranes erecting portal frames Image courtesy of Atlas Ward Structures Ltd.
Portal frames are an efficient, cost effective structural form for single-storey buildings, justifiably representing a large share of the market. This very common form of structure involves a range of structural behaviour that must be recognised and correctly addressed by designers. This presentation presents an overview of analysis and design, focussing on the key design consideration and critical detailing requirements that should be addressed in portal frame construction. The presentation covers the design rules as presented in EC3 and the UK National Annex. Request this in-house technical CPD seminar or go to online version
[top]Steel
floor construction
Decking being laid out on a steel frame
Exploring the range of options available to Engineers when considering the design of a floor plate, this presentation gives practical thoughts on the what, when, where of different options such as clear span, metal deck or precast units. It also gives an overview of how to assess the performance of steel floors over the life of the building and the impacts of fabrication and erection on the overall cost of the building. Request this in-house technical CPD seminar or go to online version
[top]Steel
grades and specifications
V-notch impact test specimen
This is a practical presentation providing guidance on a number of issues, including:
Should the Designer adopt S275 or S355 grade steel as their standard for sections? Comparison of hot rolled with cold formed hollow sections and issues to be considered before substituting one for the other. Why subgrade selection is important and the requirements of both BS5950 and EC3.
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[top]Corrosion
protection
Airless spray application of paint
This presentation is designed to provide essential information and guidance for those concerned with the corrosion protection of structural steelwork. It covers howcorrosion occurs, design detailing, methods of surface preparation, paint and metallic coatings, specifications and the importance of inspection and quality control. Request this in-house technical CPD seminar or go to online version
[top]Weathering
steel bridges
A typical weathering steel bridge over the A1 at Wetherby Weathering steel is a high strength low alloy steel that in suitable environments forms an adherent protective rust „patina‟, to prevent furth er corrosion. The corrosion rate is so low that bridges fabricated from unpainted weathering steel can achieve a 120 year design life with only nominal maintenance. Hence, a well detailed weathering steel bridge in an appropriate environment can provide an attractive, very low maintenance, economic solution in many locations. This seminar, highlights the benefits of using weathering steel , describes thelimitations, and comments on both the material availability and the appearance of such bridges. It also provides advice on a range of issues including, design and detailing, fabrication and installation, inspection and maintenance, and possible remedial measures should corrosion rates exceed those anticipated at the design stage.
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[top]Design
of floors for vibration
Impulsive response
Reviewing industry standard practice for vibration, this presentation helps Designers to understand the science behind those general rules of thumb. It then walks through the simplified approach for Design of Floors for Vibration laid out in SCI Guide P354, gives a quick method to determine whether vibration will be an issue for a proposed floor and points towards guidance for special cases such as gymnasia, hospitals and car parks. Request this in-house technical CPD seminar or go to online version
[top]Acoustics
Composite floor on steel beams
This presentation provides information on the acoustic detailing of steel framed buildings for sound insulation. The presentation begins with some fundamentals about soundand how it behaves before moving on to look at the regulatory requirements relating to sound insulation in buildings and then some of the many solutions that are available for steel framed buildings. Guidance on the principles of acoustic detailing are discussed in the presentation along with sources of design guidance such as the online acoustic performance prediction tool. In addition, case study projects are referenced which give on-site acoustic test data. This CPD is currently only available as an online version
[top]Steel
the safe solution
Example of a safe system of work with netting and edge protection Image courtesy of Richard Lees Structural Decking Ltd.
This presentation provides information on the safe erection of steel framed buildings. The presentation begins with some basics about howsafe construction practice is established before moving on to consider the specific health and safety objectives for steel erection and how these can be met. The issues that need resolving are contextualised in terms of progress through the stages of design development and construction. Guidance is given on how competence is assessed and how specifiers may contribute to the selection of a suitably competent steelwork contractor. The importance of dialogue between designers and constructors is developed. This CPD is currently only available as an online version
[top]Request
Form
To express interest in in-house technical CPD seminars delivered by our experienced team of Regional Technical Managers please click here to register and use the attached CPD request form.
[top]Fire
Engineering Seminars 2013
Unprotected steel beams at Plantation Place South, London. Image courtesy of Arup Fire
The BCSA and Tata Steel held two Fire Engineering Seminars during June 2013 to educate architects and engineers in the principles and techniques that they need to know about fire-safe designs and to explain how these are applied in practice. Leading fire engineering practitioners from the UK shared their knowledge and presented case studies. The Association for Specialist Fire Protectionoutlined their work in maintaining standards and supporting specifiers within the construction sector, and informative presentations by expert speakers from the BCSA helped to raise awareness on the most recent developments in the most cost-effective methods for designing buildings for fire. Topics covered included: Legislation and trends; the work of the Association for Specialist Fire Protection; testing, principles and practice; simplified structural fire engineering; the design of unusual steel structures, including The Shard; and how architects and engineers can gain the best value from fire engineering. Watch the online versions. Category: CPD
The free encyclopedia for UK steel construction information
Concept design
Contents
1 Multi-storey buildings
o o o o
1.1 Hierarchy of design decisions 1.2 Anatomy of building design 1.3 Floor grids 1.4 Dimensional coordination
o
1.4.1 Influence of building height 1.4.2 Horizontal coordination 1.4.3 Vertical coordination 1.5.1 Rigid frames 1.5.2 Braced frames 1.5.3 Concrete or steel cores
1.5 Structural options for stability
o o o o o o o
1.6 Columns 1.7 Structural options for floor systems 1.8 Estimating steel quantities 1.9 Factors influencing structural arrangements
1.9.1 Site conditions 1.9.2 Cranes
1.10 Service integration
2 Single storey buildings 2.1 Hierarchy of design decisions 2.2 Architectural design
o
2.2.1 Building form 2.2.2 Simple roof beam, supported on columns. 2.2.3 Portal frame 2.2.4 Trusses 2.2.5 Other forms of construction 2.3.1 Cladding types
2.3 Choice of building type
o o
2.3.1.1 Roofing 2.3.1.2 Walls
2.4 Concept design of portal frames
2.4.1 Frame stability 2.4.2 Member stability 2.5.1 Eaves connection 2.5.2 Apex connection
2.5 Connections
3 Resources 4 See also
This article presents information necessary to assist in the choice and use of steel structures at the concept design stage for modern multi-storey buildings and single storey buildings . The information is presented in terms of the design strategy, anatomy of building design and structural systems. For multi-storey buildings the primary sector of interest is commercial buildings, but the same information may also be used in other sectors. For single storey buildings the primary sector of interest is industrial buildings, but the same information can also be used in other sectors, such as commercial, retail and leisure .
Multi-storey buildings
In multi-storey buildings , the design of the primary structure is strongly influenced by many issues, as defined below:
The need to provide clear floor spans for more usable space The choice of cladding system Planning requirements, which may limit the building height and the maximum floor-tofloor zone
The services strategy and effective integration of building services Site conditions, which dictate the foundation system and location of foundations Craneage limitations and storage space for materials and components Speed of construction, which may influence the number of components that are used and the installation process.
Studies show that the cost of the building structure is generally only 10% of the total building cost - and the influence of the choice of structure on the foundations, services and cladding are often more significant. In reality, building design is a synthesis of architectural, structural, services, logistics and buildability issues. Steel frames are ideally suited for modern multi-storey commercial buildings.
Hierarchy of design decisions
The development of any proposal for a construction project requires a complex series of design decisions that are inter-related. The process should begin with a clear understanding of the client requirements and of local conditions or regulations. Despite the complexity, it is possible to identify a hierarchy of design decisions. Firstly, planning requirements are likely to define the overall building form, which will also include aspects such as natural lighting, ventilation and services. The principal design choices that need to be made in close consultation with the client are:
The depth of the floor zone and the overall structure/service interaction strategy The need for special structural arrangements in public spaces or circulation areas The provision of some tolerance between structure and services, to permit future adaptability The benefit of using longer spans, at negligible extra cost, in order to enhance flexibility of layout.
Based on the design brief, a concept design can then be prepared which is reviewed by the design team and client. It is at this early interactive stage where the important decisions are made that influence the cost and value of the final project. Close involvement with the client is essential.
[top]Anatomy
of building design
The building design is dependent on various parameters; these include:
Floor grid Building height Circulation and access space Services requirements and service integration.
[top]Floor
grids
Floor grids define the spacing of the columns in orthogonal directions, which are influenced by:
The planning grid (normally based on units of 300 mm but more typically multiples of 0.6, 1.2 or 1.5 m) The column spacing along the façades, depending on the façades material (typically 5.4 to 7.5 m) The use of the internal space, i.e. for offices or open plan space The requirements for building service distribution (from the building core).
Along the façade line, column spacings are normally defined by the need to provide support to the cladding system for example, a maximum column spacing of 6 m is normally required for brickwork. This influences the column spacing internally, unless additional columns are used along the façade line. The span of the beams across the building normally conforms to one of the following column grid arrangement:
Single internal line of columns, placed offset to the line of a central corridor. This is shown in the figure below Pairs of column lines on either side of a corridor Column-free internal spans with columns located along the façade line.
For naturally ventilated offices, a building width of 12 m to 15 m is typically used, which can be achieved by two spans of 6 to 7.5 m. A single span can also be provided with deep (400 mm or more) precast concrete hollow core units spanning the full width of the building. Natural lighting also plays a role in choice of the width of floor plate. In modern buildings, a long span solution provides a considerable enhancement in flexibility of layout. For air-conditioned offices, a clear span of 15 m to 18 m is often used. An example of the column grid for a long span option in a building with a large atrium is shown in the figure below.
Dimensional coordination
The choice of the basic building shape is usually the architect's responsibility, constrained by such issues as the site plan, access, building orientation, parking, landscaping and local planning requirements. The following general guidance influences the choice of structure.
Between sources of natural light there should be 13.5 m and 20 m intervals Naturally lit and ventilated zones extend a distance of twice the floor-to-ceiling height from the outer walls - artificial light and ventilation is required in other zones Atria improve the efficient use of the building, and reduce the running costs.
[top]Influence of building height
The building height has a strong influence on the:
Structural system that is adopted Foundation system Fire resistance requirements and means of escape Access (by lifts) and circulation space Choice of cladding system Speed of construction and site productivity.
For taller buildings, strategically placed concrete or braced steel cores are usually adopted. Ultra tall buildings are influenced strongly by the stabilising system, but are not covered here. Sizes of lifts and their speed of movement also become important considerations for tall buildings. Depending on the Regulations for fire safety, the use of sprinklers may be required for buildings of more than eight storeys (or approximately 30 m high).
[top]Horizontal coordination
Horizontal coordination is dominated by the need on plan for defined zones for vertical access, safe evacuation in fire, and vertical service distribution. Positioning of service and access cores is influenced by:
Horizontal distribution systems for mechanical services Fire resistance requirements, which may control evacuation routes and compartment sizes The need to distribute the stabilizing systems (bracing and cores) effectively throughout the building plan.
The two planning grids shown above present typical arrangements that satisfy these criteria. An atrium may be incorporated to increase lighting to the occupied space and to provide high value circulation areas at ground and intermediate levels. The design requirements for atria are:
Support to the long span roof of the atrium Access routes for general circulation Fire safety measures by smoke extraction and safe evacuation routes Light levels and servicing to internal offices.
[top]Vertical coordination
The target floor-to-floor height is based on a floor-to-ceiling height of 2.5 to 2.7 m for speculative offices, or 3 m for more prestige applications, plus the floor depth including services. The following target floor-to-floor depths should be considered at the concept design stage: Prestige office 4.0 - 4.2 m
Speculative office
3.6 - 4.0 m
Renovation project
3.5 - 3.9 m
These targets permit a range of structural solutions. If, for planning reasons, it is required to limit the overall building height, this can be achieved by use of shallow floor or integrated beam systems. Integrated beam systems are often used in renovation projects where the floor-to-floor height is limited by compatibility with the existing building or façades. For concept design of orthodox commercial multi-storey steel structures , the following 'target' floor depths may be used. Flooring system Target floor depth (mm)
Composite beam construction
800 – 1,200
Cellular beams (with service integration)
800 – 1,100
Downstand beams with precast concrete floor slabs
1,200 – 1,450
Shallow floor or integrated beams
600 – 800
Typical floor depths for multi storey buildings
[top]Structural
options for stability
The structural system required for stability is primarily influenced by the building height. For buildings up to eight storeys height, the steel structure may be designed to provide stability, but for taller buildings, concrete or braced steel cores are more efficient structurally. The following structural systems may be considered for stability.
[top]Rigid frames
For buildings up to four storeys high, rigid frames may be used in which the multiple beam to column connections provide bending resistance and stiffness to resist horizontal loads. This is generally only possible where the beams are relatively deep (400 mm to 500 mm) and where the column size is increased to resist the applied moments. Full depthend plate connections generally provide the necessary rigidity.
[top]Braced frames
For buildings up to 12 storeys high, braced steel frames are commonly used in which cross, K or V bracing is used in the walls, generally within a cavity in the façades, or around stairs or other serviced zones. Cross bracing is designed in tension only (the other member being redundant). Cross bracing is often simple flat steel plate , but angle and channel sections may also be used. When bracing is designed to work in compression, hollow sections are often used, although angle and channel sections may also be used. A steel braced frame has the two key advantages:
Responsibility for temporary stability lies with one organisation As soon as the steel bracing is connected (bolted), the structure is stable.
[top]Concrete or steel cores
Concrete cores are the most practical system for buildings of up to 40 storeys high, but the concrete core is generally constructed in advance of the steel framework. In this form of construction, the beams often span directly between the columns on the perimeter of the building and the concrete core. Special structural design considerations are required for:
The beam connections to the concrete core The design of the heavier primary beams at the corner of core Fire safety and robustness of the long span construction.
A typical layout of beams around a concrete core is shown in the figure below, with the use of heavier beams at the corner of the core. A double beam may be required to minimise the structural depth at the corner of the cores.
Typical beam layout around a concrete core
A braced steel core
Braced steel cores may be used as an economic alternative where speed of construction is critical. Such cores are installed with the rest of the steelwork package. An example of a braced steel core is shown in the figure above right.
[top]Columns
Columns in multi-storey steel frames are generally H sections , predominantly carrying axial load. When the stability of the structure is provided by cores, or discreet vertical bracing, the beams are generally designed as simply supported. The generally accepted design model is that nominally pinned connections produce nominal moments in the column, calculated by assuming that the beam reaction is 100 mm from the face of the column. If the reactions on the opposite side of the column are equal, there is no net moment. Columns on the perimeter of the structure will have an applied moment, due to the connection being on one side only. Typical column sizes are given in the table below. Number of floors supported by column section Universal Column (UKC) serial size
1
152
2-4
203
3-8
254
5 - 12
305
10 - 40
356
Typical column sizes for small and medium span composite floors
Although small column sections may be preferred for architectural reasons, the practical issues of connections to the floor beams should be considered. It can be difficult and costly to provide connection into the minor axis of a very small column section.
[top]Structural
options for floor systems
A wide range of floor system solutions is available for which typical solutions are given in the table below. Form of construction Typical solution
Downstand beams Low rise, modest spans, no restriction on construction depth precast units or composite floors Downstand beams in the façade precast concrete units (15 m), composite floors with secondary Low rise, long span e.g. 15m steel beams spanning 15 m
Medium and high rise, modest spans, no restriction on construction depth Downstand beams, composite construction
Medium and high rise, long spans (to 18 m) restricted construction depth Composite floors with cellular long span secondary steel beams
Typical floor solutions
Although steel solutions are appropriate for short spans (typically 6 to 9 m), steel has an important advantage over other materials in that long span solutions (between 12 and 18 m) can be easily provided. This has the key advantage of column-free space, allowing future adaptability, and fewer foundations.
Floors spanning onto the steel beams will normally be either precast concrete units , or composite floors. The supporting beams may be below the floor, with the floor bearing on the top flange (often known as 'downstand' beams ), or the beams may share the same zone with the floor construction, to reduce the overall depth of the zone. The available construction zone is often the determining factor when choosing a floor solution. Beams within the floor zone are known as slim floor, or integrated beams. Beams may be non composite, or composite. In composite construction shear connectors are welded to the top flange of the beam, transferring load to the concrete floor. Precast concrete units may be used for low rise frames, but composite floors are common in both low rise and high rise structures. The span range of various structural options in both steel and concrete are illustrated in the table below. Long span steel options generally provide for service integration for spans of over 12m. Cellular beams and composite trusses are more efficient for long span secondary beams, whereas fabricated beams are often used for long span primary beams.
Span range of various structural options
[top]Estimating
steel quantities
For estimating purposes in the design of office buildings, representative weights of steel may be used for buildings of rectangular plan form. These quantities will increase significantly for non rectangular or tall buildings or for buildings with atria or complex façades. The approximate quantities are presented in the table below, and are expressed in terms of the total floor area of the building. They do not include the steelwork used in the façades, atrium or roof. Approximate steel quantities
(kg/m² floor area) Form of Building Beams Columns Bracing Total
3 or 4 storey building of rectangular form
25–30
8–10
2–3
35–40
6–8 storey building of rectangular form
25–30
12–15
3–5
40–50
8–10 storey building with long spans
35–40
12–15
3–5
50–60
20 storey building with long spans and a concrete core
40–50
10–13
1–2
50–65
Approximate steel quantities
Further guidance on estimating steel quantities and cost is available.
[top]Factors
influencing structural arrangements
The construction programme will be a key concern in any project, and should be considered at the same time as considering the cost of structure, the services, cladding and finishes. The structural scheme has a key influence on programme and cost, and structural solutions which can be erected safely, quickly to allow early access for the following trades.
[top]Site conditions
Increasingly, structures are constructed on 'brownfield' sites, where earlier construction has left a permanent legacy. In city centres, a solution involving fewer, albeit more heavily loaded foundations are often preferred, which lead to longer spans for the superstructure.
[top]Cranes
The number of cranes on a project will be dominated by the site footprint, the size of the project and the use of additional mobile cranes. Multi-storey structures are generally erected using a tower crane, which may be supplemented by mobile cranes for specific heavy lifting operations. In city centre projects, tower cranes are often located in a lift shaft or atrium.
Service integration
Most large office-type structures require air conditioning or 'comfort cooling', which will necessitate both horizontal and vertical distribution systems. The provision for such systems is of critical importance for the superstructure layout, affecting the layout and type of members chosen. See SCI P166 and P273. The basic decision either to integrate the ductwork within the structural depth or to simply suspend the ductwork at a lower level affects the choice of structural member, the fire protection system, the cladding (cost and programme) and overall building height. Other systems provide conditioned air from a raised floor.
Single storey buildings
Single storey buildings use steel framed structures and metallic cladding of all types. Large open spaces can be created, which are efficient, easy to maintain and are adaptable as demand changes. Single storey buildings are a 'core' market for steel in the UK. Single storey buildings tend to be large enclosures, but may require space for other uses, such as offices, handling and transportation, overhead cranes, etc. Therefore, many factors have to be addressed in their design. Increasingly, architectural issues and visual impact have to be addressed and many leading architects are involved in the design of modern single storey buildings .
Hierarchy of design decisions
Important design factors for single storey buildings
The development of a design solution for a single storey building, such as a large enclosure or industrial facility is more dependent on the activity being performed and future requirements for the space than other building types, such as commercial and residential buildings. Although these building types are primarily functional, they are commonly designed with strong architectural involvement dictated by planning requirements and client 'branding'. The following overall design requirements should be considered in the concept design stage ofindustrial buildings and large enclosures, depending on the building form and use:
Space use, for example, specific requirements for handling of materials or components in a production facility Flexibility of space in current and future use Speed of construction Environmental performance, including services requirements and thermal performance Aesthetics and visual impact Acoustic isolation, particularly in production facilities Access and security Sustainability considerations Design life and maintenance requirements, including end of life issues.
To enable the concept design to be developed, it is necessary to review these considerations based on the type of single storey building. For example, the requirements for a distribution centre will be different to a manufacturing facility. A review of the importance of various design issues is presented in the table on the right for common building types.
[top]Architectural
design
Modern single storey buildings using steel are both functional in use and are designed to be architecturally attractive. Various examples are presented below together with a brief description of the design concept.
[top]Building form
The basic structural form of a single storey building may be of various generic types, as shown in the figure below. The figure shows a conceptual cross-section through each type of building, with notes on the structural concept, and typical forces and moments due to gravity loads.
Structural concepts
The basic design concepts for each structural type are described below:
[top]Simple roof beam, supported on columns.
The span will generally be modest, up to approximately 20 m. The roof beam may be pre-cambered. Bracing will be required in the roof and all elevations, to provide in-plane and longitudinal stability.
[top]Portal frame
A portal frame is a continuous frame with moment resisting connections to provide stability in-plane. A portal frame may be single bay or multi bay. The members are generally plain rolled sections, with the resistance of the rafter enhanced locally with a haunch. In many cases, the frame will have pinned bases. Stability in the
longitudinal direction is provided by a combination of bracing in the roof, across one or both end bays, and vertical bracing in the elevations. If vertical bracing cannot be provided in the elevations (due to industrial doors, for example) stability is often provided by a rigid frame within the elevation.
[top]Trusses
Truss buildings generally have roof bracing and vertical bracing in each elevation to provide stability in both orthogonal directions. The trusses may take a variety of forms, with shallow or steep external roof slopes. A truss building may also be designed as rigid in-plane, although it is more common to provide bracing to stabilise the frame.
[top]Other forms of construction
Built-up columns (two plain beams, connected to form a compound column) are often used to support heavy loads, such as cranes. These may be used in portalised structures, but are often used with rigid bases, and with bracing to provide in-plane stability. External or suspended support structures may be used, but are relatively uncommon.
[top]Choice
of building type
Portal frames are considered to be a highly cost-effective way to provide a single storey enclosure. Their efficiency depends on the method of analysis, and the assumptions that are made regarding the restraint to the structural members, as shown in the table below. Most efficient Less efficient
Analysis using elastic-plastic software
Elastic analysis
Cladding considered to restrain the flange of the purlins and side rails
Purlins and side rails unrestrained
Purlins and side rails used to restrain both flanges of the hot-rolled steelwork The inside flange of the hot rolled steelwork is unrestrained
Nominal base stiffness utilised
Nominal base stiffness ignored
Efficient portal frame design
The reasons for choosing simple beam structures, portal frames or trusses are shown in the table below. Simple beam Portal frame Truss
Advantages
Simple design
Long span
Very long spans possible
Designed to be stable in plane
Heavy loads may be carried
Member sizes and haunches may be optimised for efficiency
Modest deflection
Disadvantages
Relatively short span
Software required for efficient design
Generally more expensive fabrication
Bracing needed for in-plane stability
Limited to relatively light vertical loading, and modest cranes to avoid excessive deflections
Generally bracing is used for in-plane stability
No economy due to continuity
Efficient portal frame design
[top]Cladding types
The main types of roofing and wall cladding used in single storey buildings are described as follows:
[top]Roofing
'Built-up' or double layer roofing spanning between secondary members such as purlins. Composite panels (also known as sandwich panels) spanning between purlins. Deep decking spanning between main frames, supporting insulation, with an external metal sheet or waterproof membrane.
[top]Walls
Sheeting, orientated vertically and supported on side rails. Sheeting or structural liner trays spanning horizontally between columns. Composite or sandwich panels spanning horizontally between columns, eliminating side rails. Metallic cassette panels supported by side rails.
Different forms of cladding (including vertically and horizontally orientated sheets) may be used together for visual effect in the same façades. Brickwork is often used as a 'dado' or 'dwarf' wall below the level of the windows for impact resistance.
[top]Concept
design of portal frames
Steel portal frames are widely used because they combine structural efficiency with functional form. A single-span symmetrical portal frame (as illustrated in the figure below) is typically of the following proportions:
A span between 15 m and 50 m (25 m to 35 m is the most efficient) An eaves height (base to rafter centreline) of between 5 and 15 m (7.5 m or more is commonly adopted). The eaves height is determined by the specified clear height between the top of the floor and the underside of the haunch.
A roof pitch between 5° and 10° (6° is commonly adopted) A frame spacing between 5 m and 8 m (the greater frame spacings being used in longer span portal frames) Members are I sections rather than H sections, because they must carry significant bending moments and provide in-plane stiffness. Sections are generally S275 or S355. As deflections may be critical, the use of higher strength steel is rarely justified. Haunches are provided in the rafters at the eaves to enhance the bending resistance of the rafter and to facilitate a bolted connection to the column. Small haunches are provided at the apex, to facilitate the bolted connection.
Single span symmetric portal frame
The eaves haunch is typically cut from the same size Standard open sections|rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the span. The length of the haunch means that the hogging bending moment at the 'sharp' end of the haunch is approximately the same as the maximum sagging bending moment towards the apex, as shown in the figure below.
No higher resolution available.
The end frames of a portal frame are generally called gable frames. Gable frames may be identical to the internal frames, even though they experience lighter loads. If future extension to the building is envisaged, portal frames are commonly used as the gable frames, to reduce the impact of the structural works. A typical gable frame is shown in the figure below.
Typical details of an end gable of a portal frame building
Frame stability
In-plane stability is provided by frame continuity. In the longitudinal direction, stability is provided by vertical bracing in the elevations. The vertical bracing may be at both ends of the building, or in one bay only. Each frame is connected to the vertical bracing by a hot-rolled member at eaves level. A typical bracing arrangement is shown in the figure below.
Typical bracing in a portal frame
The gable columns span between the base and the rafter, where the reaction is carried by bracing in the plane of the roof, back to the eaves level, and to the foundations by the vertical bracing. If diagonal bracing in the elevations cannot be accommodated, longitudinal stability can be provided by a rigid frame on the elevation.
[top]Member stability
Restraint locations
For economic design , restraints to the rafter and column must be considered. The purlins and side rails are considered adequate to restrain the flange that they are attached to, but unless special measures are taken, the purlins and side rails do not restrain the inside flange. Restraint to the inside flange is commonly provided by bracing from the purlins and side rails, as shown in the figure below. The bracing is usually formed of thin metal straps, designed to act in tension, or from angles designed in compression if bracing is only possible from one side. The arrangement of restraints to the inside flange is generally similar to that shown in the figure below and in all cases, the junction of the inside face of the column and the underside of the haunch must be restrained.
Restraint bracing to inside flange
[top]Connections [top]Eaves connection
Typical eaves connection
A typical eaves connection is shown in in the figure below. In almost all cases a compression stiffener in the column (as shown, at the bottom of the haunch) will be required. Other stiffeners may be required to increase the bending resistance of the column flange, adjacent to the tension bolts, and to increase the shear resistance of the column web panel. The haunch is generally fabricated from a similar size beam to the rafter (or larger), or fabricated from equivalent plate. Typically, the bolts may be M24 8.8 and the end plate 25 mm thick S275.
[top]Apex connection
A typical apex connection is shown in the figure below. The apex connection primarily serves to increase the depth of the member to make a satisfactory bolted connection. The apex haunch is usually fabricated from the same member as the rafter, or from equivalent plate. Typically, the bolts may be M24 8.8 and the end plate 25 mm thick S275.
Typical apex connection
[top]Resources
Steel Buildings in Europe - Multi-storey buildings: Part 1 Architect’s guide Steel Buildings in Europe - Multi-storey buildings: Part 2 Concept design Steel Buildings in Europe - Single storey buildings: Part 1 Architect’s guide Steel Buildings in Europe - Single storey buildings: Part 2 Concept design SCI P166 Interfaces: Design of Steel Framed Buildings for Service Integration, 1997
SCI P273 Service Integration in Slimdek, 2000 Steel Buildings, 2003, (Publication No 35/03), BCSA
[top]See
Chapter 3 - Single Storey Buildings Chapter 4 - Multi-Storey Buildings
SCI P167 Architectural Teaching Resource Studio Guide, 2000
also
Multi-storey buildings Single storey buildings Composite construction Floor systems Long span beams Portal frames Moment resisting connections Service integration Building envelopes Cost of structural steelwork Cost comparison study Retail buildings Leisure buildings Structural fire resistance requirements Continuous frames Braced frames Steel construction products Structural robustness Trusses Fire and steel construction Design software and tools
Portal frames
Contents
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1 Anatomy of a typical portal frame 2 Types of portal frames 3 Design considerations
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3.1 Choice of material and section 3.2 Frame dimensions
3.2.1 Clear span and height 3.2.2 Main frame 3.2.3 Haunch dimensions 3.2.4 Positions of restraints
4 Actions
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4.1 Permanent actions 4.1.1 Service loads 4.2.1 Imposed roof loads 4.2.2 Snow loads 4.2.3 Wind actions 4.2.4 Crane actions 4.2.5 Accidental actions 4.2.6 Robustness 4.2.7 Fire 4.2 Variable actions
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7 Design
4.3 Combinations of actions
5 Frame analysis at ULS 5.1 Plastic analysis 5.2 Elastic analysis
6 In-plane frame stability 6.1 Second order effects 6.2 First-order and second-order analysis 6.3 Calculation of αcr 6.4 Sensitivity to effects of the deformed geometry
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7.1 Cross-section resistance 7.2 Member stability 7.3 Rafter design and stability
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7.3.1 Out-of-plane stability 7.3.2 Gravity combination of actions 7.3.3 The uplift condition 7.3.4 In plane stability 7.4.1 Out-of-plane stability 7.4.2 In plane stability
7.4 Column design and stability
8 Bracing
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8.1 Vertical bracing 8.1.1 Portalised bays 8.1.2 Bracing to restrain longitudinal loads from cranes 8.2.1 Restraint to inner flanges
8.2 Plan bracing
9 Connections
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9.1 Column bases
10 References 11 Further reading 12 Resources 13 See also 14 External links 15 CPD
Portal frames are generally low-rise structures, comprising columns and horizontal or pitched rafters, connected by moment-resisting connections. Resistance to lateral and vertical actions is provided by the rigidity of the connections and the bending stiffness of the members, which is increased by a suitable haunch or deepening of the rafter sections. This form of continuous frame structure is stable in its plane and provides a clear span that is unobstructed by bracing. Portal frames are very common, in fact 50% of constructional steel used in the UK is in portal frame construction. They are very efficient for enclosing large volumes, therefore they are often used for industrial, storage, retail and commercial applications as well as for agricultural purposes. This article describes the anatomy and various types of portal frame and key design considerations.
Multi-bay portal frame during construction Anatomy of a typical portal frame
Principal components of a portal framed building
A portal frame building comprises a series of transverse frames braced longitudinally. The primary steelwork consists of columns and rafters, which form portal frames, and bracing. The end frame (gable frame) can be either a portal frame or a braced arrangement of columns and rafters.
The light gauge secondary steelwork consists of side rails for walls and purlins for the roof. The secondary steelwork supports the building envelope, but also plays an important role in restraining the primary steelwork. The roof and wall cladding separate the enclosed space from the external environment as well as providingthermal and acoustic insulation. The structural role of the cladding is to transfer loads to secondary steelwork and also to restrain the flange of the purlin or rail to which it is attached.
Cross-section showing a portal frame and its restraints
Portal framed structures - overview
[top]Types
of portal frames
Many different forms of portal frames may be constructed. Frame types described below give an overview of types of portal construction with typical features illustrated. This information only provides typical details and is not meant to dictate any limits on the use of any particular structural form. Pitched roof symmetric portal frame
Generally fabricated from UKB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. 25 to 35 m are the most efficient spans. Pitched roof symmetric portal frame Lancashire Waste Development
Portal frame with internal mezzanine floor
Office accommodation is often provided within a portal frame structure using a partial width mezzanine floor. The assessment of frame stability must include the effect of the mezzanine; guidance is given in SCI P292. Crane portal frame with column brackets Portal frame with internal mezzanine floor Waters Meeting Health Centre, Bolton (Image courtesy BD Structures Ltd. and ASD Westok Ltd.)
Where a travelling crane of relatively low capacity (up to say 20 tonnes) is required, brackets can be fixed to the columns to support the crane rails. Use of a tie member or rigid column bases may be necessary to reduce the eaves deflection.
The spread of the frame at crane rail level may be of critical importance to the functioning of the crane; requirements should be agreed with the client and with the crane manufacturer. Tied portal frame
In a tied portal frame the horizontal movement of the eaves and the bending moments in the columns and rafters are reduced. A tie may be useful to limit spread in a crane-supporting structure. The high axial forces introduced in the frame when a tie is used necessitate the use of second-order software when analysing this
form of frame.
Mono-pitch portal frame
A mono pitch portal frame is usually chosen for small spans or because of its proximity to other buildings. It is a simple variation of the pitched roof portal frame, and tends to be used for smaller buildings (up to 15 m span). Propped portal frame
Where the span of a portal frame is large and there is no requirement to provide a clear span, a propped portal frame can be used to reduce the rafter size and also the horizontal Propped portal frame shear at the foundations. Rebottling Plant, Hemswell (Image courtesy of Metsec plc)
Mansard portal frame
A mansard portal frame may be used where a large clear height at mid-span is required but the eaves height of the building has to be minimised. Curved rafter portal frame
Portal frames may be constructed using curved rafters, mainly for architectural reasons. Because of transport limitations rafters longer than 20 m may require splices, which should be carefully detailed for architectural reasons. The curved member is often modelled for analysis as a series of straight elements. Guidance on the
stability of curved rafters in portal frames is given in SCI P281. Alternatively, the rafter can be fabricated as a series of straight elements. It will be necessary to provide purlin cleats of varying height to achieve the curved external profile. Cellular beam portal frame
Rafters may be fabricated from cellular beams for aesthetic reasons or when providing long spans. Where transport limitations impose requirement for splices, they should be carefully detailed, to preserve the architectural features. The sections used Cellular beam portal frame Hayes garden centre (Image courtesy of ASD Westok Ltd.)
cannot develop plastic hinges at a cross-section, so only elastic design is used.
[top]Design
considerations
In the design and construction of any structure, a large number of inter-related design requirements should be considered at each stage in the design process. The following discussion of the design process and its constituent parts is intended to give the designer an understanding of the inter-relationship of the various elements of the structure with its final construction, so that the decisions required at each stage can be made with an understanding of their implications.
[top]Choice
of material and section
Steel sections used in portal frame structures are usually specified in grade S275 or S355 steel. In plastically designed portal frames, Class 1 plastic sections must be used at hinge positions that rotate, Class 2 compact sections can be used elsewhere
[top]Frame
dimensions
Dimensions used for analysis and clear internal dimensions
A critical decision at the conceptual design stage is the overall height and width of the frame, to give adequate clear internal dimensions and adequate clearance for the internal functions of the building.
[top]Clear span and height
The clear span and height required by the client are key to determining the dimensions to be used in the design, and should be established early in the design process. The client requirement is likely to be the clear distance between the flanges of the two columns – the span will therefore be larger, by the section depth. Any requirement for brickwork or blockwork around the columns should be established as this may affect the design span. Where a clear internal height is specified, this will usually be measured from the finished floor level to the underside of the haunch or suspended ceiling if present.
[top]Main frame
The main (portal) frames are generally fabricated from UKB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. A typical frame is characterised by:
A span between 15 and 50 m An clear height (from the top of the floor to the underside of the haunch) between 5 and 12 m A roof pitch between 5° and 10° (6° is commonly adopted) A frame spacing between 6 and 8 m Haunches in the rafters at the eaves and apex A stiffness ratio between the column and rafter section of approximately 1.5 Light gauge purlins and side rails Light gauge diagonal ties from some purlins and side rails to restrain the inside flange of the frame at certain locations.
[top]Haunch dimensions
Typical haunch with restraints
The use of a haunch at the eaves reduces the required depth of rafter by increasing the moment resistance of the member where the applied moments are highest. The haunch also adds stiffness to the frame, reducing deflections, and facilitates an efficient bolted moment connection. The eaves haunch is typically cut from the same size rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the frame span. The haunch length generally means that the hogging moment at the end of the haunch is approximately equal to the largest sagging moment close to the apex. The depth from the rafter axis to the underside of the haunch is approximately 2% of the span. The apex haunch may be cut from a rolled section – often from the same size as the rafter, or fabricated from plate. The apex haunch is not usually modelled in the frame analysis and is only used to facilitate a bolted connection.
[top]Positions of restraints
General arrangement of restraints to the inside flange
During initial design the rafter members are normally selected according to their cross sectional resistance to bending moment and axial force. In later design stages stability against buckling needs to be verified and restraints positioned judiciously. The buckling resistance is likely to be more significant in the selection of a column size, as there is usually less freedom to positing rails to suit the design requirements; rails position maybe dictated by doors or windows in the elevation. If introducing intermediate lateral restraints to the column is not possible, the buckling resistance will determine the initial section size selection. It is therefore essential to recognise at this early stage if the side rails may be used to provide restraint to the columns. Only continuous side rails are effective in providing restraint. Side rails interrupted by (for example) roller shutter doors, cannot be relied on as providing adequate restraint. Where the compression flange of the rafter or column is not restrained by purlins and side rails, restraint can be provided at specified locations by column and rafter stays.
[top]Actions
Advice on actions can be found in BS EN 1991[1], and on the combinations of actions in BS EN 1990[2]. It is important to refer to the UK National Annex for the relevant Eurocode part for the structures to be constructed in the UK.
[top]Permanent
actions
Permanent actions are the self weight of the structure, secondary steelwork and cladding. Where possible, unit weights of materials should be obtained from manufacturers‟ data. Where information is not available, these may be determined from the data in BS EN 1991-1-1[3].
[top]Service loads
Service loads will vary greatly depending on the use of the building. In portal frames heavy point loads may occur from suspended walkways, air handling units etc. It is necessary to consider carefully where additional provision is needed, as particular items of plant must be treated individually. Depending on the use of the building and whether sprinklers are required, it is normal to assume a service loading of 0.1–0.25 kN/m on plan over the whole roof area.
2
[top]Variable
actions
Roof slope, α qk (kN/m²)
[top]Imposed roof loads
α < 30°
0.6
30° < α < 60° 0.6[60 - α)/30]
Imposed loads on roofs are given in the UK NA to BS EN 1991-1-1[4], and depend on the roof slope. A point load, Qk is given, which is used for local checking of roof materials and fixings, and a uniformly distributed load, qk, to be applied vertically. The loading for roofs not accessible except for normal maintenance and repair is given in the table on the right. It should be noted that imposed loads on roofs should not be combined with either snow or wind.
α > 60°
0
Imposed loads on roofs
[top]Snow loads
Snow loads may sometimes be the dominant gravity loading. Their value should be determined from BS EN 1991-1-3[5] and its UK National Annex[6] – the determination of snow loads is described in Chapter 3 of the Steel Designers’ Manual. Any drift condition must be allowed for not only in the design of the frame itself, but also in the design of the purlins that support the roof cladding. The intensity of loading at the position of maximum drift often exceeds the basic minimum uniform snow load. The calculation of drift loading and associated purlin design has been made easier by the major purlin manufacturers, most of whom offer free software to facilitate rapid design.
[top]Wind actions
Wind actions in the UK should be determined using BS EN 1991-1-4[7] and its UK National Annex[8]. This Eurocode gives much scope for national adjustment and therefore its annex is a substantial document. Wind actions are inherently complex and likely to influence the final design of most buildings. The designer needs to make a careful choice between a fully rigorous, complex assessment of wind actions and the use of simplifications which ease the design process but make the loads more conservative. Free software for establishing wind pressures is available from manufacturers. For more advice refer to Chapter 3 of the Steel Designers’ Manual and SCI P394. Wind loading calculator
[top]Crane actions
Gantry girders carrying an overhead travelling crane
The most common form of craneage is the overhead type running on beams supported by the columns. The beams are carried on cantilever brackets or, in heavier cases, by providing dual columns. In addition to the self weight of the cranes and their loads, the effects of acceleration and deceleration have to be considered. For simple cranes, this is by a quasistatic approach with amplified loads For heavy, high-speed or multiple cranes the allowances should be specially calculated with reference to the manufacturer.
[top]Accidental actions
The common design situations which are treated as accidental design situations are:
Drifted snow, determined using Annex B of BS EN 1991-1-3[5] The opening of a dominant opening which was assumed to be shut at ULS
Each project should be individually assessed whether any other accidental actions are likely to act on the structure.
[top]Robustness
Robustness requirements are designed to ensure that any structural collapse is not disproportionate to the cause. BS EN 1990[2] sets the requirement to design and construct robust buildings in order to avoid disproportionate collapse under accidental design situations. BS EN 1991-1-7[9] gives details of how this requirement should be met. For many portal frame structures no special provisions are needed to satisfy robustness requirements set by the Eurocode. For more information on robustness refer to SCI P391.
[top]Fire
Collapse mechanism of a portal with a lean-to under fire, boundary condition on gridlines 2 and 3.
In the United Kingdom, structural steel in single storey buildings does not normally require fire resistance. The most common situation in which it is required to fire protect the structural steelwork is where prevention of fire spread to adjacent buildings, a boundary condition, occurs. There are a small number of other, rare, instances, for example when demanded by an insurance provider, where structural fire protection may be required. When a portal frame is close to the boundary, there are several requirements aimed at stopping fire spread by keeping the boundary intact:
The use of fire resistant cladding Application of fire protection of the steel up to the underside of the haunch The provision of a moment resisting base (as it is assumed that in the fire condition rafters go into catenary)
Comprehensive advice is available in SCI P313.
[top]Combinations
of actions
BS EN 1990[2] gives rules for establishing combinations of actions, with the values of relevant factors given in the UK National Annex[10]. BS EN 1990[2] covers both ultimate limit state (ULS) and serviceability limit state (SLS), although for the SLS, onward reference is made to the material codes (for example BS EN 19931-1[11] for steelwork) to identify which expression should be used and what SLS limits should be observed.
All combinations of actions that can occur together should be considered, however if certain actions cannot be applied simultaneously, they should not be combined. Guidance on the application of Eurocode rules on combinations of actions can be found in SCI P362 and, specifically for portal frames, in SCI P400.
[top]Frame
analysis at ULS
At the ultimate limit state (ULS), the methods of frame analysis fall broadly into two types: elastic analysis and plastic analysis.
[top]Plastic analysis
Bending moment diagram resulting from the plastic analysis of a symmetrical portal frame under symmetrical loading
The term plastic analysis is used to cover both rigid-plastic and elastic-plastic analysis. Plastic analysiscommonly results in a more economical frame because it allows relatively large redistribution of bending moments throughout the frame, due to plastic hinge rotations. These plastic hinge rotations occur at sections where the bending moment reaches the plastic moment or resistance of the cross-section at loads below the full ULS loading. The rotations are normally considered to be localised at “plastic hinges” and allow the capacity of under utilised parts of the frame to be mobilised. For this reason members, where possible plastic hinges may occur need to be Class 1 sections, which are capable of accommodating rotations. The figure shows typical positions, where plastic hinges form in a portal frame. Two hinges lead to a collapse, but in the illustrated example, due to symmetry, designers need to consider all possible hinge locations.
[top]Elastic analysis
A typical bending moment diagram resulting from an elastic analysis of a frame with pinned bases is shown the figure below. In this case, the maximum moment (at the eaves) is higher than that calculated from a plastic analysis. Both the column and haunch have to be designed for these large bending moments. Where deflections (SLS) govern design, there may be no advantage in using plastic analysis for the ULS. If stiffer sections are selected in order to control deflections, it is quite possible that no plastic hinges form and the frame remains elastic at ULS.
Bending moment diagram resulting from the elastic analysis of a symmetrical portal frame under symmetrical loading
Portal frame analysis software (Fastrak model courtesy of CSC)
[top]In-plane
frame stability
When any frame is loaded, it deflects and its shape under load is different from the un-deformed shape. The deflection has a number of effects:
The vertical loads are eccentric to the bases, which leads to further deflection The apex drops, reducing the arching action Applied moments curve members; Axial compression in curved members causes increased curvature (which may be perceived as a reduced stiffness.)
Taken together, these effects mean that a frame is less stable (nearer collapse) than a first-order analysis suggests. The objective of assessing frame stability is to determine if the difference is significant.
[top]Second order effects
P-δ and P-Δ effects in a portal frame
The geometrical effects described above are second-order effects and should not be confused with non linear behaviour of materials. As shown in the figure below there are two categories of second-order effects:
Effects of displacements of the intersections of members, usually called P-Δ effects. BS EN 1993-1-1[11]describes this as the effect of deformed geometry. Effects of deflections within the length of members, usually called P-δ effects.
Second-order analysis is the term used to describe analysis methods in which the effects of increasing deflection under increasing load is considered explicitly in the solution, so that the results include the P-δ and P-Δ effects.
[top]First-order and second-order analysis
For either plastic analysis of frames, or elastic analysis of frames, the choice of first-order analysis or second-order analysis depends on the in plane flexibility of the frame, characterised by the calculation of the αcr factor.
[top]Calculation of αcr
The effects of the deformed geometry (P-Δ effects) are assessed in BS EN 1993–1–1[11] by calculating the factor αcr, defined as:
where: Fcr is the elastic critical buckling load for global instability mode, based on initial elastic stiffnesses FEd is the design load on the structure. αcr may be found using software or using an approximation (expression 5.2 from BS EN 1993-1-1[11]) as long as the frame meets certain geometric limits and the axial force in the rafter is not „significant‟. Rules are given in the Eurocode to identify when the axial force is significant. When the frame falls outside the specified limits, as is the case for very many orthodox frames, the simplified expression cannot be used. In these circumstances, an alternative expression may be used to calculate an approximate value of αcr, referred to asαcr,est. Further details are given in SCI P397.
[top]Sensitivity to effects of the deformed geometry
The limitations to the use of first-order analysis are defined in BS EN 1993–1–1[11], Section 5.2.1 (3) and the UK National Annex[12] Section NA.2.9 as: For elastic analysis: αcr ≥ 10 For plastic analysis:
αcr ≥ 5 for combinations with gravity loading with frame imperfections,
provided that: a) the span, L, does not exceed 5 times the mean height of the columns b) hr satisfies the criterion: (hr/ sa) + (hr/ sb) ≤ 0.5 in which sa and sb are the horizontal distances from the apex to the columns. For a symmetrical frame this expression simplifies to hr ≤ 0.25L.
2 2
αcr ≥ 10 for combinations with gravity loading with frame imperfections for clad structures provided that the stiffening effects of masonry infill wall panels or diaphragms of profiled steel sheeting are not taken into account
[top]Design
Once the analysis has been completed, allowing for second-order effects if necessary, the frame members must be verified.
Both the cross-sectional resistance and the buckling resistance of the members must be verified. In-plane buckling of members (using expression 6.61 of BS EN 1993-1-1[11]) need not be verified as the global analysis is considered to account for all significant in-plane effects. SCI P400 identifies the likely critical zones for member verification. SCI P397contains numerical examples of member verifications.
[top]Cross-section resistance
Member bending, axial and shear resistances must be verified. If the shear or axial force is high, the bending resistance is reduced so combined shear force and bending and axial force and bending resistances need to be verified. In typical portal frames neither the shear force nor the axial load is sufficiently high to reduce the bending resistance. When the portal frame forms the chord of the bracing system, the axial load in the rafter may be significant, and this combination of actions should be verified. Although all cross-sections need to be verified, the likely key points are at the positions of maximum bending moment:
In the column at the underside of the haunch In the rafter at the sharp end of the haunch In the rafter at the maximum sagging location adjacent to the apex.
[top]Member stability
Diagrammatic representation of a portal frame rafter
The figure on the right shows a diagrammatic representation of the issues that need to be addressed when considering the stability of a member within a portal frame, in this example a rafter between the eaves and apex. The following points should be noted:
There are no intermediate points of restraint for in plane flexural buckling Purlins provide intermediate lateral restraint to one flange. Depending on the bending moment diagram this may be either the tension or compression flange Restraints to the inside flange can be provided at purlin positions, producing a torsional restraint at that location.
In-plane, no member buckling checks are required, as the global analysis has accounted for all significant in-plane effects. The analysis has accounted for any significant second-order effects, and frame imperfections are usually accounted for by including the equivalent horizontal force in the analysis. The effects of in-plane member imperfections are small enough to be ignored.
Because there are no minor axis moments in a portal frame rafter, Expression 6.62 simplifies to:
[top]Rafter
design and stability
In the plane of the frame rafters are subject to high bending moments, which vary from a maximum „hogging‟ moment at the junc tion with the column to a minimum sagging moment close to the apex. Compression is introduced in the rafters due to actions applied to the frame. The rafters are not subject to any minor axis moments. Optimum design of portal frame rafters is generally achieved by use of:
A cross section with a high ratio of Iyy to Izz that complies with the requirements of Class 1 or 2 under combined major axis bending and axial compression. A haunch that extends from the column for approximately 10% of the frame span. This will generally mean that the maximum hogging and sagging moments in the plain rafter length are of similar magnitude.
[top]Out-of-plane stability
Purlins attached to the top flange of the rafter provide stability to the member in a number of ways:
Direct lateral restraint, when the outer flange is in compression Intermediate lateral restraint to the tension flange between torsional restraints, when the outer flange is in tension Torsional and lateral restraint to the rafter when the purlin is attached to the tension flange and used in conjunction with rafter stays to the compression flange.
Initially, the out-of-plane checks are completed to ensure that the restraints are located at appropriate positions and spacing.
[top]Gravity combination of actions
Typical purlin and rafter stay arrangement for the gravity combination of actions
The figure on the right shows a typical moment distribution for the gravity combination of actions, typical purlin and restraint positions as well as stability zones, which are referred to further. Purlins are generally placed at up to 1.8 m spacing but this spacing may need to be reduced in the high moment regions near the eaves. In Zone A, the bottom flange of the haunch is in compression. The stability checks are complicated by the variation in geometry along the haunch. The bottom flange is partially or wholly in compression over the length of Zone B. In Zone C, the purlins provide lateral restraint to the top (compression) flange. The selection of the appropriate check depends on the presence of a plastic hinge, the shape of the bending moment diagram and the geometry of the section (three flanges or two flanges). The objective of the checks is to provide sufficient restraints to ensure the rafter is stable out-of-plane. Guidance on details of the out-of plane stability verification can be found in SCI P397.
[top]The uplift condition
Typical purlin and rafter stay arrangement for the uplift condition
In the uplift condition the top flange of the haunch will be in compression and will be restrained by the purlins. The moments and axial forces are smaller than those in the gravity load combination. As the haunch is stable in the gravity combination of actions, it will certainly be so in the uplift condition, being restrained at least as well, and under reduced loads In Zone F, the purlins will not restrain the bottom flange, which is in compression. The rafter must be verified between torsional restraints. A torsional restraint will generally be provided adjacent to the apex. The rafter may be stable between this point and the virtual restraint at the point of contraflexure, as the moments are generally modest in the uplift combination. If the rafter is not stable over this length, additional torsional restraints should be introduced, and each length of the rafter verified.
[top]In plane stability
No in-plane checks of rafters are required, as all significant in-plane effects have been accounted for in the global analysis.
[top]Column
design and stability
Typical portal frame column with plastic hinge at underside of haunch
The most heavily loaded region of the rafter is reinforced by the haunch. By contrast, the column is subject to a similar bending moment at the underside of the haunch, but without any additional strengthening. The optimum design for most columns is usually achieved by the use of:
A cross section with a high ratio of Iyy to Izz that complies with Class 1 or Class 2 under combined major axis bending and axial compression A plastic section modulus that is approximately 50% greater than that of the rafter.
The column size will generally be determined at the preliminary design stage on the basis of the requiredbending and compression resistances. Whether the frame is designed plastically or elastically, a torsional restraint should always be provided at the underside of the haunch. This may be from a side rail positioned at that level, or by some other means. Additional torsional restraints may be required between the underside of the haunch and the column base because the side rails are attached to the (outer) tension flange; unless restraints are provided the inner compression flange is unrestrained. A side rail that is not continuous (for example, interrupted by industrial doors) cannot be relied upon to provide adequate restraint. The column section may need to be increased if intermediate restraints to the compression flange cannot be provided. The presence of a plastic hinge will depend on loading, geometry and choice of column and rafter sections. In a similar way to the rafter, both out-of-plane and inplane stability must be verified.
[top]Out-of-plane stability
If there is a plastic hinge at the underside of the haunch, the distance to the adjacent torsional restraint must be less than the limiting distance Lm as given by BS EN 1993-1-1[11]Clause BB.3.1.1. It may be possible to demonstrate that a torsional restraint is not required at the side rail immediately adjacent to the hinge, but may be provided at some greater distance. In this case there will be intermediate lateral restraints between the torsional restraints
If the stability between torsional restraints cannot be verified, it may be necessary to introduce additional torsional restraints. If it is not possible to provide additional intermediate restraints, the size of the member must be increased. In all cases, a lateral restraint must be provided within Lm of a plastic hinge. When the frame is subject to uplift, the column moment will reverse. The bending moments will generally be significantly smaller than those under gravity loading combinations, and the column is likely to remain elastic
[top]In plane stability
No in-plane checks of columns are required, as all significant in-plane effects have been accounted for in the global analysis.
[top]Bracing
Bracing in a portal frame (Image courtesy of William Haley Engineering Ltd.)
Bracing is required to resist longitudinal actions due to wind and cranes, and to provide restraint to members. It is common to use hollow sections as bracing members. Bracing arrangement in a typical portal frame
[top]Vertical
bracing
Common bracing systems
The primary functions of vertical bracing in the side walls of the frame are:
To transmit the horizontal loads to the ground. The horizontal forces include forces from wind and cranes To provide a rigid framework to which side rails and cladding may be attached so that the rails can in turn provide stability to the columns To provide temporary stability during erection.
The bracing may be located:
At one or both ends of the building Within the length of the building
In each portion between expansion joints (where these occur).
Where the side wall bracing is not in the same bay as the plan bracing in the roof, an eaves strut is essential to transmit the forces from the roof bracing into the wall bracing. An eaves strut is also required:
To ensure the tops of the columns are adequately restrained in position To assist in during the construction of the structure To stabilise the tops of the columns if a fire boundary condition exists
[top]Portalised bays
Longitudinal stability using portalised bays
Where it is difficult or impossible to brace the frame vertically by conventional bracing, it is necessary to introduce moment-resisting frames in the elevations in one or more bays. In addition to the general serviceability limit on deflection of h/300, where h is the height of the portalised bay it is suggested that:
The bending resistance of the portalised bay (not the main portal frame) is checked using an elastic frame analysis Deflection under the equivalent horizontal forces is restricted to h/1000, where theequivalent horizontal forces are calculated based on the whole of the roof area.
[top]Bracing to restrain longitudinal loads from cranes
Additional bracing in the plane of the crane girder
If a crane is directly supported by the frame, the longitudinal surge force will be eccentric to the column and will tend to cause the column to twist, unless additional restraint is provided. A horizontal truss at the level of the crane girder top flange or, for lighter cranes, a horizontal member on the inside face of the column flange tied into the vertical bracing may be adequate to provide the necessary restraint. For large horizontal forces, additional bracing should be provided in the plane of the crane girder.
[top]Plan
bracing
Plan view showing both end bays braced
Plan bracing is located in the plane of the roof. The primary functions of the plan bracing are:
To transmit wind forces from the gable posts to the vertical bracing in the walls To transmit any frictional drag forces from wind on the roof to the vertical bracing To provide stability during erection To provide a stiff anchorage for the purlins which are used to restrain the rafters.
In order to transmit the wind forces efficiently, the plan bracing should connect to the top of the gable posts.
[top]Restraint to inner flanges
Restraint to the inner flanges of rafters or columns is often most conveniently formed by diagonal struts from the purlins or sheeting rails to small plates welded to the inner flange and web. Pressed steel flat ties are commonly used. Where restraint is only possible from one side, the restraint must be able to carry compression. In these locations angle sections of minimum size 40 × 40 mm must be used. The stay and its connections should be designed to resist a force equal to 2.5% of the maximum force in the column or rafter compression flange between adjacent restraints.
[top]Connections
The major connections in a portal frame are the eaves and apex connections, which are both moment-resisting. The eaves connection in particular must generally carry a very large bending moment. Both the eaves and apex connections are likely to experience reversal in certain combinations of actions and this can be an important design case. For economy, connections should be arranged to minimise any requirement for additional reinforcement (commonly called stiffeners). This is generally achieved by:
Making the haunch deeper (increasing the lever arms) Extending the eaves connection above the top flange of the rafter (an additional bolt row) Adding bolt rows Selecting a stronger column section.
The design of moment resisting connections is covered in detail in SCI P398.
Apex connection
Eaves connection
Typical portal frame connections
Haunched connections
[top]Column
bases
Typical nominally pinned base
In the majority of cases, a nominally pinned base is provided, because of the difficulty and expense of providing a rigid base. A rigid base will involve a more expensive base detail, but more significantly, the foundation must also resist the moment, which increases costs significantly compared to a nominally pinned base. If a column base is nominally pinned, it is recommended that the base be modelled as perfectly pinned when using elastic global analysis to calculate the moments and forces in the frame under ULS loading. The stiffness of the base may be assumed to be equal to the following proportion of the column stiffness:
10% when assessing frame stability 20% when calculating deflections under serviceability loads.
[top]References
1. 2. 3. 4. 5. 6. ^ BS EN 1991, Eurocode 1: Actions on structures, BSI ^ 2.0 2.1 2.2 2.3 BS EN 1990: 2002, Eurocode - Basis of structural design, BSI ^ BS EN 1991-1-1: 2002 Eurocode 1: Actions on structures. General actions. Densities, self-weight, imposed loads for buildings , BSI ^ NA to BS EN 1991-1-1: 2002, UK National Annex to Eurocode 1. Actions on structures. General actions. Densities, self-weight, imposed loads for buildings, BSI ^ 5.0 5.1 BS EN 1991-1-3: 2003 Eurocode 1. Actions on structures. General actions. Snow loads, BSI ^ NA to BS EN 1991-1-3: 2003, UK National Annex to Eurocode 1. Actions on structures. General actions. Snow loads, BSI
7. 8. 9. 10. 11. 12.
^ BS EN 1991-1-4: 2005 +A1: 2010 Eurocode 1. Actions on structures. General actions. Wind actions, BSI ^ NA to BS EN 1991-1-4: 2005 +A1: 2010 UK National Annex to Eurocode 1. Actions on structures. General actions. Wind actions, BSI ^ BS EN 1991-1-7: 2006 Eurocode 1. Actions on structures. General actions. Accidental actions, BSI ^ NA to BS EN 1990: 2002 +A1: 2005 UK National Annex for Eurocode. Basis of structural design, BSI ^ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 BS EN 1993-1-1: 2005, Eurocode 3: Design of steel structures. General rules and rules for buildings, BSI ^ NA to BS EN 1993-1-1: 2005, UK National Annex to Eurocode 3: Design of steel structures. General rules and rules for buildings, BSI
[top]Further
reading
Steel Designers' Manual 7th Edition. Editors B Davison & G W Owens. The Steel Construction Institute 2012, Chapters 3 and 4
[top]Resources
SCI P292 In-plane Stability of Portal Frames to BS 5950-1:2000, 2001 SCI P281 Design of Curved Steel, 2001 SCI P391 Structural Robustness of Steel Framed Buildings, SCI, 2001 SCI P362 Steel Building Design: Concise Eurocodes, 2009 SCI P394 Wind Actions to BS EN 1991-1-4, SCI, 2013 SCI P397 Elastic Design of Single-span Steel Portal Frame Buildings to Eurocode 3, 2013 SCI P398 Joints in Steel Construction: Moment-resisting Joints to Eurocode 3, 2013 SCI P313 Single Storey Steel Framed Buildings in Fire Boundary Conditions, 2002 SCI P400 Interim report: Design of portal frames to Eurocode 3: An overview for UK designers, 2013
[top]See
also
Thermal performance Introduction to acoustics Steelwork specification Steel construction products Design codes and standards Member design Concept design Fabrication Braced frames Allowing for the effects of deformed frame geometry Modelling and analysis Structural robustness Structural fire resistance requirements Single storey buildings in fire boundary conditions Moment resisting connections Continuous frames Single storey industrial buildings
Retail buildings Building envelopes Design software and tools
Continuing Professional Development
Contents
[hide]
1 In-house technical CPD seminars
o o o o o o o o o o o o o
1.1 Sustainability and steel construction 1.2 Introduction to EC3 1.3 Worked examples to EC3 1.4 Design for fire 1.5 Portal frames 1.6 Steel floor construction 1.7 Steel grades and specifications 1.8 Corrosion protection 1.9 Weathering steel bridges 1.10 Design of floors for vibration 1.11 Acoustics 1.12 Steel the safe solution 1.13 Request Form
2 Fire Engineering Seminars 2013
In-house technical CPD seminars
Primarily aimed at engineers and architects, these free in-house technical CPD seminars last around 50 minutes, ending with a question & answer session, and are designed to be presented around the lunchtime period. They may either be delivered online, or by Tata Steel's experienced team of Regional Technical Managers.
[top]Sustainability
and steel construction
Vulcan House in Sheffield Steel framed: BREEAM Excellent
This presentation sets out the principal sustainable construction drivers in the UK and identifies key things that structural engineers can do to deliver sustainable buildings. It covers operational carbon emissions, building assessment , waste, materials and planning. Summary results from the Target Zero programme are presented and steel construction sustainability credentials demonstrated.
Request this in-house technical CPD seminar or go to online version
[top]Introduction
to EC3
Buckling curves for members in compression
This presentation highlights the more significant technical features of EC3, highlighting the major changes in presentation compared to BS 5950. The presentation covers the Eurocode system of determining of ultimate loads and then introduces the Eurocode approach to the assessment of frame stability and choice of steel sub-grade. An introduction to the calculation of member resistances is given, including the calculation of flexural buckling resistance(members in compression) and lateral-torsional buckling resistance (members in bending). A number of useful support resources are highlighted. This presentation is complemented by the presentation of worked examples to EC3. Request this in-house technical CPD seminar or go to online version
[top]Worked
examples to EC3
Erecting precast floor planks Image courtesy of Atlas Ward Structures Ltd.
A series of numerical worked examples are presented, demonstrating the application or EC3 to common design situations. The use of the expressions given in the Standard is demonstrated, but also the use of look-up tables and other support resources. Each worked example is complemented by using the resistance tables in the ‘Blue Book’. The examples cover the design of struts, restrained beams and unrestrained beams, and members subject to both compression and bending. The examples incorporate the influence of the UK National Annex.
Request this in-house technical CPD seminar or go to online version
[top]Design
for fire
Large compartment fire test
This presentation examines the regulatory background to fire precautions in buildings in the UK and the most common methods of meeting the demands of these regulations. It describes the most common forms of structural fire protection and explains the role of fire testing. It also describes the special case of fire precautions in single storey buildings. Finally, the role of fire safety engineering is explained and its role in providing more economical solutions for fire safety in buildings than is explored. Request this in-house technical CPD seminar or go to online version
[top]Portal
frames
Cranes erecting portal frames Image courtesy of Atlas Ward Structures Ltd.
Portal frames are an efficient, cost effective structural form for single-storey buildings, justifiably representing a large share of the market. This very common form of structure involves a range of structural behaviour that must be recognised and correctly addressed by designers. This presentation presents an overview of analysis and design, focussing on the key design consideration and critical detailing requirements that should be addressed in portal frame construction. The presentation covers the design rules as presented in EC3 and the UK National Annex. Request this in-house technical CPD seminar or go to online version
[top]Steel
floor construction
Decking being laid out on a steel frame
Exploring the range of options available to Engineers when considering the design of a floor plate, this presentation gives practical thoughts on the what, when, where of different options such as clear span, metal deck or precast units. It also gives an overview of how to assess the performance of steel floors over the life of the building and the impacts of fabrication and erection on the overall cost of the building. Request this in-house technical CPD seminar or go to online version
[top]Steel
grades and specifications
V-notch impact test specimen
This is a practical presentation providing guidance on a number of issues, including:
Should the Designer adopt S275 or S355 grade steel as their standard for sections? Comparison of hot rolled with cold formed hollow sections and issues to be considered before substituting one for the other. Why subgrade selection is important and the requirements of both BS5950 and EC3.
Request this in-house technical CPD seminar or go to online version
[top]Corrosion
protection
Airless spray application of paint
This presentation is designed to provide essential information and guidance for those concerned with the corrosion protection of structural steelwork. It covers howcorrosion occurs, design detailing, methods of surface preparation, paint and metallic coatings, specifications and the importance of inspection and quality control. Request this in-house technical CPD seminar or go to online version
[top]Weathering
steel bridges
A typical weathering steel bridge over the A1 at Wetherby Weathering steel is a high strength low alloy steel that in suitable environments forms an adherent protective rust „patina‟, to prevent furth er corrosion. The corrosion rate is so low that bridges fabricated from unpainted weathering steel can achieve a 120 year design life with only nominal maintenance. Hence, a well detailed weathering steel bridge in an appropriate environment can provide an attractive, very low maintenance, economic solution in many locations. This seminar, highlights the benefits of using weathering steel , describes thelimitations, and comments on both the material availability and the appearance of such bridges. It also provides advice on a range of issues including, design and detailing, fabrication and installation, inspection and maintenance, and possible remedial measures should corrosion rates exceed those anticipated at the design stage.
Request this in-house technical CPD seminar or go to online version
[top]Design
of floors for vibration
Impulsive response
Reviewing industry standard practice for vibration, this presentation helps Designers to understand the science behind those general rules of thumb. It then walks through the simplified approach for Design of Floors for Vibration laid out in SCI Guide P354, gives a quick method to determine whether vibration will be an issue for a proposed floor and points towards guidance for special cases such as gymnasia, hospitals and car parks. Request this in-house technical CPD seminar or go to online version
[top]Acoustics
Composite floor on steel beams
This presentation provides information on the acoustic detailing of steel framed buildings for sound insulation. The presentation begins with some fundamentals about soundand how it behaves before moving on to look at the regulatory requirements relating to sound insulation in buildings and then some of the many solutions that are available for steel framed buildings. Guidance on the principles of acoustic detailing are discussed in the presentation along with sources of design guidance such as the online acoustic performance prediction tool. In addition, case study projects are referenced which give on-site acoustic test data. This CPD is currently only available as an online version
[top]Steel
the safe solution
Example of a safe system of work with netting and edge protection Image courtesy of Richard Lees Structural Decking Ltd.
This presentation provides information on the safe erection of steel framed buildings. The presentation begins with some basics about howsafe construction practice is established before moving on to consider the specific health and safety objectives for steel erection and how these can be met. The issues that need resolving are contextualised in terms of progress through the stages of design development and construction. Guidance is given on how competence is assessed and how specifiers may contribute to the selection of a suitably competent steelwork contractor. The importance of dialogue between designers and constructors is developed. This CPD is currently only available as an online version
[top]Request
Form
To express interest in in-house technical CPD seminars delivered by our experienced team of Regional Technical Managers please click here to register and use the attached CPD request form.
[top]Fire
Engineering Seminars 2013
Unprotected steel beams at Plantation Place South, London. Image courtesy of Arup Fire
The BCSA and Tata Steel held two Fire Engineering Seminars during June 2013 to educate architects and engineers in the principles and techniques that they need to know about fire-safe designs and to explain how these are applied in practice. Leading fire engineering practitioners from the UK shared their knowledge and presented case studies. The Association for Specialist Fire Protectionoutlined their work in maintaining standards and supporting specifiers within the construction sector, and informative presentations by expert speakers from the BCSA helped to raise awareness on the most recent developments in the most cost-effective methods for designing buildings for fire. Topics covered included: Legislation and trends; the work of the Association for Specialist Fire Protection; testing, principles and practice; simplified structural fire engineering; the design of unusual steel structures, including The Shard; and how architects and engineers can gain the best value from fire engineering. Watch the online versions. Category: CPD
