Diagnostic Load Test of Continuous Prestressed Box Girder Bridge

Published on July 2016 | Categories: Documents | Downloads: 49 | Comments: 0 | Views: 181
of 8
Download PDF   Embed   Report

Diagnostic Load Test of Continuous Prestressed Box Girder Bridge

Comments

Content

Diagnostic Load Test of Continuous Prestressed Box Girder Bridge
Sayan
SIRIMONTREE
Associate Professor
Thammasat University
Phathumthani,Thailand

Kridayuth
CHOMPOOMING
Assistant Professor
Thammasat University
Phathumthani,Thailand

[email protected]

[email protected]

Sayan Sirimontree received his
PhD degree in civil engineering
from Khon Khaen University,
Thailand.

Kridayuth Chompooming
received his PhD degree in civil
engineering from Utah State
University, USA.

Wacharapong
PRASARNKLIEO
Lecturer
King Mongkut’s Institute of
Technology Ladkrabang,
Thailand
[email protected]
Wacharapong Prasarnklieo
received his MEng degree in civil
engineering from Thammasat
University, Thailand.

Summary
A main objective of the present investigation is to obtain structural responses of a continuous box
girder bridge employing a diagnostic load test. The bridge under consideration is a 7-span,
prestressed concrete structure with the total length of 665 meters. Four 3-axle trucks are employed
in the diagnostic load test. Strains and vibratory motions of the girder are measured and reported.
Varying speeds of test trucks are considered during the dynamic test. Natural frequencies and mode
shapes of the box girder are determined based on modal analysis. Correlation between natural
frequencies of the box girder obtained from field measurement and a finite element model is also
illustrated. Wavelet analysis is employed to decompose girder strains into static and dynamic
components, and dynamic amplification factors are determined and reported. In addition, the results
of long-term measurement of pot bearing movements subjected to variations of surrounding
temperature are also presented.
Keywords: box girder bridges; diagnostic load test; dynamic test; dynamic amplification factors;
modal analysis; wavelet analysis.

1.

Introduction

The structure under investigation is a main bridge of the First Thai-Lao Friendship Bridge crossing
over the Mekong River on the highway route between Nong Khai in Thailand and Vientiane in Lao
People’s Democratic Republic. The bridge was opened and has been under services since 1994.
Under continuously increasing traffic volume and load intensity, the bridge becomes vulnerable to
the conditions of deterioration and damage. Bridge inspection and structural evaluation is
established as part of corrective and preventive maintenance to ensure serviceability, load-carrying
capacity, and durability of the bridge structure by the Department of Highways, Thailand.
Diagnostic load tests can be considered as part of bridge evaluation and load-rating procedures for
identification of actual live load distribution, unintended composite action, conditions of support
movement, and participation of non-structural components [1]. A number of research and field
measurement studies have been reported on the applications of diagnostic load tests [2,3]. One of
the main objectives of the investigation is to gain insights into physical behavior and obtain actual
performance of the bridge structure based on load testing. Static and dynamic tests under
predetermined loads are considered. The measurement procedures and results of the diagnostic load
test are presented in the following.

2.

Description of Bridge Structure

The main bridge of the First Thai-Lao Friendship Bridge under consideration is a 7-span,
continuous prestressed structure with the total length of 665 meters. The bridge configuration and
overall dimensions are depicted in Fig. 1. Regarding the girder segment cross-sectional dimensions,
the width of the top slab is approximately 12.4 meters with the depth varied between 2.1 and 6.1
meters as shown in Fig. 2. The bridge supports two traffic lanes and a single track of 1-meter gauge
for railway along the center line.

Fig.1: General configuration of the main bridge structure

Fig.2: Cross section of the box girder of the main bridge

3. Diagnostic Load Test
3.1
Test Trucks
To accomplish the objectives of the diagnostic load test adopted in this investigation, four 3-axle
trucks are employed. Weights and dimensions of the test trucks are listed in Table 1.
Table 1: Weights and Dimension of Test Trucks

3.2

Static Test

3.2.1 Load Patterns of Test Trucks
A load pattern of test trucks as shown in Fig. 3 is employed in the static load test of the bridge. The
pattern of test trucks consists of two rows of trucks. The front row contains two trucks (No. 1 and 2)
traversing side-by-side on each traffic lane followed by the other two (No. 3 and 4). The test trucks
of the pattern described above are moved across the test spans, consisting of Span 1 and 2, and
make a one-minute stop at every distance of ¼ of the span length for recording the bridge responses.

Fig. 3: Load pattern of test trucks for static load test
3.2.2 Strains
Fig. 4 shows the strain gages installed at Section 1 and 2 of the test spans of the box girder. The
measurement results are illustrated in Figs. 5 and 6 for strains on Section 1 and 2 of the box girder,
respectively, as the test trucks crossing the bridge. Reversals of the bending moment directions can
be observed as the trucks moving across the test spans. For example, as the test trucks moved into
Span 1 (Position 1 in the figures), the positive bending moment (indicated by compressive or
negative strain on top of the girder) is induced over Section 1 (Fig. 5) and the negative bending
moment (indicated by tensile or positive strain on top) is induced over Section 2 (Fig. 6). And, the
bending moments are then changed in their directions as the test trucks moved into Span 2 (Position
2 in the figures). A maximum magnitude of the strains under loading of test trucks is approximately
35 microstrain corresponding to tensile strain at the bottom of girder Section 2.

Fig. 4: Positions of strain gages installed in test spans, Span 1 and 2

Fig. 5: Strains on Section 1 under test trucks

3.3

Fig. 6: Strains on Section 2 under test trucks

Dynamic Test

3.3.1 Truck Loading
A dynamic test is employed to determine modal properties and dynamic effects due to vibratory
motion of the moving trucks and the bridge structure. One of the test trucks is considered to traverse
across the bridge in each traffic lane with three varying speeds. The patterns of the moving truck for
the dynamic test are described in Fig. 7.

Fig. 7: Load patterns for dynamic test; one test truck traversing across the bridge in each traffic
lane with varying speeds

3.3.2 Strains and Dynamic Amplification
Fig. 8 shows the results of strain at the bottom of Section 2 of the box girder. A wavelet analysis is
employed to decompose the strains into static and dynamic components [4]. A dynamic
amplification factor, defined as the ratio between the maximum dynamic response and the
corresponding static response, is adopted to determine dynamic effects. It should be noted that, due
to safety concern, a maximum limit of 40 km/hr is considered for truck speed during the dynamic
test. The dynamic amplification factors (DAFs) for the strains obtained from SG23 and SG24
measured at the bottom of Section 2 of the box girder subjected to one test truck are described in
Fig. 9. DAFs obtained vary from 0.07 to 0.15.

Fig. 8: Strain at bottom of Section 2 of the box girder and strain decomposition

Fig. 9: Dynamic amplification factors for strains at the bottom of girder Section 2

3.3.3 Natural Frequencies
Bridge motions are measured using a set of accelerometers placed along the bridge spans. The
natural frequencies and the corresponding mode shapes of the box girder are determined based on
modal analysis with a consideration of Fourier coefficients [5]. The results obtained are listed in
Table 2. An example of response time-histories and the corresponding power spectral density (PSD)
of the girder vertical accelerations is depicted in Fig. 10 along with the illustrations of the
corresponding mode shapes obtained based on finite element analysis. Correlation between the
natural frequencies obtained from the field measurement and a finite element model adopted is
illustrated in Table 2.

Table 2: Natural Frequencies of Continuous Box Girder

Fig. 10: Acceleration time-history, PSD, and examples of mode shapes of the box girder

4. Long-Term Measurement of Support Movements
The longitudinal movements of the pot bearings on Pier P1, P2, P7 and P8 are continuously
monitored for the duration of 16 weeks. The results of support movements for Pier P1 and P2 are
shown in Fig. 11. The variations of bridge surrounding temperature are also recorded. The dotted
lines in the figure represent the averages of the measurements taken over 7-day intervals. A positive
movement corresponds to the longitudinal displacement of the pot bearings toward the fixed
support at Pier P4. It can be observed that as the temperature decreased, the bearings tend to move
toward the fixed support due to thermal contraction of the bridge structure and vice versa.
Correlation between the support movements and temperature are illustrated in Fig. 12.

Fig. 11: Longitudinal movement of bearings on Pier P1 and P2, and temperature

Fig. 12: Correlation between averages of longitudinal movemenst of bridge bearings and
temperatures

5. Conclusions
A diagnostic load test procedure and the results obtained for a 7-span, continuous box girder bridge
are described. Four 3-axle test trucks are employed in the static load test. The results of strains of
the box girder are reported. Reversals of the bending moment directions on girder sections can be
illustrated as the test trucks moving across the continuous spans. Based on the results of the
dynamic load test subjected to one truck moving with varying speeds, the natural frequencies and
the corresponding mode shapes of the girder are evaluated. Strains obtained for the box girder are
decomposed into static and dynamic components to evaluate dynamic amplification factors. DAFs
obtained are varied with truck speed and range from 0.07 and 0.15. The results of long-term
measurement of pot bearing movements and surrounding temperature for the duration of 16 weeks
are presented. The results illustrate the correlation between the movements of bridge supports and
temperature variations.
It should also be mentioned that the results of the diagnostic load test described herein are to be
employed for further investigation, regarding structural analysis and performance evaluation, of the
bridge structure.

6. References
[1]
[2]

[3]

[4]
[5]

American Association of State Highway and Transportation Officials, The Manual for
Bridge Evaluation, 2nd Ed., Washington, DC, 2011, p. 8-1 to 8-16.
CHAJES M. J. and SHENTON H. W. III, “Using Diagnostic Load Tests for Accurate Load
Rating of Typical Bridges”, Proceedings of Structures Congress 2005: Metropolis and
Beyond, ASCE, New York, April 20-24, 2005, pp. 1-11.
GANGONE M. V., WHELAN M. J., and JANOYAN K. D., “Wireless Monitoring of a
Multispan Bridge Superstructure for Diagnostic Load Testing and System Identification”,
Computer-Aided Civil and Infrastructure Engineering, Vol. 26, Issue 7, 2011, pp. 560-579.
NEWLAND D. E., An Introduction to Random Vibrations, Spectral and Wavelet Analysis,
3rd Ed., Longman Scientific & Technical, 1993, p. 295-339.
HE J. and FU Z. F., Modal Analysis. Butterworth-Heinemann, 2001, p. 163-164.

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close