diagnostic load test of continuous prestressed box girder bridge

8/10/2019 Diagnostic Load Test of Continuous Prestressed Box Girder Bridge

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Diagnostic Load Test of Continuous Prestressed Box Girder Bridge

SayanSIRIMONTREEAssociate ProfessorThammasat UniversityPhathumthani,Thailand

[email protected]

Sayan Sirimontree received hisPhD degree in civil engineeringfrom Khon Khaen University,Thailand.

KridayuthCHOMPOOMINGAssistant ProfessorThammasat UniversityPhathumthani,[email protected]

Kridayuth Chompoomingreceived his PhD degree in civilengineering from Utah StateUniversity, USA.

WacharapongPRASARNKLIEOLecturerKing Mongkuts Institute ofTechnology Ladkrabang,Thailand

[email protected]

Wacharapong Prasarnklieoreceived his MEng degree in civilengineering from ThammasatUniversity, Thailand.

Summary

A main objective of the present investigation is to obtain structural responses of a continuous boxgirder 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 employedin 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 modeshapes of the box girder are determined based on modal analysis. Correlation between naturalfrequencies of the box girder obtained from field measurement and a finite element model is alsoillustrated. Wavelet analysis is employed to decompose girder strains into static and dynamiccomponents, and dynamic amplification factors are determined and reported. In addition, the resultsof long-term measurement of pot bearing movements subjected to variations of surroundingtemperature 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 crossingover the Mekong River on the highway route between Nong Khai in Thailand and Vientiane in LaoPeoples 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 tothe conditions of deterioration and damage. Bridge inspection and structural evaluation isestablished as part of corrective and preventive maintenance to ensure serviceability, load-carryingcapacity, 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 foridentification of actual live load distribution, unintended composite action, conditions of supportmovement, 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 ofthe main objectives of the investigation is to gain insights into physical behavior and obtain actualperformance of the bridge structure based on load testing. Static and dynamic tests underpredetermined loads are considered. The measurement procedures and results of the diagnostic loadtest 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 andoverall 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 6.1meters 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.


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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-axletrucks are employed. Weights and dimensions of the test trucks are listed in Table 1.

Table 1: Weights and Dimension of Test Trucks


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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. Thepattern 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 trucksof the pattern described above are moved across the test spans, consisting of Span 1 and 2, andmake a one-minute stop at every distance of 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. Themeasurement 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 intoSpan 1 (Position 1 in the figures), the positive bending moment (indicated by compressive ornegative strain on top of the girder) is induced over Section 1 (Fig. 5) and the negative bendingmoment (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 (Position2 in the figures). A maximum magnitude of the strains under loading of test trucks is approximately35 microstrain corresponding to tensile strain at the bottom of girder Section 2.


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

Fig. 5: Strains on Section 1 under test trucks Fig. 6: Strains on Section 2 under test trucks

3.3

Dynamic Test

3.3.1 Truck Loading

A dynamic test is employed to determine modal properties and dynamic effects due to vibratorymotion of the moving trucks and the bridge structure. One of the test trucks is considered to traverseacross the bridge in each traffic lane with three varying speeds. The patterns of the moving truck forthe dynamic test are described in Fig. 7.


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Fig. 7: Load patterns for dynamic test; one test truck traversing across the bridge in each trafficlane 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 isemployed to decompose the strains into static and dynamic components [4]. A dynamicamplification factor, defined as the ratio between the maximum dynamic response and thecorresponding static response, is adopted to determine dynamic effects. It should be noted that, dueto safety concern, a maximum limit of 40 km/hr is considered for truck speed during the dynamictest. The dynamic amplification factors (DAFs) for the strains obtained from SG23 and SG24measured at the bottom of Section 2 of the box girder subjected to one test truck are described inFig. 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


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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. Thenatural frequencies and the corresponding mode shapes of the box girder are determined based onmodal analysis with a consideration of Fourier coefficients [5]. The results obtained are listed inTable 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 thecorresponding mode shapes obtained based on finite element analysis. Correlation between thenatural frequencies obtained from the field measurement and a finite element model adopted isillustrated in Table 2.

Table 2: Natural Frequencies of Continuous Box Girder


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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 bridgeare described. Four 3-axle test trucks are employed in the static load test. The results of strains ofthe box girder are reported. Reversals of the bending moment directions on girder sections can beillustrated as the test trucks moving across the continuous spans. Based on the results of thedynamic load test subjected to one truck moving with varying speeds, the natural frequencies andthe corresponding mode shapes of the girder are evaluated. Strains obtained for the box girder aredecomposed into static and dynamic components to evaluate dynamic amplification factors. DAFsobtained are varied with truck speed and range from 0.07 and 0.15. The results of long-termmeasurement of pot bearing movements and surrounding temperature for the duration of 16 weeksare presented. The results illustrate the correlation between the movements of bridge supports andtemperature 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 thebridge structure.

6. References

[1] American Association of State Highway and Transportation Officials, The Manual forBridge Evaluation, 2ndEd., Washington, DC, 2011, p. 8-1 to 8-16.

[2] CHAJES M. J. and SHENTON H. W. III, Using Diagnostic Load Tests for Accurate LoadRating of Typical Bridges,Proceedings of Structures Congress 2005: Metropolis and

Beyond, ASCE, New York, April 20-24, 2005, pp. 1-11.

[3] GANGONE M. V., WHELAN M. J., and JANOYAN K. D., Wireless Monitoring of aMultispan Bridge Superstructure for Diagnostic Load Testing and System Identification,

Computer-Aided Civil and Infrastructure Engineering, Vol. 26, Issue 7, 2011, pp. 560-579.

[4] NEWLAND D. E.,An Introduction to Random Vibrations, Spectral and Wavelet Analysis,3rdEd.,Longman Scientific & Technical, 1993, p. 295-339.

[5] HE J. and FU Z. F.,Modal Analysis. Butterworth-Heinemann, 2001, p. 163-164.

diagnostic load test of continuous prestressed box girder bridge

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