A Load Test Study on Monolithic T-Girder of 40m Spans

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1 A Load Test Study on Monolithic T-Girder of 40m Spans Chuandong Gao 1,2 1 National and Local Joint Engineering Laboratory of Traffic Civil Engineering Materials Chongqing Jiaotong University Chongqing , China 2 Institute of Civil engineering Chongqing Jiaotong University Chongqing , China Abstract The prefabrication quality of monolithic T-girder directly influences the project quality of the finished bridge state. The test study is conducted on the simply-supported and newly-precast monolithic T-girder which has met the service life in order to study its loads and bearing capacity as well as the cross-link function of T-girder bridge. Moreover, the measured value and the theoretical value are compared for the analytical research. This paper takes the Xiaogou Bridge on the highway from Liangping County to Zhong County in Chongqin City as the example and conducts a comparative analysis on the measured value and the theoretical value collected from the simply-supported and newly-precast monolithic T-girder. Thus, this paper provides help for further studying the cross-link function in the finished bridge state of T-girder bridges and is of great significance in guaranteeing the project quality of the finished bridge in the later period. Keywords - monolithic T-girder; 40m; static load; theoretical analysis I. INTRODUCTION In recent years, as the highway construction develops rapidly in mountainous areas, the bridge mileage covers an increasingly large percentage of the total highway mileage. With the continuous development of the concrete technology and the pre-stressing technique, the bridge development is influenced by composite factors including construction schedule, construction cost, and construction etc. The fabricated T-girder easily adopts the serialization and standardization, so the pre-stressing concrete T-girder bridges through prefabrication and hoisting represent a big percentage and such bridges are frequently used in China s highways [1,2]. It has become an important means to guarantee the bridge quality and determine whether the project quality of monolithic pre-stressing T-girder can meet the specifications and design requirements. In the bridge construction, the bearing capacity of the bridge was originally evaluated by using methods such as the correction coefficients in the finished bridge state, relative residual and crack etc. The monolithic T-girder prefabrication quality directly influences the project quality of the finished bridge state. This paper discusses how to guarantee the project quality of the finished bridge in the construction[4,5] and also studies the stress state of the simply supported monolithic T-girder. This paper uses the typical newly-precast monolithic 40m T-girder Bridge meeting the service life as an example for the analytical research. II. PROJECT OVERVIEW AND RESEARCH CONTENTS & METHODS A. Project Overview Located in the highway from Liangping County to Zhong County in Chongqin City, Xiaogou Bridge consists of the separate layout on the left and right. This paper studies the T- girder located in the left-side bridge girder of the second block (bridge girder between two adjacent expansion joints). Superstructure: The whole bridge consists of two blocks and is divided into the left and right layouts. The left bridge layout is the simply supported-continuous pre-stressed concrete m T-girder. 5 girders are used in the monolithic width with the space between girders of 2.4m and girder height of 2.5m. The girder is prefabricated with the C50 concrete. Bridge deck system: The left and right widths are both 12m in the deck arrangement. The bridge deck width is as follows: 0.5m (crash barrier) +11m (roadway) +0.5m (crash barrier) =12.0m. The 8cm-thick C50 concrete and the 10cmthick modified asphalt are respectively used in the flat bed and the pavement layer. Substructure: The pier part features the double-column bridge piers and the excavated pile foundation, and C40 concrete is used for cast-in-situ. 0# abutment is the lightweight abutment plus pile foundation. 8# abutment is the gravity U-type abutment plus the open-cut foundation and C40 concrete is used for cast-in-situ. Automobile load design: The road is the bi-directional four-lane highway I Class. The designed flood frequency is 1/100, the seismic fortification intensity is, and the ground motion peak acceleration is 0.05g. The research status is as DOI /IJSSST.a ISSN: x online, print

2 follows: when the hoisting of the monolithic precast T-girder is finished and the horizontal-linkage hinge joint is not running, the monolithic T-girder is in the status of simple support. This study makes a static load test of the monolithic 8-4# T-girder that has not been linked horizontally and vertically after the Xiaogou Bridge finishes the hoisting. The bridge façade picture and the T-girder picture are shown in Figure 1 and Figure 2 respectively. The structural diagram of the T- girder cross section is shown in Figure 3. The cross section diagram of the left-side bridge is shown in Figure 4. Figure 3. Cross-section structural diagram of the bridge (unit: mm) Figure 1. Bridge facade picture Figure 4. Cross section diagram of the left-sided bridge(unit: cm) 8-4#beam Figure 2. Bridge deck picture B. Research Contents & Methods (1)Research contents In the research, the static test is adopted and the cross section with the least force-carrying capability is selected for the strain test, the deflection test and the crack observation and detection. (2) Research methods 1 Strain: The super structure of the bridge is the prestressed T-girder, so the strain sensor is adhered to the concrete surface of the beam bottom in the cross section in the strain test. Moreover, the static strain indicator has a resolution of (±1με)when the indicator is used in the strain test. 2Deflection: The high-precision electronic level is used for testing the transversal arrangement of the cross section and the vertical displacement measuring points along the test bridge span, and the cross-section deflection of the bridge span is tested in the loading procedure. 3Crack observation and detection: The crack detection is conducted around the areas near the prestressed test cross section in the loading procedure. The crack width is detected by the crack viewer with a test resolution of ±0.02mm. DOI /IJSSST.a ISSN: x online, print

3 The test data collection system composition of strain and deflection (displacement) is shown in Figure 5. Figure 5. Static strain and deflection data collection system. C. Main Instruments and Equipment TABLE I. List of main instruments and equipment. Number Name Specifications & models QTY Equipment 1 Laptop LENOVO-E40 1 Q101 2 Static strain test system TDS Q102 3 Electric resistance strain gauge B AA several Q103 4 Precision level DSZ2 1 Q105 5 Indium steel ruler FS1 1 Q109 6 Crack width viewer DJCK-2 1 Q106 7 Digital camera Cannon 1 Q107 8 Steel tap 5m 1 Q108 9 Load car Iron horse 3 -- III. STATIC TEST AND RESULT ANALYSIS A. Identification of Research Cross Section According to the structure of the bridge, the finite element method is used for the element division. The bridge structure is discretized into plane elements for the analysis and calculation. Based on the calculation result of the bridge internal force, the cross section with the least force-carrying capacity is selected as the object of the load test. The test mainly includes controlling the cross-section strain and deflection. The testing section and the position diagram are shown in Figure 6 and 8-4# beam is located in the section Z1. Figure 6. Control section position diagram (Unit: m) DOI /IJSSST.a ISSN: x online, print

4 B. Measuring Point Layout Two strain measuring points are arranged in the lower part of the testing section (without including the temperature compensation) and two deflection measuring points are arranged in the upper part of the testing section. (a) Arrangement diagram of strain measuring point (b) Arrangement diagram of deflection measuring point Figure 7. Arrangement diagram of strain and deflection measuring points for the testing section (Unit: cm). For the convenience of detection and data collection, the strain and deflection measuring points of the testing section are arranged as shown in Figure 7. C. Identification of Test Loads In accordance with Design Specifications on Highway Reinforced Concrete and Prestressed Concrete Bridge and Culvert(JTG D ) and Test methods of Long Span Concrete Bridge(which were passed in the fifth expert meeting held in Berlin in October 1982), the least favorable load analysis is conducted for automobile-highway class I, together with calculating the concrete stress increments of the horizontal wet joint, the diaphragm, the screed-coat weight of the 8cm-thick concrete bridge deck, the 10cmthick asphalt concrete bridge deck pavement weight,which are generated in the lower part of the T-girder testing section[7,8], as well as determining the control force and deflection of the control section. Meanwhile, the common finite element software Dr. Bridge V3.03 for bridges is used in the main bridge and monolithic T-girder to calculate and analyze the sectional properties of the bridge and the internal forces of the static load test. In this study, the strain unit is μ ε and the stress unit is MPa. Meanwhile, the stress takes tension as positive and compression as negative. The strain is conversed into the rebar elasticity modulus of the stress MPa and the concrete elasticity modulus of the stress MPa. The deflection takes the downward as positive and the upward as negative with mm as the unit. The bending moment takes the tension of the downside beam as positive and the compression as negative with kn m as the unit. The bridge calculation parameters and the traverse distribution coefficients of the bridge middle beam are shown in Table 2 and Table 4. TABLE II. parameter list for bridge calculation. Number Parameter items Parameter value 1 Design load classes Automobile: highway-i class 2 Bridge type 4 40m prestressed T-girder 3 Concrete strength grade of girder C50 4 Calculated span(m) Continuous state: ; Simple support state: Lane 2 lanes on the left; 2 lanes on the right 6 Impact coefficientμ sagging moment effect 0.222,hogging moment effect0.320 TABLE III list of bridge section properties. Section type Sectional inertia moment(m4) Sectional areas(m2) Beam depth(m) Distance from neutral axis to beam bottom YS(m) 4# beam Notes: 1the sectional properties of the finished bridge state are used for calculating the traverse distribution coefficients and the impact coefficients. 2The influence of plain bars is not considered in the data of the table above. TABLE IV traverse distribution coefficients of the middle beam load. Loading conditions 2# 3# 4# Positive load Designed automobile load Notes: The traverse distribution coefficients are used as the maximum value of the middle beam in the load test. DOI /IJSSST.a ISSN: x online, print

5 (1) Test loading According to the provisions of Procedures on Highway Bridge Loading Capacity Detection and Assessment (JTG/T J ), the least favorable effect value in the specific working condition, which is generated based on the live load of the design standard, is obtained through the equivalent conversion in accordance with the principles of the following expression (as shown in Formula 1): S 0.95 s q 1.05 S (1 ) (1) Where, is the static test load efficiency; q S is the corresponding internal force of the loading s control section or the deflected maximum calculation effect value; S is the internal force of the same loading control section or the deflected calculation value of the least favorable effect which is generated in the control load; is dynamic increment coefficient adopted in the design calculation. When the load test efficiency coefficient η satisfies 0.95 ~1.05 in the testing section, two heavy-duty trucks are used as the test load through the calculation in the test, which can meet the traffic demands. The weight of the loading vehicles and the parameters of the vehicles are shown in Table 5. TABLE V Parameter list of loading vehicles. Mid-rear axle Front-mid axle Vehicle Gross wt(t) Front axle load(t) load(t) distance(m) Mid-rear axle distance (m) Transverse wheelbase(m) (2) Loading efficiency Working condition conditon1 TABLE VI loading efficiency list of the static load test. Internal force of designed Test load internal force Load Control category control(kn m)finished bridge state (kn m)simple support state efficiency Positive load of sagging moment in Z1section (3# beam control) It can be seen in Table 6 that the load efficiency is 0.95, meeting the requirements that the load efficiency should range from 0.95 to It means that the test study effectively meets the requirements. D. Test Distribution Load To ensure the validity of the test load, the test is loaded in the least favorable position of the bridge span structure. The automobile loading arrangement diagram for the test conditions is shown in Figure 8. Figure 8. Automobile arrangement diagram at Z1 test section (Unit: cm) E. Test Procedures The following preparations should be made before the test: (1) According to the requirements of the testing plan, the test vehicles are rented and loaded for weighing. Meanwhile, the original data of the test vehicles are recorded for further analysis. The loading positions and measuring point installation positions are marked after clearing the bridge deck. (2)The tested spans are laid off according to the former strain and deflection arrangement mode and the displacement of strain gage is made on the concrete surface of the beam bottom. Before the displacement, all the measuring points are polished, leveled and scrubbed, and then the strain gage is adhered to the surface together with damp-proof treatment. To exclude the influence of the atmospheric temperature change in the test, the temperature compensation strain gage of the same temperature field is laid in each section. (3) The test instrument is laid with the sensor connected with the connecting wire for the on-line debugging of the instrument. The circuits of all strain gages and the displacement meter are checked. (4) The pre-strain is conducted to check the reading of the strain gages. When everything is correct, the load test is done according to the working conditions. To achieve the stability of the test data and the test safety, the following arrangement is made for the loading procedures according to the experience: (1) Before the test, a test vehicle travels to and from the bridge several times at a low speed to remove the remaining strain and achieve the stability of the test data. (2) To ensure the test safety and avoid the bridge damage caused by overload, four-level loading is adopted on the test vehicles: 25%, 50%, 75%, and 100%. The strain and deflection of the control sections is calculated in advance for the test vehicles of different levels. When the test vehicle of this level is in place, the corresponding strain and deflection are measured and compared with the calculated value. DOI /IJSSST.a ISSN: x online, print

6 According to the principles of the elastic mechanics, the strain of the test vehicle is checked and compared with the estimated calculation to determine whether the strain is within the range of the estimate calculation before moving on to the following load. (3) The observation is made after the load of each level is in place for about 15 minutes. The load of the following level cannot be started until the load of the preceding level stabilizes the transformation of the bridge. The remnant observation and zero setting are done after the unloading is over for about 30 minutes and then the following work condition is continued. (4) After the test is over, the wire is dismantled. The detection instrument is checked with detailed inspection records. F. Test Results According to the research conditions in the tests, the measure value and the theoretical value of the strain and deflection in the test sections are listed and the corresponding correction coefficient is calculated. Subsequently, the coefficient is compared with the relevant standard and regulations and it is the main evidence to determine whether the bearing capacity and rigidity of the bridge meet the design requirements. (1) Strain test result In the test condition, the strain test result of the test section is shown in Table 7. In the table, the stretching strain is taken as positive. TABLE VII Strain and correction coefficient of test section in the test load. Positive load of maximum sagging moment in Z1 section Working condition Working condition 1 Measure point Measure value( )1 Residue value( )2 Elastic value( ) 3=1-2 Theoretical value( )4 Relative residual 2/1*100 (%) Correction coefficient3/ According to the actual strain value and the theoretical calculation value of all measure points in the test section in the table above, the comparison chart of these two strain values is drawn as shown in Figure 9. (2) Deflection test result In the test detection condition, the deflection and correction coefficient of the test section are shown in Table 8. In the table, the deflection takes the downward deformation as positive. Figure 9. The comparison chart of actual strain value and calculation value in working condition 1 TABLE VIII. Result and analytical sheet of deflection test in the static load test. Working condition Working condition 1 Measure Point Total Positive load of maximum sagging moment in Z1 section Residue Elastic Theoretical Relative residual2/1 *100(%) Correction coefficient3/ 4 deformation( )3= value( ) deformation( )1 deformation( ) According to the actual deflection value and the theoretical calculation value of all measure points in the test section in the table above, the comparison chart is shown in Figure 10 to reveal the transverse distribution curve of deflection for the girder in the load. Figure 10. the comparison chart of measure value and calculation value for the defection in working condition 1 DOI /IJSSST.a ISSN: x online, print

7 G. Summary (1) Strain correction coefficient In the test load, the strain correction coefficient of Z1 test section is 0.66~0.69 and the actual strain value is smaller than the theoretical value in the test section. Moreover, the strain correction coefficient of the control section is within the reasonable range. After finishing the unloading, the maximum relative residue is 4.04%in the test section, which means that the bridge has good working performances and that the structural strength of the bridge meets the requirements of the design load. (2) Deflection correction coefficient In the test load, the deflection correction coefficient of Z1 test section is 0.75~0.82 and the actual deflection value is smaller than the theoretical value in the test section. Moreover, the deflection correction coefficient of the control section is within the reasonable range. After finishing the unloading, the maximum relative residue is 0.27%in the test section and the residue deformation meets the requirements. It means that the bridge has good working performances and better deformation recovery capability after unloading, and the whole bridge is in elastic state. The test shows that the structural strength of the bridge meets the requirements of the design load. (3) Crack detection of girder The crack observation and detection is made in the areas near the studied sections of the girder and the critical parts of T-girder before and after the test as well as in the loading procedure. No visible cracks are detected and it means that the anti-cracking capability of the test bridge span meets the operating requirements. IV. CONCLUSIONS In the test load, the 8-4# monolithic T-girder of Xiaogou Bridge generally has good working performance and is in elastic operating state. Moreover, its actual bearing capacity meets the design load standard (automobile load: highway --- I Class). It is proposed that the smoothness of the bridge deck should be maintained to reduce the impact of vehicles on the bridge structure in the use procedure of the finished bridge. According to the relevant regulations in Highway Bridge Maintenance Specifications (JTG H ), the check, detection and maintenance should be done regularly in the use of the bridge. The study tests whether the bearing capacity and the structural deformation of the monolithic T-girder structure meet the design and use requirements. The deformation law of T-girder structure is investigated and its actual strained state and working conditions are evaluated in the load test, which provides scientific evidence for the following operation, maintenance and management. The study is limited because it only focuses on the static load study without involving the dynamic properties of the monolithic T-girder structure. REFERENCES [1] LIU Shao-tang, SHA Cong-shu. Deformation measurement and pre-camber arrangement for the MSS test in construction of bridge. Journa of HENAN Polytechnic University( Natural Science), vol. 31, No. 04, pp , [2] ZHENG Hai-bo. Study on the construction technology of posttensioned pre- stressed T- beam. Concrete, No. 10, pp , [3] LI Xiang-ping. Installation of 45 Meters Long Prestressed Concrete T-shape Beams of the Rongjiang Nanhe River Ultra-large Water Bridge. Construction Technology, vol. 34, No. 05, pp , [4] ZHOU Yi-tang, ZHU Wei-ming, GUO Xiao-xu, et al., Loading Test for Traffic Bridge of Tuca River Hydropower Station. Journal of Kunming University of Science and Technology ( Science and Technology), vol. 34, No. 05, pp , [5] ZHONG Shu-liang,GAO Fang-qing. Carrying capacity study of monolithic prestressed concrete T beam. China Measurement & Test, vol. 36, No. 06, pp , [6] GE Su-juan, ZHANG Gao-kui, CHEN Huai. Influence of horizontal clapboard on load transverse distribution of hollow plate bridge. Journa of Railway Science and Engineering, vol. 9, No. 02, pp , [7] PENG Ze-you. Analysis of Load Transverse Distribution of Fabricated T-Beam. Hjghway. No. 03, pp , [8] TANG Feng, TANG Zhenzhan. Static load experimental analysis of a continuous T beam bridge. Highway Engineering. vol. 35, No. 06, pp , DOI /IJSSST.a ISSN: x online, print