DESIGN AND FIELD TEST OF BAQIHU BRIDGE IN QUANZHOU CITY, CHINA

Transcription

1 DESIGN AND FIELD TEST OF BAQIHU BRIDGE IN QUANZHOU CITY, CHINA Jian-gang WEI & Bao-chun CHEN College of civil engineering in Fuzhou University, Fuzhou, China Key words: CFST, arch bridge, design, field test, rigid-frame, Baqihu, Quanzhou, China Abstract: The rigid-frame tied through arch bridge is a main form in CFST arch bridge. Most of this type bridges have a single-span, in which the arch ribs are fixed to the piers to form a rigid frame and high strength strands are employed as tired bars by pre-stressed to produce horizontal compression forces to balance the thrust of the arch ribs. Some of them have more than one spans, three spans are the most popularity. This paper presents the design and field test of a typical three span through rigid-frame tied CFST arch bridge--baqihu Bridgeis, which has a span arrangment of 51m+80m+51m. It is located in Quanzhou City of Fujian Province, China. The bridge was constructed from March, 2006 to Oct

2 1. INTRODUCTION Quanzhou is located in south-east coastline of Fujian Province, China. From Tang Dynasty, it is well-known as the jumping-off point of Maritime Silk Road and had the name of the biggest open port in east. With the rapid development of economy, the city is expandsed along the coast and many roads have been constructed. As the key project in the main trunk road connecting the west area to the east districts of Quanzhou city, Baqihu Bridge will play an important role in local economic developing and road network. The total 488m length bridge consists of main bridge and two side approaches. Through an opening design competition, a through rigid-frame tied arch bridge was selected as the solution for the main bridge. 2. STRUCTURES The rigid-frame tied through arch bridge is a main form in CFST arch bridge. Most of this type bridges have a single-span, in which the arch ribs are fixed to the piers to form a rigid frame and high strength strands are employed as tired bars by pre-stressed to produce horizontal compression forces to balance the thrust of the arch ribs. Some of them have more than one spans, three spans are the most popularity [1][2]. The main bridge of Baqihu Bridge has a total length of 182m. It consists of three spans, a central span of 80m and two side spans of 51m (Figure 1). The bridge is composed of two individual bridge, each one is 21.2m wide and carries four lanes and two side walkways. Ratio of rise to span of the arch rib is 1/5. The bridge adopts cantenary curve with a parameter m of as its arch axis. The distance of two rib s center is 17.9m. CFST arch ribs are fixed at the springing to piers and their pile foundation, form a rigid-frame structure. Independed tied bars are served for each arch ribs, anchored at the ends of the two side spans and crossed anchored at the piers. Figure 1: General layout of Baqihu Bridge (cm) The arch rib consists of two steel tubes with 900mm diameter and a wall thickness of 14mm for center arch rib and 750mm diameter and 12mm thickness for side arch rib, as shown in Figure 2. The upper and lower tubes are welded together by two Q345c steel plates of which the thickness is 14mm for and 12mm for side span. Differ with other dumbbell CFST cross section, there is no concrete filled in the web space (except spring of arch), only the two tubes are filled with C50 concrete. Tied bars are 6 steel strands of 15φ15.2mm for and 4 steel strands of 12φ15.2mm for side span with a strength of 1860MPa and protected by Polyethylene. There are total 30 pairs of hangers, each one has a steel wire of 73φ7mm coated by zinc with a

3 standard strength of 1570MPa. The hangers are also protected by two PE layer coats and have adjustable chilled cast anchors at its two ends. In longitudinal direction, the distance between two neighbor hangers is 5.0m. The cross beams suspended by cable hangers are made of prestressed concrete with a calculation span of 17.9m. Pre-cast RC π-shaped deck slabs of 0.4m in deep supported by cross beams were covered by 10cm thick steel fiber concrete to form the road wearing layer. (a)cross section of enter arch Figure 2: Cross section of arch rib (cm) (b)cross section of side arch 3. CONSTRUCTION The construction of substructure and foundation is relatively straightforward. Two temporary bridges were constructed along the two sides of the bridge to be built. Working platform at each pier was built for piles and substructures. The scaffolding method is adapted to erect CFST arch ribs. Landing stage of scaffold were set up separately for difereent ribs (Figure 3). One rib was divided into three segments to hoist and has a weight of 35t. The segments were temporarily connected by the inner Flange. After the measurement and adjustment of the axis, they were welded together to make steel tubular arch rib. And then bracings were erected to connect the two arch ribs to form a spatial structure. Figure 3: Landing stage of scaffold

4 After that, concrete was pumped into the steel tubes to form CFST arch ribs. Then fabricated transverse girders and deck slabs were erected. The Baqihu Bridge was constructed from March, 2006 to Oct The completed bridge is shown in Figure FIELD TESTS Figure 4: View of Baqihu Bridge After the Baqihu Bridge was completed, a filed test was carried out to check its quality and to establish a baseline record for future health monitoring. 4.1 Satic test The cross section of quartile, crown and spring of the arch ribs in both centeral and side spans were chosen to be the measured sections as shown in Figure 5. The loading cases were decided by FEM results, as listed in Tab.1. Strains and deformations were measured in Case 1 to Case 3, but only strains in Case 4 and 5. Figure 5: Measurement points setup in static test Case Number Arrangement of vehicle Testing item Case 1 Case2 Case 3 Case 4 Case 5 2 lines, 2 columns 4 lines, 1 column 2 lines, 2 columns 2 lines, 2 columns 2 lines, 2 columns Table 1: Static testing cases Maximum positive bending moment of crown of rib Maximum negative bending moment of spring of rib Maximum positive bending moment of quartile of rib Maximum positive bending moment of crown of side span rib Maximum negative bending moment of spring of side span rib

5 The test results of deflections of the arch rib are listed in Table 2. It can be found that the test results agree well with the design calculation. Therefore, the static characteristics of the bridge meet the design requirement. Case No. Cross-section Test result Calculation Test/Calculation Case 1 Crown Case2 Quartile Case 3 Quartile Table 2: Deflections (unit: mm) 4.2 Dynamic test Dynamic test includes ambient stochastic and forced vibration tests. In ambient stochastic vibration test, the vibration characteristics of the bridge, such as nature frequency, vibration mode and damping, should be obtained by minute vibratory response. There are eight in-plane acceleration vibration pickups are set in the deck and rib of. Anther out-plane acceleration vibration pickups are set in the same place to obtain out-plane dates. Dynamic tests were conducted with vehicle traveling over the bridge with or without an obstruct at speeds of roughly 20km/h, 30km/h, 40km/h and 50km/h. The obstruct in test is a 10cm high barrier laid on deck in the. The vibration responses under vehicle running are recorded by strain gauges and acceleration sensors. A view on the measurement instrumentations in dynamic test is shown in Figure 6. (a)1'-1'cross section (b)2'-2'cross section (c)3'-3'cross section (d)4'-4'cross section resistance strain gauge Figure 6: Measurement points setup in dynamic test

6 4.3 Test results Strains and deflections of the arch rib from static tests are listed in Tables 3 and 4. It can be seen from these tables that most of the ratios of test results to calculation results are satisfied the requirement of (0.7~1.0) defined by the relative criterion except the strain in quater section of the arch rib in. Test case Test result Calculation Test/Calculation Section Case Crown Case Quarter section Case Quarter section Table 3: Deflections of arch rib in central span (unit: mm) Test case Test result Calcualtion Test/Calculation Section Case Crown of rib Case Spring of rib Case Quarter section of rib Case Crown of side span rib Case Spring of side span rib Table 4: Strains (unit: ) The identified and calculted natural frequency, mode shapes and damping of the bridge are listed in Table 5. The testing results of the basic dynamic characteristics of the bridge agree well with the calculation ones. All of the first four vibration mode shapes occured in arch of the central span. It means that though it is a three continue tied rigid-fram arch bridge, but the central span with the longest span will dominate the low modes and three spans will vibrate independently. The first and second modes are out-plane vibration in symmetric and anti-symmetrec shapes, while the third and fouth modes are in-plane vibration in anti-symmetrec and symmetric shapes. Therefore, the out-plane stiffness of the arch is smaller than that of in-plane, which is coincide with the stability calculation results. The impact factos are listed in Table 6. It can be obtained from the China Design Code JTG D [3] that the impact factorμof this bridge is 0.1. The test impact factorsμare smaller than 0.1 as from the design calculation when the vehicle running through it with a speed slowerr than 30km/h. However, when the speed is over 30km/h, the tested impact factors are lager than 0.1. In other words, the present China Design Code could underestimated the impact factor of a CFST arch bridges and larger ones should taken into account in the design

7 Calculation Mode Frequency (Hz) Test results Frequency (Hz) Damping Vibration mode shape Table 5: Mode shapes and damping of the bridge Speed (km/h) Test position Extrados of crown of Intrados of spring of Extrados of crown of Intrados of spring of Extrados of crown of Intrados of spring of Extrados of crown of Intrados of spring of Maximum strain( 10-6 ) Table 6: Impact factor Impact factor (1+μ) Impact factor of standard load (1+μ)

8 5. CONCLUSION Baqihu Bridge located in Quanzhou City, Fujian Province, China, is a three-span through rigid-frame tied CFST arch bridge. This paper presents the design and field test of this bridge. Both the static and dynamic test results agree well with the calculation ones, indicate that the calculation model in design is correct and can serve as basis for finit element model updating and structural health monitoring. And at the same time, analysis results show that the bridge behaves as predicted in design and the quality is basicly satisfied. Although it is a three continue tied rigid-fram arch bridge, however from the dynamic tests, all of the first four vibration mode shapes occured in arch of the central span and the out-plane stiffness of the arch is smaller than that of in-plane. The present China Design Code could underestimated the impact factor of a CFST arch bridges when the vehicle running with a quick speed and larger impact factors should taken into account in the design. REFERENCES [1] Chen, B. C A. Concrete filled steel tube arch bridges 2nd Ed., China Communications Press, Beijing, China. (in Chinese) [2] Yang, Y. L. and Chen, B. C Rigid-frame tied through arch bridge with concrete filled steel tubular ribs, Proc., Fifth International Conference on Arch Bridge, Madeira, Portugal, [3] Ministry of Communication of China, Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts, JTG D Beijing, China (in Chinese)