Precast segmental design and construction in China

Transcription

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Precast segmental design and construction in China Dong, Xu, Huichi Li & Chao Liu Dept. of Bridge Engineering, Tongji University, Shanghai, China ABSTRACT: In 2001, Liuhe River Bridge in Shanghai was the first span-by-span precast segmental bridge constructed by a launching gantry in China. In 2003, Humin Viaduct in Shanghai was the first project using short line segment casting for urban elevated viaducts. The biggest PC beam bridge using balanced cantilever precast segmental construction in China is Jiujiang Bridge in Guangdong province, which has two 160 m main spans and was completed in In July 2001, the first 4-span cable-stayed bridge in China, Yiling Bridge in Hubei province, using precast segmental PC box girder construction, was completed. Although China has these successful references, the precast segmental construction, originated from the contractor system, was not well followed in time by design codes. Lack of specified design codes in China usually causes owners and even designers hesitate to use this construction method. Longitudinal mild reinforcing bars do not continue through the joints. The new concept of making closed horizontal shear reinforcement, with the addition of normally arranged vertical stirrups, to carry the horizontal and vertical components of the diagonal tensile stress was proposed in the reinforcement design of precast segments for the approach bridges of the Sutong Bridge. 1 INSTRUCTIONS Precast segmental construction for prestressed concrete bridges has been widely used in the world. This method has many advantages, such as qualityguaranteed standard precast segments, fast speed construction time and the least site work. Lots of prestressed concrete bridges in the world were built by precast segmental construction method in recent 20 years [1]. Although more and more precast segmental bridges have been successfully built in China, this construction method is still relatively new to Chinese engineers. One reason is that the precise control of the dimensions of the bridge makes the construction much more complicated. Another reason is that the precast segmental construction method, originated from the contractor system, is not well followed in time by design codes. This construction method even well influences the structural design including the profiles of internal and external prestressing tendons, the detail dimensions of segments and the arrangements of segments and deviators, etc. Due to these reasons, lack of the regarding specified design codes usually causes owners and even designers to hesitate to use this construction method. 2 RECENT CONSTRUCTION REFERENCES IN CHINA 2.1 Precast segmental span by span construction New Liuhe Bridge in Shanghai The new Liuhe Bridge in Shanghai consisted of 3 42 m single spans. The bridge had dual carriageways, each of which included 3 lanes of traffic. The upward bridge was m in width, while the downward bridge was m. In order to be familiar with the design method of segmental construction with external prestressing, the precast segmental construction was adopted. The upward and downward bridges consisted of two parallel single-cell box girders, respectively. Each box girder was longitudinally divided into 13 segments. The weight of a single span was 515 tons, and the weight of a 3.33 m standard segment was 40 tons. The bridge segments were precasted with long-line method. A launching gantry was used to carry and install all the segments into a span. This bridge was the first span-by-span precast segmental bridge constructed by a launching gantry in China, as shown in Figure 1. Mixed tendons were used for the longitudinal prestressing in the bridge. Most 795

2 Figure 1. New Liuhe River Bridge, installation of precast segments. Figure 3. Humin Viaduct, installation of precast segments. Figure 2. New Liuhe River Bridge, external tendons. of the prestressing tendons were internal bonded tendons, while four 19T15 external unbonded tendons were used in each span, occupying 24% of the whole prestressing in a single span, as shown in Figure 2. The bridge was completed in Oct Humin Elevated Viaduct in Shanghai Humin Elevated Viaduct in Shanghai was the second bridge using span-by-span precast segmental construction using launching gantry in China, as shown in Figure 3. The total length of the viaduct was 5.56 km and the average span was m. The 30 m spans were made of 11 precast segmental box girders, while the 35 m spans were made of 13 ones. The girder segments were 25 m in width, 2.1 m in height, 2 m, 2.5 m and 3 m in length. The weight of the precast segments at pier tops was up to 130 tons and the standard segments were about 110 tons. The segments were short line precasted. Figure 4 shows the external tendons and deviators in this bridge. Although half of the 5.56 km viaduct was designed to adopt this method, most of the spans were built by the traditional monolithic scaffolding construction because of the restricted construction period. Only Figure 4. Humin Viaduct, external tendons and deviators. Figure 5. Second Jiujiang Bridge. two sections were left to use precast segmental construction, one in the main viaduct and another in a ramp. Each section included ten spans. The whole viaduct, which has an appealing arc bottom shape, was completed in the end of

3 Figure 8. Launching gantry to assemble the segments. Figure 6. Approach bridges in deep water. Figure 9. Installation of the deviator segment. Figure 7. Precast segments storage yard. 2.2 Precast segmental balanced cantilever construction Second Jiujiang Bridge At present, the biggest bridge using precast segmental balanced cantilever construction in China was the Second Jiujiang Bridge, completed in 1996 and located in Guangdong Province, as shown in Figure 5. The main bridge had 6 spans (50 m m m m + 50 m), including the two biggest 160 m precast segmental spans. In this bridge, the maximum cantilever length reached to 78.5 m, leaving a 3 m cast-in-situ closure. The width of the deck segments was m. All the prestressing system was internally arranged Deep water approach bridges of the Sutong cable-stayed bridge Sutong Bridge, with a total length of 8146 m over the Yangtze River including a 1088 m cable stayed main span, was completed on June 28, 2007 and will open to traffic at the end of April this year. The approach spans were wholly prestressed concrete continuous beams including about 25 spans of 75 meters in deep water (Fig. 6). Short line segment precasting and balanced cantilever construction were adopted (Fig. 7). Launching gantry was used to assemble the cantilever segments, as shown in Figure 8. The bridge is the largest bridge project using precast segmental cantilever construction, launching gantry assembling and short line precasting in China. Special segments, including the anchorage segments and deviator segments (Fig. 9) were carefully designed and precasted. In order to keep the weight of all segments within 150 tons, the anchorage cross beam, which occupies half of the weight of the pier segment, is casted in-situ. Each span consists of six 25T15 external tendons. A new external prestressing system was adopted in this bridge. The adopted external prestressing system was developed by the joint R&D center supported by OVM (Liuzhou) Machinery Co. Ltd. and Tongji University, as shown in Figure 10 and Figure 11.The system uses unbonded monostrands and has two features. The first is the deviator device, which can comb the individual monostrands and guide 797

4 Figure 10. External prestressing tendons. Figure 13. Profile of the deviator after testing. Figure 11. External prestressing tendons. Figure 14. Connection of the deviator and the straight part. Figure 12. Detail of the deviator of the system. them one by one in deferent vertical and lateral curvatures (Figs 12 13). The second feature is that the straight part of the tendon profile does not need to be grouted (Fig. 14). It significantly reduces the self weight of the tendons and facilitates the inspection and monitoring of the strands during service time (Fig. 15). 2.3 Precast segmental cable-stayed bridge Yunyang Bridge over the Hanjiang River Yunyang Bridge, completed in 1993 and located in Hubei Province, was a P.C. cable-stayed bridge with double pylons. The bridge was the first long span earth anchored prestressed concrete cable stayed bridges in Figure 15. Easy inspection of the tendons. China, with 3 spans (86 m m + 86 m). The concrete girder was box-section with three cells, using three-dimension prestress technology. The length of the segments were 3.7 m and 4.3 m and the maximum weight of the precast segments was around 100 tons. (Fig. 16) Yiling Bridge over the Yangtze River Yiling Bridge, crossing Yangtze River in Yichang City in Sichuan Province, was a 4-span cable-stayed bridge 798

5 Figure 16. Figure 17. Yunyang Bridge under construction. Yiling Bridge. with 2 main spans of 348 m and side spans of 120 m with a total length of 936 m. The concrete girder was three cells box-girder, which was 23 m in width. The precast segments were 3 m in length. The cast-in-situ length of the main girder was 131 m on both side spans and 22 m around the central pylons; the two 324 m main spans were precasted and erected using the balanced cantilever method. (Fig. 17) 3 SOME TECHNICAL CONSIDERATIONS 3.1 Flexural behavior For externally prestressed concrete structure, the flexural resistance in ultimate stage has relationship with many factors, including span depth ratio, the distance between deviators, the type of external prestressing strands, i.e., bonded or unbonded, etc. Joint is also a major factor influencing the flexural behavior of precast segmental structures. Although FEM method has become popular for analysis, the numerical model for joints is difficult to make. Meanwhile, designers used to get a quick estimation of the flexural ultimate carrying capacity of the bridges, especially in preliminary stage. As a matter of fact, section equilibrium method can be borrowed from the calculation of internally prestressed concrete structures to calculate the flexural ultimate carrying capacity of externally unbonded prestressed concrete structures. The key factor to use this simplified method is to get the ultimate stress increment of external prestressing tendons at the ultimate stage. Although many codes in the world include those equations, they are quite different because many parameters that have influences on the ultimate stress of external prestressing tendon cannot be totally simulated in experiments. Zero ultimate stress increment was recommended for the external tendons in Sutong approach bridges. Although a little bit conservative, it was practical and not a controlling factor in this case because the unbonded monostrands external system were adopted and much more internal tendons than external tendons were used in the mixed tendon arrangement in the design. In China, there are already some theoretical and experimental achievements in the design theories of the flexural behavior for monolithic and precast segmental construction. Some numerical analysis models considering the geometric and material nonlinear, as well as the joint mechanism have been developed. In order to create more precise numerical models, which can reflect the mechanism of joints between segments, further research and experimental tests are still being made. The technical guides are hopefully to be issued based on these researches in the following years. 3.2 Reinforcement design near joints of the precast segments in Sutong Bridge Although dry joints between precast segments have been used in a lot of bridges around the world, the mechanism of this kind of construction is still not very clear, especially the shear strength. Fortunately, if the glued and multiple-key joints are used between precast segments, the web crack pattern will be the same as that in the monolithic structures in ultimate stage [3]. Nevertheless, there is still a concern that because longitudinal mild reinforcing bars do not continue through the joints in precast segmental bridges, minor deficiency of shear resistance might happen in the local areas along the joints. In order to minimize the uncertain area as much as possible, in the design of the reinforcement near joints of the precast segments in the Sutong approach spans, horizontal reinforcements were suggested to be made closed so as to get enough development length similar to the vertical stirrups as in Figure

6 Bridge, made Chinese Engineers to be more familiar with this construction method. At the same time, the theoretical and experimental research achievements will further accelerate the establishment of the detailed specifications for the design and construction of precast segmental bridges. REFERENCES Figure 18. Reinforcement arrangement in a web near joints of the precast segments. 4 CONCLUSIONS [1] Naaman, A.E. & Breen, J.E External prestressing in bridge. ACI Detroit: SP 120. [2] Xiang Hanfan Major Bridges in China, Compilation Board of Major Bridges in China. Beijing, China: The People s Transportation Press. [3] Podolny, W. & Muller, J.M Construction and Design of Prestressed Concrete Segmental Bridges. New York, USA. Practical experiences gained from major engineering projects, especially in the approach spans of Sutong 800