Caija Bridge – Chongqing, China
Located close to the center of China, Chongqing is the largest of the country’s four municipalities, with a population of approximately 32 million people. For the past three decades, China has been pursuing a policy of relocating close to 80% of its impoverished rural population to existing cities or new urban centers being constructed to accommodate them. Roughly 10 million people have been moved to urban areas to date, making extensive infrastructure projects an imperative. While numerous new roads and bridges have been built, the problems of traffic congestion, due in part to increased private vehicle ownership, and pollution persist. In response, Chongqing officials have been planning to construct an extensive mass transit network.
One of the highest metro line bridges in the world at close to 100 meters above water level, the Caijia Bridge is one of Chongqing’s most recent rail- only bridges. Passengers can now make the expedited trip from Beibei to the Chongqing city center in about 40 minutes, a tremendous improvement considering that a bus or automobile trip could take hours in the past. Each six-car train can carry 1,410 passengers, reducing traffic congestion and lowering exhaust emissions generated by vehicular traffic. The Caijia Bridge has also become a much-admired city landmark, and has already contributed significantly to the development of the northwestern area of this fast-growing municipality.
Innovation of Design and/or Construction
The 1,240-meter-long Caijia Bridge was designed to have a 640-meter-long main section that consists of five spans. The main span is flanked by end sections: a southern end section comprising two 45-meter-long spans and a northern end section with six 60-meter spans and three 50-meter spans. Under a 100-year flood, the water level of the Jialing River can also rise to an elevation of 198.43 meters, or about 30 meters above the normal 168.87-meter level. As a result, the main-span girder is located 100 meters above the normal water level.
The majestic towers of the Caijia Bridge ascend 186 meters from the top of the pile cap, with 105 meters below the deck and 78 meters above it. The two legs of the lower portion of the towers are connected to form a box cross section that extends above the highest high-water level. This enables the lower portion of the tower to better withstand vessel impacts, using the impact of a 1,000-ton barge as the criterion.
The tower cross beam is prestressed to overcome the tensile force created by the horizontal components. The upper portions of the tower legs have cross-sectional dimensions of 2.80 meters (transverse) by 6.0 meters (longitudinal). The lower portions of the legs vary in dimensions from 2.0 by 6.0 meters at the deck level to 4.0 by 9.0 meters at the bottom. At the top of the pile cap, the tower is 12.24 meters wide in the transverse direction.
A double-plane arrangement for the cables was chosen to give the bridge a more robust appearance. The cables are spaced 8.0 meters apart at the deck. A transverse beam at each cable location brings the cable forces from the edge of the deck back to the girder box. These transverse beams are 0.5 meters thick and 1.5 meters deep at the centerline of the box. All are made of prestressed concrete.
The bridge girder has a single-cell box cross section and a constant depth of 3.5 meters over the entire five-span main section. The deck is 15 meters wide, including two 4.7-meter-wide tracks, two 1.3-meter-wide maintenance paths, and two 1.5-meter-wide cable anchorage zones.
The bottom of the box is 8.0 meters wide. In a typical section, the webs are 0.50 meters thick and the bottom slab is 0.3 meters thick, a haunch on each side tapering to 0.5 meters at the inner face of the web. At the north approach spans, the deck widens from 15 meters to 18.09 meters to accommodate the curvature of the tracks.
Rapid Construction
The bridge girder has a single-cell box cross section and a constant depth of 3.5 meters over the entire five-span main section. The deck is 15 meters wide, including two 4.7-meter-wide tracks, two 1.3-meter-wide maintenance paths, and two 1.5-meter-wide cable anchorage zones. The box girder in the cable-stayed portion was constructed segmentally using form travelers to expedite construction. The segment length was 8.0 meters to match cable spacing. After each segment was completed, a cable pair was installed, one cable on each side of the deck. The box girder was prestressed longitudinally and transversely, and the slab atop the girder was prestressed transversely. The construction cycle for each segment averaged 12 days.
The side and approach spans were constructed using a self-advancing underslung truss. To facilitate the advance of the truss, the middle portion of each pier top was hollowed out and the box girder was cast on top of the truss. After each span was placed and prestressed, the truss was advanced to the next span. Construction of each span took approximately 25 days on average.
The towers were constructed using jump forms, with C50 concrete (cube compressive strength of 50 MPa, equivalent to a cylinder compressive strength of about 39 MPa) mixed at the jobsite. A temporary work bridge was built to pump material from the shore to the tower location and for constructing the bridge girder.
Anchored in the box section of the tower legs are the upper ends of the 112 cables used on the Caijia Bridge. Installation began with the external pipe. Strands were then pulled, one at a time, from the lower end to the upper end and stressed individually. A lead strand was installed first, and the force in this strand was monitored by a gauge. Each subsequent strand was then stressed to the force indicated by the first strand. After installation of all strands, the force in the entire cable was adjusted to the specified force with a single hydraulic jack.
The cables comprise 15.2-millimeter-diameter, seven-wire strands with a breaking strength of 1,860 MPa. Each cable strand is encased in its own high-density polyethylene pipe to enable individual replacement and ensure that the cable force in each can be adjusted in the future without interrupting traffic. Individual strands are further protected by an outer pipe of high-density polyethylene.
Aesthetics and/or Harmony with the Environment
The Caijia Bridge is located at the northeastern part of Chongqing, a region slated for development as a new business center. This region is also one of the municipality’s most scenic, featuring hot springs, recreational facilities, and a beautiful, rolling landscape that attracts hikers. Because of this, bridge design aesthetics were considered of significant importance.
The cable-stayed design incorporating segmental construction offers the advantages of both a visually-appealing profile and the appropriate deflection control. The cable forces can also be adjusted anytime in the future, if required, making it possible to adjust the elevation of the girder.
Cost Competitiveness
The construction cost of the entire bridge was 251 million yuan (roughly $38 million). This amounts to an average of $189 per square foot of deck area.
Jury Comments
Challenging design requirements for limited live load deflections were met by combining segmental with cable-stayed technology demonstrating how a segmental bridge solution addressed the unique demands for a heavy rail bridge structure. In addition to outstanding aesthetics, the engineering team should be commended for the rigor required to analyze and design this cable stayed bridge built using cast-in-place balanced cantilever construction. Special studies of this light rail transit bridge with direct fixation track included rail-structure interaction, dynamic response of the bridge for the moving transit loads, and wind studies that included the rolling stock.
2015 Bridge Award of Excellence
Category: International
Owner:
Chongqing Rail Transit Group Co., Ltd.
Owner’s Engineer:
T.Y. Lin International
Designer:
T.Y. Lin International
Contractor:
China First Highway Engineering Co., Ltd.
Construction Engineering Services:
T.Y. Lin International
Construction Engineering Inspection:
Chongqing Construction Science Research Institute
Form Travelers for Cast-in-Place Segments:
China First Highway Engineering Co., Ltd.
Erection Equipment:
China First Highway Engineering Co., Ltd.
Post-Tensioning:
Liu Zhou OVM Machinery Co., Ltd.
Stay Cable Materials:
Liu Zhou OVM Machinery Co., Ltd.
Bearings:
Chengdu Xinzhu Road & Bridge Machinery Co., Ltd.
Expansion Joints:
Chengdu Xinzhu Road & Bridge Machinery Co., Ltd.
Epoxy Supplier:
Liu Zhou OVM Machinery Co., Ltd.
