200704171229Introduction of Bih-Tan Bridge
The seventy-year old suspension bridge at Bih-Tan has always been a draw for sightseers with its rainbow-like arch. Rated as one of the top 12 scenic wonders in Taiwan, the bridge is beginning to show signs of age. Now the sparkling new Bih-Tan Bridge rises only 250 meters away, adding its own version of modern engineering elegance and beauty to the surroundings.
Bih-Tan Bridge had ever been a very famous Project during the construction of 2nd Northern Expressway and it was designed by T.Y. Lin International. The Project owner is the Taiwan Area National Expressway Engineering Bureau (TANEEB), Ministry of Transportation and Communications. T.Y. Lin Taiwan was appointed by the owner to act as the Engineer for supervising the construction of the work on behalf of TANEEB. Construction of the Project was undertaken by one of the most important Contractors in Taiwan Continental Engineering Corporation (CEC) from Sept. 1990 to Dec. 1995. The total amount of CEC’s works on this project is approximately US$ 50 million.
Bih-Tan Bridge is a structure of rare appearance and a unique example of innovative design and advanced bridge construction technology. The superstructure and arch girders of piers P4 and P5 of the main bridge must be constructed by a grounded shoring method. The central portion of superstructure between both arch girders on Hsin-Tien Creek is constructed by the cantilever segment method of DSI (DYWIDAG SYSTEMS INTERNATIONAL). The writer was assigned to be deputy site manager of CEC of the above project in early stage and site manager in late stage during the period from Sept. 1992 to Mar. 1997. This article is to introduce some important Construction Issues of the Project.
2. OUTLINES OF THE BRIDGE
The Reinforced Concrete post tensioned Bih-Tan bridge is one of the major bridges of Taiwan’s Second Northern Freeway. It is located 8 kilometers south of Taipei city center in the famous scenic suburban “Bih-Tan” Creek area, which many tourists visit for recreation purposes, and therefore it was necessary to build a unique-looking bridge with a long span in order to match the surrounding environment. The bridge has a height of 20 meters above the waterway level. The total length of the bridge is 800 meters with a 750-meter radius horizontal alignment. This has been a unique prestressed concrete bridge in Taiwan until now, which is no any expansion joint on the deck slab within full length of 800 meters. The bridge crosses Hsin-Tien Creek in two halves of 16.35 meters in width with 3 lanes each running on both north and south bounds. Refer to figure 1 the main bridge includes several piers designated as P4, P5 and the longest span in between which is 160 meters in length. This span was to be constructed using the cantilever segment method with 3 meters of each segment due to passing over Hsin-Tien Creek and the length of the span.
Figure 1. Elevation of BIH-TAN Main Bridge of 2nd Northern Expressway
3. PILING CONSTRUCTIOn
Advanced technique of full casing piling, rarely used before, is used for drilling 2-meter diameter bored piles for the foundation. A total 215 piles with average drilling depth of 30 meters were carried out to ensure the quality of piles rather than using reverse circulation piling or caisson. Static loading tests revealed very minor settlements less than 4mm indicating full casing piling to be the better piling method, more suitable for foundation construction in gravel and sand rock stratums. Most of the piles must penetrate into rock layers with a depth of 10~14.5 meters in order to form a rock socket. Post grouting of a flat jack under some piles was also done after the pile concrete had set completely in order to increase the end bearing capacity and reduce the residual sediment.
4. the biggest steel caisson ever built IN TAIWAN
The foundation for 4 main piers for the center 160-meter span was constructed inside the waterway of Hsin-Tien Creek. The size of each foundation is 24m × 19m × 4.5m. Since the bottom of foundation is 6.25 meters below water level, a coffer dam of huge steel caisson with the area 82.2m × 47.2m was built for each two pier foundations on both sides of Hsin-Tien Creek by using 17 sets of synchronized hydraulic jacks to lower down the steel caisson to designated excavation depth. This is the biggest steel caisson ever built for a bridge pier foundation in Taiwan area. After the caisson was set in position, tremie seal concrete of 3.5-meter depth was placed and a foundation of 4.5-meter depth was constructed on top of the tremie seal. The volume of concrete poured in each foundation was about 2,000 cubic meters, which should be continuously constructed and completed within a day.
5. ALUMA SHORING SYSTEM
Bih-Tan Bridge is located in a 750-meter radius horizontal alignment and the cross section of bottom slab of the main bridge is varied with a 5% superelevation. Therefore, the outward appearance of the bridge is a complex curve in three dimensions. The concrete weight of box girder each meter is about from 75 to 100 tons and some structural shoring frames must be on the riverbed of Hsin-Tien Creek. For the safety of construction the shoring system should be carefully selected and checked.
CEC would spend much money to import ALUMA Shoring System from CANADA in order to promote the construction quality and ensure the construction progress. ALUMA is a Shoring System of Aluminum alloy material with the features of lightness, bigger strength, stainless and adjustable accessories on both sides of each segment. The maximum vertical bearing load of shoring frame can be 5 tons per square meter but its unit cost per square meter is about NT$ 16,000, which is 3 times of general shoring system.
6. A unique-looking bridge with a SPECIAL CONSTRUCTION METHOD FOR SUPERSTRUCTURE
Because Bih-Tan Bridge is located in a scenic area, which many tourists visit for recreation purposes, it is necessary to build a unique-looking bridge with a long span in order to match the surrounding environment. The bridge has a height of 20 meters above normal waterway level. It adopts a divided twin bridge design concept with 3 lanes for a single cell continuous concrete box girder of 3.5-meter depth. The travelers segment cantilever construction methodology is being used to build the center long span with 750-meter radius. Due to these factors, several things need to be paid attention to in construction.
l Prestressing for transverse direction must be done every 60cm first in order to reduce the thickness of the bridge deck as well as the dead load itself.
l Due to large curvature of the bridge, which creates large dead torsion, horizontal draped cable in deck must be placed with prestressing at both ends. Also, deeper deck section is necessary for putting heavier cable in the outside girder web to contract part of the torsion.
7. dynamic analysis AFTER DESIGN
After the design was completed, a dynamic analysis was carried out by using firstly a scaled down version of spectrum analysis with an equivalent shear of 0.15w, which is equal to the original static 0.15w earthquake design. A second analysis was carried out using DORT’s 0.22g outside the basin spectrum analysis. The purpose of dynamic analysis is to compare the caisson’s worst combination results and examine those results with the original specified design capacities at work stress level. Analysis also determined and tabulated moment and shear capacities for critical structural elements. The demands of each element are within the capacities and cause the element to behave below yielding strength without exceeding the ductility factor for the static equivalent dynamic analysis.
Bih-Tan Bridge is a structure of rare appearance and a unique example of innovative design and advanced bridge construction technology. It broke our traditional concept of building a bridge with considering its beauty and fashion design in addition to its safety, function and economic factors. Some of advanced bridge construction technology above mentioned can be a very good example for future use in the similar project.