Should special design be catered for in bridge piers upon jacking up of superstructure for installation of bearings in Incremental Launching method?

After the completion of launching process, the superstructure has to be lifted up to allow for installation of bearings. This is usually achieved by means of jacks to raise 5-10mm successively at each pier.

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Central prestressing is normally required during construction in Incremental Launching method. Why?

The erection condition plays an important role to the structural design of bridges when incremental launching method is adopted.

Each section of superstructure is manufactured directly against the preceding one and after concrete hardens, the whole structure is moved forward by the length of one section. When the superstructure is launched at prefabrication area behind one of the abutments, it is continually subjected to alternating bending moments. Each section of superstructure (about 15m to 25m long) is pushed from a region of positive moment and then to a region of negative moment and this loading cycled is repeated. As such, tensile stresses occur alternately at the bottom and top portion of superstructure section. For steel, it is of equal strength in both compression and tension and it has no difficulty in handling such alternating stress during launching process. However concrete could only resist small tensile stresses and therefore, central prestressing is carried out to reduce the tensile stress to acceptable levels.

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What is the purpose of leveling pad in bridge bearing?

Bridge bearings should be installed to lie horizontally on bridge piers and columns so that it would not induce eccentricity forces on substructure. However, the bridge superstructure requires different longitudinal and transverse level and gradient in order to keep in line with the geometry of the road.

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