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Bridge cantilever construction - precast

Balanced cantilever construction is an economical method when access from below is expensive or practically impossible. The cross-section is normally a box. Segments may be cast in-situ or precast. Construction starts from the top of a pier, with the segment normally fixed to the pier either permanently or temporarily during the construction. (Construction can be carried out from an abutment provided there is a balancing weight to counteract the segments in the span).

Subsequent segments are post-tensioned to the previous sections on alternate sides of the pier so that the out-of-balance moment is kept to a minimum. A temporary support is sometimes added on one side of the pier; if the pier/deck connection is fully fixed this is not necessary.

In segmental balanced cantilever construction the precast segments are transported to the bridge and placed and held at the right position before post-tensioning back to the rest of the bridge. A moving gantry with lifting capability for the heaviest segment is required to lift and hold the segment in position. The precast segments normally have a shear key and match cast in a casting yard. Epoxy resin is normally used in the joint.

For further information see Concrete Society Current Practice Sheet 153, Balanced cantilever bridges, published in CONCRETE in November 2007. Copies of the Current Practice Sheet may be downloaded from the Members’ Area of the Concrete Society web site or from the Concrete Bridge Development Group (CBDG) website.

Typical erection sequence for type a gantry

Acknowledgement: Gifford Consulting

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