Bridge deck segmentation
The segments are generally precast and erected by crane or by special erection equipment, such as a gantry or lifting frame. They may also be cast in situ, although this is rare.
Precast I or T beams are normally topped with a concrete layer that is cast in situ to tie together all of the elements. This also provides the running surface for the traffic as well as forming a compression member for the structure when in service.
Example of transverse segmentation
span by span on falsework
span by span on a movable scaffolding system (MSS)
the free cantilever method (FCM)
the incremental launching method (ILM)
Precast segmental schemes are constructed using the methods of:
span by span, erected over falsework or using erection gantries (overhead or underslung)
the free cantilever method (FCM), using cranes, lifting frames or gantries
fully precast deck, using special launching equipment
The economics of building a bridge depend upon the balance between various parameters including span lengths, site constraints, available budget, aesthetics, deck height and the choice of bridge type.
Depending on the construction method, it is generally accepted that the most economical type of bridge for a given span length follows some general guidelines:
Post-tensioned precast or in-situ bridges for span lengths from 30m to 150m. Precast beam bridges can span up to 50m and are recognised as being the most economical. Precast segmental bridges are visually more pleasing and allow longer spans. Within that range, span-by-span construction allows bridges to be built with spans up to about 50m, while the balanced cantilever method allows them to reach 150m or more.
Arch bridges, for spans up to 500m
Cable-stayed bridges, for spans up to about 1,000m
Suspension bridges, for spans of up to approximately 2,000m