Bearings, movement and expansion

They are also subject to movement due to thermal expansion and contraction caused by change in ambient temperature. These forces and movements are absorbed and controlled by supporting the bridge on a system of appropriate fixed, rotating, elastomeric and/or sliding bearings.

Lateral forces must be transferred to suitable supporting structures in such a way that stability is maintained at all times whilst no undue forces are induced in the bridge deck or supporting structure. For beam type bridges, usually one support has fixed bearings, to locate the bridge in position in the longitudinal direction (along the deck) while other supports have bearings that allow movement in the longitudinal direction to accommodate thermal expansion/contraction of the deck. These sliding bearings usually incorporate a PTFE (polytetrafluoroethylene) surface and have suitable guides that restrict the movement to the longitudinal direction, restrain lateral movement, and transmit the lateral forces to the supporting structure. Alternatively, for shorter spans, the bridge may be supported on elastomeric bearings that accommodate movement by shear deformation of their rubber/steel plate sandwich construction (Lee, 1994).

Because the bridge supports are fixed in position but the deck may expand or contract considerably along its length, generally at least one expansion joint is required. Depending on the configuration and type of support additional movement joints may also be necessary

The type of expansion joint used depends on the overall length of the bridge. For short spans, expansion joints may, for instance, consist of a compressible polymer profile or pre-formed sealing strip installed between metal nosings fixed to the deck and/or wall of the abutment. For large movements, a system of sliding plates or opposed tooth or comb blocks may be used to cover the larger expansion gap. The main movement joint occurs in the deck at the support(s) remote from the fixed beaming (Lee, 1994).