Choice of Bearings

Bridge bearings are devices for transferring loads and movements from the deck to the substructure and foundations.
In highway bridge bearings movements are accommodated by the basic mechanisms of internal deformation (elastomeric), sliding (PTFE), or rolling. A large variety of bearings have evolved using various combinations of these mechanisms.

Design Considerations

The functions of each bearing type are :

1. Elastomeric
   The elastomeric bearing allows the deck to translate and rotate, but also resists loads in the longitudinal, transverse and vertical directions. Loads are developed, and movement is accommodated by distorting the elastomeric pad.

2. Plane Sliding
   Sliding bearings usually consist of a low friction polymer, polytetrafluoroethylene (PTFE), sliding against a metal plate. This bearing does not accommodate rotational movement in the longitudinal or transverse directions and only resists loads in the vertical direction. Longitudinal or transverse loads can be accommodated by providing mechanical keys. The keys resist movement, and loads in a direction perpendicular to the keyway.

3. Roller
   Large longitudinal movements can be accommodated by these bearings, but vertical loads only can generally be resisted.

The designer has to assess the maximum and minimum loads that the deck will exert on the bearing together with the anticipated movements (translation and rotation). Bearing manufacturers will supply a suitable bearing to meet the designers requirements.

Bearings are arranged to allow the deck to expand and contract, but retain the deck in its correct position on the substructure. A 'Fixed' Bearing does not allow translational movement. 'Sliding Guided' Bearings are provided to restrain the deck in all translational directions except in a radial direction from the fixed bearing. This allows the deck to expand and contract freely. 'Sliding' Bearings are provided for vertical support to the deck only.