Q. Explain types of bearings?

The bearing transfers the forces coming from the superstructure to the substructure. It also allows for necessary movements in the superstructure which are caused by temperature variations. The following types of bearings are generally used.

a) Sliding bearing - permitting rotation and translation

b) Rocker and roller bearings - permitting rotation and translation respectively.

c) Elastomeric bearings - Made of natural or synthetic rubber of shore hardness of approximately 50 to 70. They are very stiff in resisting volume change but are very flexible when subjected to shear. They are generally reinforced with steel plates in alternate layers to reduce bulging. When used with a steel or concrete girder these permit moderate longitudinal movements and small rotations at the ends. These are provided for bridges having RCC or Pre stressed girders and can take deflection a well as movement

d) P.T.F.E. Bearings - The PTFE can be unfilled or filled with glass fibre or other reinforcing material. These are used either to provide rotation by sliding over cylindrical or spherical surfaces or to provide horizontal sliding movement over flat surface or a combination of both. Where there are large displacements accompanied with relatively small loadings, as in case of centrifugal loads, wind loads or seismic loads, PTFE sliding bearings are utilised.