Define Seismic Isolation Bearings  - Bridge Bearing?

Many bridges had been built before the proper seismic design specifications were known to us. These bridges require strengthening and enhancing the ductility of substructures in order to sustain the earthquake forces. Conventional retrofitting methods are costlier (more than 3 times) than providing 'Seismic Isolation Bearings' (SIB). The studies have shown that:

(a) The SIB eliminated the in-plane torsional rotation of the bridge and resulted in a more uniform distribution of seismic forces among substructures in the transverse direction.

(b) The modifications in the response of the bridge resulted as a result of use of SIB because the effective fundamental period of the bridge get elongated due to the flexibility provided by the SIB.

(c) Average retrofitting cost using SIB is calculated as only 30% of that using conventional retrofitting methods.

An isolation bearing induce sufficient lateral flexibility in a bridge structure and increase the period of vibration, resulting in a reduced force response. The increased lateral flexibility is associated with larger horizontal displacements at the bearing level. In the longitudinal direction, the stiffness of a bridge with isolation bearings is the sum of the stiffness of all piers and abutments including the bearings. In a bridge with conventional bearings. However, the longitudinal stiffness incorporates the stiffness of a fixed support alone. The isolation bearings should also include a restoring force to return the superstructure to its pre-earthquake position.