SEISMIC BASE• ISOLATION OF MULTISTOREY BUILDINGS USING ELASTOMERIC BEARINGS

Abstract

In this work the effectiveness of base isolation has been studied, using the ELctôm.erLr Rubber Bear-tnes of varied isolation abilities, for multistorey buildings of different heights, by interposing these bearings directly under the second raft, carrying the superstructure. The P-A viscoelastic structural model for the bearings and the simple lumped mass model for the superstructure has been adopted. Three different earthquakes viz the El-centro (NS), the Koyna (L) and the Taft (NE) have been used to simulate the three different types of earthquake forcings, in order to predict the behavior of such base isolated buildings of different isolation periods and heights. The analysis has been carried out both with and without P-A effects to know the essence of the P-A effects of the bearings due to different structure heights and earthquake forces. The Isolation works well in general and the response reduction increases with the increase in the flexibility of the bearing but correspondingly base displacements also increase. While these base displacements are controllable for low isolation periods,for high flexibilities of the isolation bearing some additional displacement control devices would be necessary, in order to limit the excessive base displacements. In certain cases it is seen that the advantage derived due to isolation is the highest such as for less high buildings when subjected to large earthquakes. The P-A effects of the bearing which are prominent in case of high buildings and increase with the earthquake forces, cannot be neglected, particularly when the buckling safety factor is low. It is seen that the bearings are subjected to a large variation of the compression load and their height reduction is generally low, except in certain extreme cases and hence no stability problems arise.