Base isolation is a modern earthquake-resisting technique in which a superstructure is decoupled from foundation with the help of some isolation device to effectively reduce the transfer of ground motion. Base isolation reduces strength and ductility demand on the superstructure, resulting in lower dependence on post-yield behavior. The earthquake forces do not get directly transferred to the superstructure in the case of base isolated structures, as they do in the case of fixed base structures. Variation in damping of bearings can further control the magnitude of earthquake forces transferred to the superstructure. An effort has been made to study the effect of variation in the damping of bearing on the response of two base isolated RC frames of different configurations against earthquake forces. Damping in bearings is varied from 5% to 40% and the response of the structures is compared in terms of period shift, base shear, storey drift and acceleration-response with equivalent fixed base frames. Response spectrum analysis of these frames is carried out with Manual STAAD Pro (2005). The response of base isolated frames showed a significant reduction in various parameters as compared with fixed base frame. It is found that the rate of response reduction is high up to 20% damping in bearing. Beyond 20% damping, the rate of response reduction is less.

Base isolation is a technique of passive control of structures in which a superstructure is decoupled from the foundation to prevent it from absorbing earthquake energy. The superstructure is supported on isolators whose characteristics are such that they can not only decouple the building from the damaging component of earthquake but also support the building effectively. These isolators are also designed to provide damping. The system isolates building from the horizontal components of the ground motion by interposing bearings with low horizontal stiffness between structure and foundation. This gives the structure a fundamental frequency that is much lower than both its Fixed Base (FB) frequency and fundamental frequency of ground motion.

The concept of base isolation has become a successful reality within the last 25 years with the development of multilayer elastomeric bearings, which are made by bonding of sheets of rubber to vulcanized thin steel reinforcing plates (Ferrito, 1991; and Naeim and Kelly, 1999). There are many Base Isolated (BI) buildings all over the world, which are constructed using elastomeric and other types of bearings. Some of them are Foothill Communities Law and Justice Centre (California), Fire Command and Control Facility (Los Angeles), West Japan Postal Center (Japan), and Bhuj Hospital (India). The satisfactory performance of these buildings is encouraging the adoption of Base isolation technique in the field of earthquake-resistant design.

Structural Engineering Journal, Base Isolated Buildings, Base isolation, Base Isolated Structures, Response Spectrum Analysis, Multilayer Elastomeric Bearings, Base Isolation Techniques, Seismic Isolation Designs, Seismic Isolated Structures, BI Frame, Isolation Bearings.