DYNAMIC BEHAVIOR OF BASE ISOLATED BUILDINGS

Abstract

The main theme of the isolation technique is to reduce the large interstorey drifts of the building by providing a flexible system between the superstructure and the foundation. The building should move to an extent to skip out from the forces coming to the structure due to seismic loading. To get such a movement in the structure it should be sufficiently flexible. It is not possible, since it will create high floor accelerations due to high interstorey drifts of the structure. Reducing high floor accelerations and providing the design displacements to the structure with fixed base condition is impossible. A flexible material between the superstructure and its foundation will create high displacements at its level. But it should be with stand with column loads. So much research is over on this topic. In this study, the literature behind the development and the investigations of base isolation is reviewed. The theory behind the analysis of base isolated buildings is studied. As there are no code specifications for the analysis and design of isolated structure in Indian codes, a study is made to understand the design specifications of UBC-97 and by comparing them with IS 1893 (part 1): 2002, some formulation for the analysis is obtained. Base isolated buildings are modeled using ANSYS 5.4. Here the flexibility of isolation system is given through springs, which are provided in the three orthogonal directions. The modeling is compared with the theoretical analysis. The isolation provided with springs has good performance to provide the required displacement response at base level. A study is made on the behavior of base isolated buildings under various parameters like flexibility of isolation system, stiffness of the structure, height of the building, damping in the isolation system. for this a single, five, ten and twenty storey isolated building frames are modeled and the corresponding behavior is discussed. A hospital building is analysed using computer modeling and designed it with elastomeric bearings in the isolation system.