BASE ISOLATION OF URM INFILLED RC FRAME BUILDINGS

Abstract

Unreinforced masonry (URM) infilled RC frame buildings are common structures in India. Masonry infill is used in buildings for its good sound and heat insulation property. During earthquakes, the infills are subjected to both in-plane and as well as out-of-plane forces, for which their behaviour becomes quite complex. Generally the infills first crack under in-plane forces and then collapse due to out-of-plane inertia forces. It has been observed that even in the buildings for which RC frame is intact after earthquake, collapse of infill is almost unavoidable. This is not at all desirable for hospital buildings, which are of utmost post-earthquake importance and must be functional after the earthquake. Other non-structural components, such as medical equipments, complex network of electrical and mechanical facilities also must be intact after earthquake. Floor response and interstorey drift, together these two components, cause damage to the building contents, architectural facades, partition walls, piping and ductwork, false ceilings, building equipments and elevators. The only practical way of reducing floor accelerations and interstorey drift simultaneously is to use base isolation. The isolation system provides the necessary flexibility, with the displacements concentrated at the isolation level. Thus seismic isolation provides a viable economic solution to the difficult problem of reducing earthquake damage of non-structural elements. Two buildings, one 4 storey and one 8 storey, having identical plan have been considered in this study. 114mm and 229mm thickness of masonry infill has been considered. Two types of base isolation systems have been designed for each of the buildings, namely Lead-Rubber Bearing (LRB) and Friction Pendulum System (FPS). The effect of isolators on dynamic characteristics, seismic performance, interstorey drift ratio and peak floor acceleration has been studied and compared with its fixed base counterparts with the help of Non-Linear Static Push-Over Analysis