Effect of Soil Flexibility on Seismic Fragility of Code-Designed Reinforced Concrete Framed Buildings

Abstract

The seismic response of structures is very complex owing to the behaviour of soils during an earthquake. Traditional seismic designs rely on the assumption that the base of the foundation is fixed. However, the foundation tends to displace owing to the flexible nature of the soil. Indian seismic code and other codes though incorporate the effect of soil-structure-interaction (SSI), but in a very broad sense, and thus are inadequate. This study aims to understand the seismic performance of reinforced concrete (RC) framed buildings designed according to the Indian code, considering soil flexibility. The seismic vulnerability of buildings is investigated through pushover and fragility analysis. Six RC framed buildings with different heights, encompassing low-, mid- and high-rise buildings, covering a wide range of lateral periods (short-, medium- and long-periods), are considered. These buildings are considered to be resting on raft foundations. The soil flexibility for four types of soft clay soil is computed based on shear velocity, shear modulus and density. A variational study for a range of soft soils in combination with short-, medium- and long-period structures is carried out. Fragility curves and damage probability matrices are developed using push over analysis incorporating SSI. The overall conclusions from the fragility curves are that the soil flexibility may significantly increase the seismic vulnerability of low-rise RC framed buildings, and the performance of high-rise buildings may be improved by taking this into consideration.