Soil-structure interaction effects on seismic response of open ground storey buildings

Bhuj earthquake in India, 2001 had witnessed spectacular failures of a class of reinforced concrete multi-storied buildings termed as Open Ground Storey (OGS) buildings, necessitated by a functional demand to provide a parking space within a building plan. Due to the absence of brick-infilled walls at the ground floor, a sudden reduction in the storey stiffness can cause enormous seismic displacement demand in the ground storey itself. Further, the column-side sway mechanism is developed due to the presence of soft storey between the stiff upper storey and rigid base by assuming the soil support is stiff enough. The present study focuses on the effect of soil flexibility on the seismic response of open ground storey buildings. Analytical studies on typical open ground storey building models considering the soil flexibility have been carried out in SAP2000 software and static nonlinear analysis (pushover analysis) has been used to study the lateral response. Variation in boundary conditions are incorporated by simulating three different soil conditions hard, medium and soft, classified as mentioned in IS 1893 (Part 1) 2016. It is observed from the present study that the soil flexibility increases the lateral displacement and secondary forces associated with the P-Delta effect. Further, a parametric study is carried out to study the influence of soil flexibility in OGS buildings of various slenderness ratios. The importance of considering the influence of soil-structure interaction has been highlighted for obtaining a realistic performance point of the building. In addition, for preliminary and quick seismic assessment of huge stock of existing OGS buildings in Indian urban regions, a simplified methodology for estimating the lateral behaviour of a flexible base open ground storey building has been developed. This methodology is useful to segregate the highly vulnerable OGS buildings that undergo a detailed assessment prior to retrofit. The developed methodology is validated with the detailed analytical studies made on open ground storey buildings.