SOIL STRUCTURE INTERACTION EFFECT ON DYNAMIC BEHAVIOR OF 3D BUILDING FRAMES WITH RAFT FOOTING

Abstract

The soil flexibility effect is generally not considered in seismic design of building frames and the design is done based on results of dynamic analysis taking fixed base condition. Flexibility effect of soil causes lengthening of lateral natural period due to overall reduction in lateral stiffness of the structure. Such lengthening lateral natural period (T) may considerably vary the seismic response of building frames resting on raft foundation. Hence it is necessary to unite the flexibility of soil on which the foundation rests during analysis such study being termed as soil structure interaction (SSI). In the present study the dynamic behavior of building frames over raft footing under seismic forces uniting soil structure interaction is considered. The analysis is carried out using FEM software SAP2000 *Ver14. For the interaction analysis of space frame, foundation and soil are considered as parts of a single compatible unit and soil is idealized using the soil models for analysis. The soil system below a raft footing is replaced by providing a true soil model (continuum model). In continuum model, soil is considered as homogeneous, isotropic, elastic of half space for which dynamic shear modulus and Poisson’s ratio are the inputs. Influence of number of parameters such as number of storey’s, soil types and height ratio for seismic zone-V is considered in present study. Building responses are considered for bare frame with and without accounting for soil flexibility. The responses in terms of lateral natural period and seismic base shear, lateral displacement (story drift), with and without soil flexibility is compared to evaluate the contribution of soil flexibility on building frames.