Study on Soil Structure Interaction (SSI)

The effect of soil flexibility is generally ignored in the seismic design of building. The design is generally carried out based on the results of dynamic analysis considering fixed base condition. Flexibility of soil causes lengthening of natural period due to overall decrease in stiffness of the structural system. Such lengthening alters the seismic response of the building frame to some extent. It is therefore, necessary that the dynamic inter-relationship between soil and structure to be taken into account in the seismic analysis of structures. For such analysis the buildings with 5, 10, 15 stories having the same plan are modeled for Ordinary Moment Resisting Frame (OMRF) and Special Moment Resisting Frame (SMRF) by two approaches, namely frames with fixed support and frames with supports accounting for soil flexibility. The soil stiffness for various types of soil and foundation based on the properties, namely shear modulus, poisons ratio of the soil and shape and size of footings are computed. The soil stiffness is computed as per FEMA 356 guidelines. Modeling the structural system accordingly, a comparative study of response is made in terms of natural period, base shear, displacements and support reactions. The results are presented in the form of tables and graphs. In order to evaluate the seismic response of the structure, the dynamic properties of the combined soil structure system needs to be established. Owing to the difference in responses of a rigidly supported structure and elastically supported structure, it is essential to study the effect of soil stiffness on structure for the realistic design.