Effect of Soil Flexibility on Dynamic Behavior of Building Frames Resting on Strip Foundation

The effect of soil flexibility is generally ignored in seismic design of buildings. The design is generally carried out based on the results of dynamic analysis considering fixed base condition. Flexibility of soil causes lengthening of lateral natural periods due to overall decrease in lateral stiffness. Such lengthening may considerably alter the seismic response of the building frames. It is therefore, necessary that the dynamic inter relationship between soil and structure to be taken into account in the seismic analysis of structures. In order to evaluate the seismic response of a structure, at a given site, the dynamic properties of the combined soil structure system must be established. Due to the differences in responses of a rigidly supported structure and elastically supported structure, it is essential to study the effect of soil flexibility on structure for safety and economy. The seismic load acts during a very small interval of time. Hence, during the action of such loads, instead of consolidation settlement, the instantaneous settlement is expected to occur. This behavior of soil can be conveniently simulated using a set of elastic springs. This study has been carried out for numerous building frames resting on strip foundations and comparison between the nature of change in lateral natural periods and base shears due to the incorporation of soil flexibility relative to that at fixed base condition has been presented. Such study may help to provide guidance to assist more accurately the seismic vulnerability of the building frames resting on strip foundations and may be useful for seismic design.