Effect of Soil Flexibility on Seismic Performance of 3-D Frames

In most of the designs of rigid structural frames, the analysis is carried out by assuming fixity at base, which means that the building is idealized to rest on hard rock. This paper discusses the importance of considering the effect of soil stiffness on the seismic performance of rigid structural frame system resting on it. Flexibility of soil causes lengthening of lateral natural period of the structural system due to overall decrease in lateral stiffness. Such lengthening of lateral natural period may considerably alter the seismic response of the building frames resting on isolated footings. Hence, focus of this paper is to bring out the effect of flexible foundation soil on the performance of 2D and 3D frame-foundation systems and their overall dynamic behavior from pushover analysis, a static non-linear analysis. Gazetas [1] model is used for representing stiffness of soil and the foundation resting on it as a spring. For analysis, design and pushover analysis of frame SAP2000 v15 [2] software is used. It is observed that modulus of soil has considerable effect on natural period of system and overall performance of structural system, indicating that idealization of fixity at the base may be seriously erroneous.