Multistorey frame on grid foundation system is very common for the purpose of housing in the third world countries like India. The effect of soil-structure interaction is generally ignored in the design process of such buildings. But, in reality, supporting soil medium allows movement to some extent due to its flexibility. This may decrease the overall lateral stiffness of the building frames resulting in a subsequent increase in the natural period of the system and hence, may alter the response under earthquake and wind loading considerably. To investigate the possibility of the same, buildings are modeled by four alternate approaches, viz., (1) bare frame with fixed supports, (2) bare frame with spring supports accounting for soil-flexibility, (3) frame with brick infill having fixed supports and (4) frame with brick infill having spring supports representing soil-flexibility. For each category, two cases, namely, one without plinth beams and the other with plinth beams are considered. The stiffnesses of the springs attached to the foundation base to account for the soil-flexibility are determined for various types of soil and foundation based on their properties, e.g. shear modulus, Poission's ratio of the soil, and shape and size of the footings as prescribed in the standard literature. Representative case studies of building frames, modeling the soil-foundation-structure system so rigorously with four alternatives mentioned above, are reported in details. The free vibration characteristics as well as response under design spectrum-consistent seismic excitation input are presented in each case. The trends in behaviour observed from the study are presented and discussed. The trends recognized in the study may be helpful in arriving at useful design guidelines for improved seismic behaviour of such buildings.
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