Effect of soil-flexibility on dynamic behaviour of building frames on raft foundation

Abstract

The overall lateral stiffness of any building decreases due to the compressibility of soil. This leads to a subsequent increase in the natural periods of the structural system. The seismic lateral response may change due to natural periods while seismic torsional response arising out of mass or stiffness eccentricity depends on the ratio of uncoupled torsional-to-lateral natural period of the building. Hence, the effect of soil–structure interaction on lateral natural period, seismic base shear and fundamental torsional-to-lateral period ratio of building frames resting on raft foundation needs a detailed investigation. Present study is a limited effort to accomplish these objectives. Influence of the variation of a number of parameters such as (a) different soil conditions, (b) number of stories, (c) number of bays, (d) the ratio of flexural stiffness of columns to that of beams and (e) ground excitation frequency is considered in the present study. Buildings are modelled by two parallel approaches, viz., (1) frame with brick in-fill having fixed supports and (2) frame with brick in-fill having supports accounting for soil-flexibility. The soil-flexibilities for various types of soil and foundation based on their properties, e.g., shear modulus, the Poisson ratio of the soil, and shape and size of the rafts are computed following the well-accepted literature. The lateral natural period, torsional-to-lateral natural period ratio and seismic base shear of a variety of building frames at fixed base condition are compared to the same considering soil-flexibility due to variation in different influencing factors. The trends observed are attempted to be interpreted physically. The curves and tables presenting the change in lateral natural period and torsional-to-lateral period ratio due to effect of soil-flexibility of various building frames seem to be useful for calculating base shear of low-rise buildings through a simple methodology. Such simple methodology may be useful in the design offices for its simplicity and accuracy.