Assessing lateral period of building frames incorporating soil-flexibility

Abstract

Flexibility of soil medium below foundation decreases the overall stiffness of the building frames resulting in a subsequent increase in the natural periods of the system. It is well established that the seismic lateral response may considerably alter due to the change in lateral natural periods. Hence in the present study, an attempt has been made to observe the effect of soil–structure interaction on the change in lateral natural periods of building frames resting on isolated and grid foundations. Variation of a number of factors 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) frequency of the ground excitation is considered. For such analysis, buildings are modelled by four alternate approaches, namely, (1) bare frame with fixed supports, (2) bare frame with supports accounting for soil-flexibility, (3) frame with brick in-fill having fixed supports and (4) frame with brick in-fill having supports accounting for soil-flexibility. For each category, two cases, viz ., one without tie beams, and the other with tie beams at plinth level are considered. The soil-flexibility 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 footings are computed by methodology prescribed in well-accepted literature. Modelling the system so rigorously, a comparative study of variations of lateral natural periods of various building frames due to variation in different influencing factors are made and interpreted physically. The study shows that the presented variation curves for dynamic characteristics may be used for reasonably accurate assessment of the effect of soil–structure interaction on any building frame with the help of simple linear interpolation. These curves are also useful for incorporating the effect of soil-flexibility in calculating base shear through a simple methodology and may prove useful in the design offices for its simplicity and accuracy.