INFLUENCE OF SOIL-STRUCTURE INTERACTION ON THE EARTHQUAKE RESPONSE OF BUILDINGS

In this paper, the influence of the soil properties on the dynamic response of a building subjected to different earthquakes is quantitatively analyzed. The proposed mathematical model for foundation and surrounding soil consists of (1) an equivalent soil-foundation mass M0, (2) a translational spring of stiffness K0, (3) a rotational spring of stiffness Kθ, and (4) dashpots for damping in translational and rocking modes. By using existing theory, values for K0 and Kθ have been selected for a rigid foundation supported by semi-infinite soil mass. A numerical incremental-iterative type of scheme is devised and a linear acceleration method is utilized to obtain the elastic and inelastic structural response. Digitized earthquake records from El Centro 1934 and 1940 have been utilized to study the influence of foundation conditions on the structural response of a multi-storey building. A range of soil properties and soil damping characteristics are chosen and various response curves are presented; this leaves complete freedom to a soil and foundation engineer to choose any desired value of the soil properties.Numerical analyses of a three-storey building show that for the foundation, the largest relative displacement occurs in the translational mode and the largest foundation rotation can be obtained by combining translational and rotational modes. It is shown that the natural frequency for a building in which the foundation effects are considered is always lower than that obtained for a building in which foundation effects are neglected. It is found that the soil damping characteristics have a major influence on the structural response, but Poisson's ratio of the soil has a minor influence. Various other recommendations regarding earthquake foundation designs are also suggested.