Importance of Dynamic Soil-Structure Interaction Analysis in Design of Tall Buildings and Infrastructure Projects

Abstract

After 1964 Niigata earthquake (M 7.5), it was evident that damage to the structure not only depends on the behaviour of super structure but also on the sub-soil below it. It was also observed that large concentration of damage in specific areas during an earthquake is due to site dependent factors related to surface geological conditions and local soil. Since then, many researchers have studied the behaviour of the soil subjected to the dynamic loading. Besides field observations, investigations were done experimentally, analytically and numerically. From these investigations, it was understood that the response of soil to dynamic loads plays a major role in the damage of structures. The behaviour of soil becomes much complex and several factors needs to be considered. Since past few decades, soil-structure interaction (SSI) has been recognized as an important factor that may significantly affect the relative building response, the motion of base and motion of surrounding soil. In general, building-soil interaction consists of two parts; kinematic and dynamic (or inertial) interaction. The former is a result of wave nature of excitation and is manifested through the scattering of incident waves from building foundation and through filtering effect of the foundation that may be stiffer than the soil and therefore may not follow the higher frequency deformations of soil. This interaction depends on frequency, angle of incidence and type of incident waves, as well as shape of foundation and on the depth of embedment. It develops due to presence of stiff foundation elements on or in soil cause foundation motion to deviate from free-field motions. The later is due to inertia forces of building and of the foundation which act on soil due to contact area. And it depends on the mass and height of the building and the mass and depth of foundation, on the relative stiffness of soil compared with the building and on the shape of foundation. It develops in structure due to its own vibrations which gives rise to base shear and base moment, which in turn cause displacements of the foundation relative to free field. At low level of ground shaking, kinematic effect is more dominant causing increase of period. Observations from recent earthquakes have shown that the response of the foundation and soil can greatly influence the overall structural response. There are several cases of severe damages in structures due to SSI in the past earthquakes (1989 Loma Prieta Earthquake, 1995 Kobe Earthquake and 2001 Bhuj). SSI analysis procedures are important in various cases of structural and soil conditions. Some of them are briefly outlined here. Type A structures like Rigid Tower, in which the supporting soil media will go to nonlinearity and the structure will remain in linear state only. Type B structures like pile supported Jetties, in which the supporting pile and soil will go to nonlinearity and the structure will remain in linear state only. Type C structures like Frame Buildings, in which the pile, soil and structure will go to nonlinear state under strong seismic shaking. Type D structures like Pipes, in which the supporting soil media will go to nonlinear state under differential settlement and pipe, will also go to nonlinear state, etc.