INFLUENCE OF SOIL MODULUS ON DYNAMIC RESPONSE OF BUILDING

Abstract

The structures resting on soil may fail due to excessive settlement, tilting or sinking into sub-soil, despite being structurally safe during earthquakes. This devastating effect on buildings have been observed during several past earthquakes. Generaly while design of structure for seismic loadings, it is assumed as structure rests on a rigid base, which is not true for structures resting on soft soils. The considerable deformation of sub-soil system, often endangers the safety and stability of whole structure, otherwise safe. Continuous efforts are made to evaluate the extent of influence of underlying subsoil stratum on dynamic response of building structures. In present study soil structure interaction effects have been studied for a typical seven storey R.C. framed structure, resting on raft foundation. In order to provide a parametric study of soil modulus effects, six different type sub-soils (loose sand, silt, clay, dense sand, sand stone and basalt i.e. from softer range to harder rocks ), are chosen. The soil medium is represented as a set of statically equivalent springs. All four frames ( longitudinal exterier, longitudinal interier, transverse exterier and transverse interier) are analysed for dynamic response and for different sub-soil characteristics. Results of the study indicate that the design forces for a specified seismic input, vary with the nature of sub-soil medium i.e. soil structure interaction has a dominant role. Initialy as the shear modulus of sub-soil increases ( from loose sand to dense sand ), there is a rapid decrease in the dynamic response ( shear force, moment, displacement, time period, axial force ) of the members. This rate of decrease of response reduces very gently as foundation soil becomes more rigid ( dense sand to basalt ). Hence it is not going to be much favourable to have very rigid sub-soil mediurn.Furthermore, independent analyses are desirable for the options of various sites in order to arrive at a cost effective design with respect to the site.