Dynamic soil structure interaction behaviour of building frames with raft foundation

Abstract

The soil flexibility effect is generally ignored in seismic design of building frames and the design is carried out based on results of dynamic analysis considering fixed base condition. Flexibility of soil causes lengthening of lateral natural period due to overall decrease in lateral stiffness of the structure. Such lengthening of lateral natural period(T) may considerably alter the seismic response of the building frames resting on Raft foundation. Hence it is necessary to incorporate the flexibility of soil on which the foundation rests during analysis, such study being termed as Soil Structure Interaction. The present paper deals with dynamic behaviour of building frames asymmetric in plan with Raft footings under seismic forces incorporating soil-structure interaction. The analysis is carried out using FEM software SAP 2000*V14. The structure is idealized as 3D space frame where in beam and column are modeled as 2 noded line elements with six degrees of freedom at each node and slabs are modeled as thin shell with four noded plate elements having six degrees of freedom at each node. The soil is modeled as equivalent springs with single(Winkler) and six degrees of freedom(Modified winkler) the stiffness of which varies with type of soil based on the properties, viz., dynamic shear modulus and poissons ratio of soil. The raft foundation for winkler and Modified Winkler are modeled as thin shells with four noded plate elements having 6 degrees of freedom at each node and is discritised, such that the aspect ratio of the elements are equal to one. The response in terms of fundamental Natural Period, Seismic Base Shear and Max. Lateral Displacement, for a variety of building frames with and without consideration of soil flexibility is compared to evaluate the contribution of soil structure interaction on building frames. Influence of of parameters such as different soil types (i.e, Soft, Medium and Stiff), different span lengths, number of bays and number of stories are considered in the parametric study for Zone V. It is found that the effect of soil flexibility on building frames appreciably alters the fundamental lateral natural period and seismic base shear of the system. Due to the effect of soil flexibility, the lateral natural period and seismic base shear increase with the decreasing stiffness of soil. It is also observed that the lateral period and base shear of the building increases with increase in number of bays. The Maximum lateral displacement increases with increase in number bays and stories. © 2013 Taylor & Francis Group.