Considering SSI by an equivalent linear method for nonlinear analysis of concrete moment frames

This paper evaluates the effects of Soil-Structure Interaction (SSI) on the seismic response of multi-story concrete frames subjected to earthquake ground motions. To calculate the structural response, an iterative Wave-Propagation (WP) based method has been used. The exact solution of SSI problems, in addition to high computational costs, due to general complexity of finite element analysis, require an advanced computer system. According to the method proposed herein, buildings were modelled as an extension of the layered soil media by considering each story as a layer in the WP path. The motion in each layer could be determined from the motion in any other layer. This permitted to use an operation called deconvolution, which could be considered to investigate the effect of substructure soil layers on the structural response. One of the capabilities of WP method is to facilitate the computation of bedrock motion from a known free surface motion and consequently determination of correct foundation input motion. While most of the current methods assume constant values for basic parameters such as; stiffness and damping, in the present method in order to investigate the nonlinear behaviour, stiffness and damping are proportional to the displacement level of soil layers or structural floors. The computational speed and accuracy of the proposed method are also verified by a benchmark frame analysis. Also, based on the Park-Ang Damage Index, this paper evaluated the measures of damage (the relationship between earthquake characteristics (PGA) and damage level) which govern structural degradation and/or collapse obtained from different well-known destructive earthquakes.