Nonlinear seismic analysis of multi-storied reinforced concrete buildings with masonry infill

Abstract

Masonry infill wall in frame structures have long been known to affect strength, stiffness and post peak behavior of the infilled framed structures. The interaction between the wall and the frame under lateral load dramatically changes the overall characteristics of the composite structure. In seismic areas, ignoring the composite action of infill panel and frame is not always on the safe side. So, response to seismic loads creates a major source of hazard during seismic events. The present study consists of two parts. At first, lateral load – displacement curve for column and masonry wall is obtained using non-linear FE analysis from which lateral stiffness of column and infills has been found. Secondly, this stiffness which is equivalent to the stiffness of a nonlinear spring is then used in a multi-degree of freedom (MDOF) spring-mass-dashpot system. The MDOF system is modeled in MATLAB where dynamic equilibrium equations have been solved by state-space method. Detailed parametric study with different PGA values, earthquake frequency and varying number of stories has been performed to obtain a guideline of collapse of different types of buildings under seismic loading. After performing nonlinear dynamic analysis, behavior of these structures is compared. It is evident that natural frequency is completely different from codeprovision of natural frequency due to the presence of infill. Soft-storey frame collapses earlier than bare and infilled frame for all seismic excitations. As, number of storey increases, soft storey collapse initiates earlier than buildings with smaller number of stories.