A simplified procedure for the seismic analysis of RC moment resisting setback frames

Abstract

Irregularity in elevation of buildings is increasingly popular in modern urban construction. One of the prominent forms of vertically irregular frames is the setback building frames. The seismic performance of buildings with irregular distribution of mass, stiffness, and strength along the height may be significantly different from that of regular buildings. The present paper focuses on quantification of irregularity and studying the effect of irregularity on the seismic performance of setback building frames. An index namely Geometric Regularity Index (GRI), calculated based on the geometric parameter of the building frame, is proposed in the study to quantify the vertical irregularity of setback building frames. 55 numbers of 3 bay frames with stories 5, 7, 9, 11 and 13, with varying levels of irregularity, are considered in the study. Linear dynamic analysis of the frames is performed in ETABS using response spectrum method. The proposed index is correlated with the actual seismic response of the structure such as base shear and overturning moment. Based on the results of analysis, a simple method which employs the GRI of the frame is proposed to estimate the seismic responses such as base shear and overturning moment at the base. The proposed method is found to estimate the seismic response of setback frames with reasonable accuracy.