Determination of inelastic seismic demands of RC moment resisting setback frames

In this paper, an extensive parametric study is conducted on plane RC moment resisting frames with setbacks. Firstly, a parameter called as ‘irregularity index’ is proposed based on the dynamic characteristics of the frame to quantify the setback irregularity. Secondly, this paper aims to determine the affect of setback presence on inelastic deformation demands. To achieve this purpose, building frames with different arrangements of setbacks are modeled and designed in accordance with the European standard code of practice. These frames are subjected to an ensemble of 13 ground motions scaled to different intensities in order to obtain different performance levels as prescribed by SEAOC 1995 and analyzed by time history analysis. Results of the analytical study indicate strong influence of the parameters like beam–column strength ratio, number of stories, geometrical irregularity and the performance level under consideration on inelastic seismic demands. Furthermore, a seismic response database consisting of 13,728 non-linear dynamic analyses is generated, and non-linear regression analysis is performed on this database to propose simple formulae to estimate different seismic parameters in terms of the proposed irregularity index. The applicability of author proposed equations in PBD and DBD is briefly discussed.