Effect of Increased Number of Openings on Response Modification Factor in Steel Building Frames with Concentric Braces

In the present study, the method of determining response modification factor was examined in structures first. Subsequently, a nonlinear static analysis was conducted on several simple steel frames with concentric braces and different number of openings and stories to calculate their response modification factors. Finally, the variations in ductility factor values and added structure resistance were studied. The frames were designed based on Iran standard no.2800. Then a nonlinear static analysis was conducted under lateral loads to develop a code and calculate the added resistance coefficients. P-delta effect was considered in calculating the added resistance coefficients and frame ductility. The calculated values of response modification factor were larger than the values specified in the code for the simple frames with concentric steel braces. Moreover, the values were different for different number of openings. The data was analyzed using ETABS 9.1.4 and SAP2000 14 software. The results showed that ductility factor increased as the number of openings increased. Besides, the results proved it wrong to consider a constant coefficient for all concentric braced steel structures with simple frames, which would render the building non-economic in most cases or fail to provide good resistance to seismic force.