The Effect of Increased Opening Height on Response Modification Factor in Steel Building Frames with Concentric Braces

Nowadays, the seismic design of structures in structural codes is commonly produced based on equivalent static method and design base shear using linear spectrum. One of the effective factors used to determine the base shear is the response modification factor or behavior factor. 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 stories and openings. The findings call into question the use of a constant coefficient for all braced structures. Nonlinear static analysis can indicate the general behavior of a structure and demonstrate how a failure mechanism develops in a structure. The added resistance and ductility values obtained in the present study can provide a solid ground for judging the behavior of structures.