Recently, designing irregular structures has been interested among civil engineers. The existing seismic codes like Iranian one do not present the separated response modification factor for regular and irregular structures. In this study, a procedure is presented to estimate the response modification factor of irregular Reinforced Concrete Moment Resisting Frames (RCMRFs). To do so, firstly, several irregular RCMRFs with various types of irregularity are designed based on the behavior factor of Iranian seismic code. Then, the inter-story drifts of these frames subjected to the proportional seismic ground motions are achieved and evaluated by the acceptance criteria. Secondly, the behavior factors of the aforementioned frames resulting from pushover analysis are acquired and the frames are redesigned based on these behavior factors. After that, the inter-story drifts of redesigned frames are again obtained and it is shown that these frames satisfy the admissible restrictions. In addition, two new relations are presented to acquire the behavior factor and the main period of irregular RCMRFs. Six new irregular RCMRFs are designed using the behavior factors achieved via the proposed relations. The behavior factors and the fundamental periods of these frames, which are computed from pushover and modal analyses respectively, are compared with those achieved via the presented relations. This comparison validates the accuracy of the suggested relations.
