This study investigates the influence of masonry infill walls (MIWs) on the response modification factor of moment-resisting reinforced concrete frames (RCFs). To this end, several moment-resisting RCFs with a wide range of stories and bays are considered, and their base-shear versus roof-displacement curves are derived using incremental dynamic analysis. The frames are examined first with sixteen earthquake excitations, including the impact of masonry infill walls, and then without this effect. Four classes of clay brick masonry properties are utilized to cover a diverse range of masonry walls. Two cases are examined for infilled frames: in the first, all bays are infilled, and in the second, some bays in each story are infilled. The response modification factors of these frames are subsequently compared. The results indicate that MIWs significantly increase the secant stiffness and the strength of moment-resisting RCFs. The response modification factors of the infilled moment-resisting RCFs are found to be higher than those of bare frames. In addition, for most RCFs, the values of the response modification factor achieved for infilled moment-resisting RCFs are greater than the Iranian seismic code’s prescribed value, whereas, for some bare frames, the code’s recommended value is not met. Furthermore, several relations are developed to predict the response modification factors of infilled moment-resisting RCFs based on the bare frames’ fundamental period and response modification factor. Four new frames subjected to seismic ground motions distinct from the primary earthquake excitations used to derive these relations and an experimental case are used to confirm their validity. Moreover, several graphs for the response modification factor of infilled frames with various clay brick masonry walls were developed. Using these graphs, building designers can determine the prism strength of MIWs required to meet the seismic code’s prescribed value based on the fundamental
