عنوان مقاله [English]
Many seismic codes permit a reduction in design loads, taking advantage of the fact that the structures possess significant reserve strength (overstrength) and the capacity to dissipate energy (ductility). Overstrength and ductility are incorporated in the structural design through a force reduction factor or a response modification factor. The factor represents the ratio of the forces that would develop under the specified ground motion if the structure were to behave elastically to the prescribed design forces at the strength limit state. Such a design concept is based on the assumption that well-detailed structures can develop lateral strength in excess of their design strength and sustain large nonlinear deformation without collapse. The role of the force reduction factor is essential in designing the earthquake load-resisting elements. In documents such as ATC-19 and ATC-34 , the behavior factor was calculated as the product of three factors: overstrength factor, ductility factor, and redundancy factor. Many researchers have studied parameter influence on the seismic modification factors of different systems. Since the loading pattern can affect the structure of the hinges of the formed plastic, it can also influence the seismic responses of the structure, such as the behavior factor. In this research, the effect of lateral loading patterns on overstrength, ductility, and the seismic modification factors of moment-resistant frames with medium ductility, is investigated by nonlinear static analysis (pushover) and nonlinear dynamic analysis. Also, the effects of any other parameters influencing the value of the R factor, such as type of consumer sector, are considered, i.e; Iranian (IPE, IPB), American (W)), number of bays and height. The results show that the value variations of nonlinear dynamic analysis generally matched well with those obtained from the pushover analysis. Also, the results show that the value of ductility and the R factor obtained from the frames made for the American sector was more than the Iranian sectors under the same condition. Finally, according to the analysis results, the modified rectangular and the rectangular-triangular loading patterns make the highest R factor for American sectors and Iranian sectors, respectively.