عنوان مقاله [English]
The seismic linear demand of structures is usually reduced by employing a force-reduction factor in most force-based seismic regulations. This approach is simple enough to be widely used by designers. However, calibration of the employed force-reduction factor has a high level of importance, since it is supposed that this factor can transfer the structural linear response to the structural nonlinear response. Current force reduction factors in the ASCE 41-06 standard, which is the latest regulation for seismic rehabilitation programs, results in conservative designs in the case of foundations when compared to design regulations, e.g., IBC 2000. This conservation source comes from the fact that the force reduction factor is limited between one and two in the case of force-controlled members.The aim of the current paper is to evaluate the influence of SFSI effects on the force-reduction factor, based on the ASCE 41-06 standard, focusing on force-controlled members. For this purpose, the beam on the nonlinear inkler oundation approach is used, which is a simple and efficient method. First, a collection of 3, 6, 10 and 15 story concrete moment-resisting frames, founded on soft, medium and hard soil, are designed and analysed for the case of fixed-base and flexible-base assumptions. Eight ground motion records were chosen in order to estimate the median response of the considered frames for a pre-defined seismic scenario. A comparison has been made between the results of the nonlinear response history analysis of each frame, under flexible and fixed base conditions, with the response based on the equivalent linear static approach. The results show that the equivalent linear static approach load ombinations, in the case of foundations, can lead to conservative designs. Finally, by adjusting linear to nonlinear results, a set of new force-reduction factors has been proposed in order to deal with this problem.