The seismic linear demand of structures is usually reduced by employing a forcereduction factor inmost of force-based seismic regulations. The current force reduction factors in ASCE/SEI 41-06 standard results in conservative design in the case of foundations when compared to the conventional design regulations e.g. IBC 2000. The aim of the current paper is to evaluate the influence of the Soil-Foundation- Structure-Interaction (SFSI) effects on the force-reduction factor based on ASCE/ SEI 41-06 standard. Therefore, a comparison has been made between the results of the nonlinear response history analysis of a set of 3, 6, 10 and 15-storeyed concrete 2-D frames with the result of the equivalent linear static approach. The results show that the equivalent linear static approach load combinations, in the case of foundations, can lead to conservative designs. Finally, a set of new force-reduction factor has been calculated in order to cope with this problem.
