Although the newly developed performance-based seismic analysis approaches made significant contribution to improve the prediction of structural damages associated with a given level of earthquake, however, none of the current invariant force distributions employed in these approaches considers the effect of foundation flexibility. For a structure supporting on flexible foundation, it was understood the structural response could be beneficially or detrimentally affected by a phenomenon referred as Soil-Structure-Interaction. This effect has been taken into account in pushover analysis of SDOF systems supporting on flexible foundations, but MDOF structures are less investigated. The modal pushover analysis method (MPA) is generally recognized as a useful method for estimating the dynamic response of Key Words : soil structure interaction, complex modal pushover, force distribution pattern.
