Factored Modal Combination for Evaluation of Earthquake Load Profiles

To accurately predict the inelastic earthquake response of a structure by pushover analysis, the prescribed load profiles should be consistent with the actual force profiles which develop maximum design values during the time-history response of the structure. In the present study, a new modal combination method, factored modal combination (FMC), was developed to predict the earthquake load profiles of building structures affected by higher dynamic modes. In the FMC, multiple story load profiles are predicted by combining the modal spectrum responses multiplied by the modal combination factors. Based on the results of parametric studies on moment-resisting frames and walls, the modal combination factors were defined according to the hierarchy of each mode affecting the dynamic responses of structures. The FMC was applied to prototype buildings with and without vertical irregularity. The results showed that the FMC predicted the actual story load profiles which developed during the elastic and inelastic time-history responses of the structures.