Higher Mode Effects on Seismic Behavior of MDOF Steel Moment Resisting Frames

In this paper, the effects of higher modes on seismic response of multi degree of freedom (MDOF) steel moment-resisting frames (SMRF) are investigated. Modification factors to the response of first mode SDOF system are presented in order to estimate seismic MDOF system demands. The study is based on spectral analysis and linear and nonlinear dynamic time history analysis of 4, 10, 15, 20 and 25 storey SMRFs under Elcentro, Tabas, Naghan and Manjil earthquake loading. A modification factor is defined in order to estimate roof elastic displacement demands of an MDOF frame from first mode elastic displacement spectra. Base shear modification factor (to compute MDOF strength reduction factors), maximum story drift demand modification factor , and maximum story dynamic ductility demand modification factor are defined and presented for SDOF system responses in order to estimate the main MDOF system nonlinear seismic responses, including higher mode effects.