Modal spectra combination method for pushover analysis of special steel moment resisting frames

The nonlinear static procedures (NSPs) proposed by design codes do not lead to reliable results especially for tall buildings. They generally provide inconsistent estimates of inelastic seismic demands, especially for the top floors due to their inabilities in considering the higher modes effects. In this paper, a new enhanced pushover procedure is proposed which is based on the envelope of the structural responses resulting from two separate pushover analyses as a combination rule. Also, the suggested pushover analyses are performed using a newly proposed modal load pattern, i.e., the Modal Spectra Combination (MSC), and the ASCE41-06 required first mode load pattern. The MSC load pattern is consisted of a number of mode shapes combined with appropriate weighting factors that depend on their modal participation factors, modal frequencies and design spectral values. A number of 2-D steel moment resisting frame models with different number of stories are used to investigate the efficiency of the proposed method. The inter-story drifts and the maximum plastic beam moment and curvature responses are used as a measure to compare the results obtained from the nonlinear time-history analyses (NL-THA) and some other NSPs. The results obtained through rigorous nonlinear dynamic analyses show that the application of the proposed method leads to acceptable results for steel MRF systems in comparison to other available enhanced NSPs. The OpenSees program is used for numerical analysis.