Effect of frame irregularity on accuracy of modal equivalent nonlinear static seismic analysis

Abstract

This study investigates the effect of frame set-back irregularity in height on accuracy of Modal Pushover Analysis (MPA) for predicting target displacement, story drifts, and base shear. In order to have a thorough investigation of frame irregularity, 21 irregularity layouts are considered for a 5-story steel moment resisting frame. Each irregular frame is designed to represent low and high values of response reduction factor (R). For each frame model, nonlinear dynamic analyses for 14 ground motions (7 pairs) and nonlinear static pushover analysis up to the MPA-predicted target displacement are performed using computer program IDARC-2D with fiber beam - column elements. The results of nonlinear static analyses are compared with the results for nonlinear dynamic analyses and FEMA440 modified coefficient method. Correlations of predicted measures with respected to results from nonlinear dynamic analyses are computed for all models and also each model. For Irregular frames, MPA is inconservative for estimating median of dynamic responses for selected ground motions though error quantity depends on models R value and kind of response. MPA estimated results have good correlation with respect to FEMA in displacement and drift results for all models under all ground motions are considered.