Evaluation of modal pushover analysis using generic frames

The recently developed modal pushover analysis (MPA) has been shown to be a significant improvement over the pushover analysis procedures currently used in structural engineering practice. None of the current invariant force distributions accounts for the contribution of higher modes-higher than the fundamental mode-to the response or for redistribution of inertial forces because of structural yielding. By including the contributions of a sufficient number of modes of vibration (generally two to three), the height-wise distribution of responses estimated by MPA is generally similar to the 'exact' results from non-linear response history analysis (RHA). Although the results of the previous research were extremely promising, only a few buildings were evaluated. The results presented below evaluate the accuracy of MPA for a wide range of buildings and ground motion ensembles. The selected structures are idealized frames of six different heights: 3, 6, 9,12,15, and 18 stories and five strength levels corresponding to SDF-system ductility factor of 1, 1.5, 2, 4, and 6; each frame is analysed for 20 ground motions. Comparing the median values of storey-drift demands determined by MPA to those obtained from non-linear RHA shows that the MPA predicts reasonably well the changing height-wise variation of demand with building height and SDF-system ductility factor. Median and dispersion values of the ratios of storey-drift demands determined by MPA and non-linear-RHA procedures were computed to measure the bias and dispersion of MPA estimates with the following results: (1) the bias and dispersion in the MPA procedure tend to increase for longer-period frames and larger SDF-system ductility factors (although these trends arc not perfect); (2) the bias and dispersion in MPA estimates of seismic demands for inelastic frames are usually larger than for elastic systems; (3) the well-known response spectrum analysis (RSA), which is equivalent to the MPA for elastic systems, consistently underestimates the response of elastic structures, e.g. up to 18% in the upper-storey drifts of 18-storey frames. Finally, the MPA procedure is simplified to facilitate its implementation in engineering practice-where the earthquake hazard is usually defined in terms of a median (or some other percentile) design spectrum for elastic systems-and the accuracy of this simplified procedure is documented. Copyright (C) 2003 John Wiley Sons, Ltd.