SUMMARYIn spite of important di erences in structural response to near-fault and far-fault ground motions, this paper aims at extending well-known concepts and results, based on elastic and inelastic response spectra for far-fault motions, to near-fault motions. Compared are certain aspects of the response of elastic and inelastic SDF systems to the two types of motions in the context of the acceleration-, velocity-, and displacement-sensitive regions of the response spectrum, leading to the following conclusions. (1) The velocity-sensitive region for near-fault motions is much narrower, and the acceleration-sensitive and displacement-sensitive regions are much wider, compared to far-fault motions; the narrower velocitysensitive region is shifted to longer periods. (2) Although, for the same ductility factor, near-fault ground motions impose a larger strength demand than far-fault motions-both demands expressed as a fraction of their respective elastic demands-the strength reduction factors Ry for the two types of motions are similar over corresponding spectral regions. (3) Similarly, the ratio um=u 0 of deformations of inelastic and elastic systems are similar for the two types of motions over corresponding spectral regions.
The modal pushover analysis (MPA) procedure, which includes the con tributions of all significant modes of vibration, estimates seismic demands much more accurately than current pushover procedures used in structural engineering practice. Outlined in this paper is a modified MPA (MMPA) pro cedure wherein the response contributions of higher vibration modes are computed by assuming the building to be linearly elastic, thus reducing the computational effort. After outlining such a modified procedure, its accuracy is evaluated for a variety of frame buildings and ground motion ensembles. Although it is not necessarily more accurate than the MPA procedure, the MMPA procedure is an attractive alternative for practical application because it leads to a larger estimate of seismic demands, improving the accuracy of the MPA results in some cases (relative to nonlinear response history analy sis) and increasing their conservatism in others. However, such conservatism is unacceptably large for lightly damped systems, with damping significantly less than 5%. Thus the MMPA procedure is not recommended for such sys tems.
SUMMARYThe recently developed modal pushover analysis (MPA) has been shown to be a signiÿcant 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 su cient 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 di erent heights: 3; 6; 9; 12; 15, and 18 stories and ÿve 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 nonlinear 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 are 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 simpliÿed to facilitate its implementation in engineering practice-where the earthquake hazard is usually deÿned in terms of a median (or some other percentile) design spectrum for elastic systems-and the accuracy of this simpliÿed procedure is documented.
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