The characterization of strength of earthquake demands for seismic analysis or design requires the specification of a level of intensity. Numerous ground motion intensity indices that have been proposed over the years are being used for normalizing or scaling earthquake records regardless of their efficiency. An essential point of this study is that a ground motion index is appropriate, or efficient, as long as it can predict the level of structural response. This study presents correlations between 23 ground motion intensity indices and four response variables: elastic and inelastic deformation demands, and input energy and hysteretic energy; nonlinear responses are computed using elastoplastic, bilinear, and bilinear with stiffness degradation models. As expected, no index is found to be satisfactory over the entire frequency range. Indeed, indices related to ground acceleration rank better in the acceleration-sensitive region of the spectrum; indices based on ground velocity are better in the velocity-sensitive region and, correspondingly, generally occur in the displacement-controlled region. Despite frequent criticism, the peak ground motion parameters passed the test successfully. A ranking of indices is presented, thus providing a choice of the most appropriate one for a particular application in the frequency range of interest.

SUMMARYPerformance-based seismic design requires reliable methods to predict earthquake demands on structures, and particularly inelastic deformations, to ensure that speciÿc damage-based criteria are met. Several methods based on the response of equivalent linear single-degree-of-freedom (SDOF) systems have been proposed to estimate the response of multi-degree-of-freedom structures. These methods do not o er advantages over the traditional Veletsos-Newmark-Hall (VNH) procedure, indeed, they have been shown to be inaccurate. In this study, the VNH method is revised, considering the inelastic response of elastoplastic, bilinear, and sti ness-degrading systems with 5% damping subjected to two sets of earthquake ground motions. One is an ensemble of 51 earthquake records in the Circumpaciÿc Belt, and the other is a group of 44 records in California. A statistical analysis of the response data provides factors for constructing VNH inelastic spectra. Such factors show that the 'equal-displacement' and 'equal-energy' rules to relate elastic and inelastic responses are unconservative for high ductilities in the acceleration-and velocity-sensitive regions of the spectrum. It is also shown that, on average, the e ect of the type of force-deformation relationship of non-linear systems is not signiÿcant, and responses can be conservatively predicted using the simple elastoplastic model.

SUMMARYThe inelastic response of single-degree-of-freedom (SDOF) systems subjected to earthquake motions is studied and a method to derive hysteretic energy dissipation spectra is proposed. The amount of energy dissipated through inelastic deformation combined with other response parameters allow the estimation of the required deformation capacity to avoid collapse for a given design earthquake. In the ÿrst part of the study, a detailed analysis of correlation between energy and ground motion intensity indices is carried out to identify the indices to be used as scaling parameters and base line of the energy dissipation spectrum. The response of elastoplastic, bilinear, and sti ness degrading systems with 5 per cent damping, subjected to a world-wide ensemble of 52 earthquake records is considered. The statistical analysis of the response data provides the factors for constructing the energy dissipation spectrum as well as the Newmark-Hall inelastic spectra. The combination of these spectra allows the estimation of the ultimate deformation capacity required to survive the design earthquake, capacity that can also be presented in spectral form as an example shows.

Most recent seismic codes include response modification factors in the definition of the equivalent lateral forces that are used for the design of earthquake resistant buildings. The response modification factors (R) are used to reduce the linear elastic design spectrum to account for the energy dissipation capacity of the structure. The evaluation of these response modification factors for various sets of earthquake records and ductility factors is presented herein. Special attention is given to the short period range where the reduction of linear elastic response spectra is smaller than the values for intermediate and long period structures. An idealized and simple variation of the response modification factor as a function of the period of vibration, suitable for seismic codes formulation, is also presented.

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