SUMMARY
The inelastic (design) spectra characterizing seismic hazard at a site are generally obtained by the scalingdown of the elastic (design) spectra via the use of response modification factors. These factors depend significantly on strength reduction factors (SRFs), where SRF represents the ratio of elastic strength demand to the inelastic strength demand of a single-degree-of-freedom oscillator, with the inelastic deformations limited to a specified ductility demand ratio. SRF spectrum gives the variation of this factor with the initial period of the oscillator. This study considers the scaling of SRF spectrum in case of an elasto-plastic oscillator undergoing strength and stiffness degradations. A new model is proposed in terms of the pseudo-spectral acceleration (PSA) values, when normalized to unit peak ground acceleration (PGA), and ductility demand ratio and a ductility supply-related parameter. Least-square estimates of the coefficients are obtained through linear regression analyses of the data for 956 recorded accelerograms in western USA. Parametric studies carried out with the help of the proposed model show that higher earthquake magnitude and/or alluvium site geology may result in higher SRFs for medium- to long-period structures

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