Inelastic Response to Site-Modified Ground Motions

A building with a period equal to that of a site may be more susceptible to yielding during a moderate earthquake, but the larger yielding during a major earthquake is much the same as for a building having a different period. This conclusion results from analyzing one-degree-of-freedom, elastoplastic structures using ground motions (both real and calculated) whose elastic response spectra have peaks attributable to site conditions. Inelastic response spectra for site-modified motions do not show pronounced peaks at the period of the site; rather, they are as smooth as inelastic spectra computed from motions unaffected by site conditions. Inelastic spectra for design may be based upon the same ratios of spectral acceleration to peak acceleration and spectral velocity to peak velocity as for normal motions. Thus, the amount by which a site modifies peak acceleration and peak velocity is important, and the period of a site is not significant by itself.