Inelastic displacement ratio of near-source pulse-like ground motions

Near-source pulse-like records resulting from rupture's directivity have been found to depart from so-called ordinary ground motions in terms of both elastic and inelastic structural seismic demands. In fact, response spectra may be strong if compared with what is expected from common ground motion prediction equations. Moreover, because not all spectral ordinates are affected uniformly, a peculiar spectral shape, with an especially amplified region depending on the pulse period, may follow. Consequently, inelastic seismic demand may show trends different to records not identified as pulse-like (i.e., ordinary). This latter aspect is addressed in the study reported in this short communication, where a relatively large dataset of identified impulsive near-source records is used to derive an analytical-form relationship for the inelastic displacement ratio. It is found that, similar to what was proposed in literature for soft soil sites, a double-opposite-bumps form is required to match the empirical data as a function of the structural period over the pulse period ratio. The relationship builds consistently on previous studies on the topic, yet displays different shape with respect to the most common equations for static structural assessment procedures.