Equivalent Linear Computation of Response Spectra for Liquefiable Sites: The Spectral Envelope Method

Abstract

AbstractA simplified methodology is proposed for the computation of elastic response spectra at liquefiable sites, based on the widely used equivalent linear (frequency domain) site response analysis method. The actual response spectrum is obtained as the upper-bound envelope of the thus calculated response spectra corresponding to the preliquefaction and the postliquefaction segments of the seismic excitation, the former computed for the natural soil properties and the latter for the properties of the liquefied soil. The time of liquefaction onset, which separates the two seismic excitation segments, is defined in terms of the minimum factor of safety against liquefaction within the liquefiable layer. Additional guidelines are provided for the estimation of the excess pore pressure buildup and the associated soil softening during the preliquefaction segment of the seismic excitation, as well as for the definition of the liquefied soil stiffness and the hysteretic damping ratio. The proposed methodology h...