Ground motion model for median directional inelastic spectral displacements

Abstract

Peak inelastic displacement of single-degree-of-freedom (SDOF) systems has been shown to be an efficient ground motion (GM) intensity measure (IM) for estimating the structural demand. Most ground motion models (GMMs) are developed to estimate the arbitrary or geometric mean of the two horizontal components of the response spectra obtained from the two orthogonal as-recorded directions of ground motion. Meanwhile, more recent GMMs have opted in estimating orientation-independent horizontal component definitions. This horizontal component definition has the advantage of removing the sensor orientation as a contributor to the estimated ground motion uncertainty. This is the definition adopted in this study, where the median directional inelastic spectral displacements were considered as an IM. To do this, a set of bilinear SDOF systems and a subset of the NGA-West2 database were used to fit a GMM to estimate the medians and dispersions of Sdi,RotD50 from shallow crustal earthquakes. These were calculated by rotating the ground motions through all non-redundant rotation angles in the horizontal plane. A set of functional forms was developed using the mixed-effects regression approach, with the predictor variables being the elastic vibration period, T, the force reduction factor, R, and a set of ground motion causal parameters. The results show how the model is able to predict the data quite well, while keeping the dispersions at a reasonable level.