DIRECTIONALITY OF STRONG GROUND MOTION DURATIONS

Directionality of ground motions is an important aspect to be considered in seismic design and is incorporated in the Pacific Earthquake Engineering Research (PEER) center’s Next Generation Attenuation Ground Motions (NGA) projects and ASCE7-10. However, the NGA models and ASCE7-10 are limited to ground motion response spectra. Many empirical relationships for strong ground motion durations have been developed, but few of these relationships have considered the directionality of strong ground motion durations. In this study, the directionality of strong ground motion durations is examined using a total of 3177 horizontal motion sets from the PEER Ground Motion Database. As a measure of strong motion duration, significant durations are considered, which are defined as the time interval between 5% to 75% or 95% of Arias intensity. Using the non-linear mixed-effects regression method, the empirical relationships correlating orientation-independent strong motion durations to earthquake magnitude, source-to-site distance, and local site conditions are developed. The orientationindependent durations are defined as the median (50 th percentile), maximum (100 th percentile) and minimum (0 th percentile) durations of a horizontal motion for all non-redundant rotations. For about 65 – 80% of the PEER Ground Motion Database, the ratios of the maximum to minimum significant durations over all orientations range from 1.1 to 1.4. Based on the empirical relationships developed in this study, the ratios generally decrease with increasing magnitude and distance. In comparing the empirical relationships by this study and the previous studies, there was no significant difference between the medians of the 50 th percentile significant durations over all orientations and the medians per the previous studies solely based on the asrecorded orientations.