A seismological method for estimating the long-period transition period T L in the seismic building code

Abstract

Many changes have been made to the design response spectrum used in the ASCE 7 Standard in recent years. One parameter that has not been investigated or revisited since its first appearance in FEMA 450-1/2003 is the long-period transition period parameter, T_L. The long-period transition period parameter was introduced and defined as the corner period that marks the transition from the constant velocity to the constant displacement segments of the design response spectrum. The long-period transition period parameter is primarily important for long-period structures such as high-rise buildings and bridges. The most current estimation of T_L used in engineering design standards is loosely based on a correlation between modal magnitude M_w and T_L that does not account for stress drop Δσ or the crustal velocity in the source region β. This study aims to include both Δσ and β in its estimation of T_L. Modal magnitude is obtained from disaggregation data from the 2018 National Seismic Hazard Model (NSHM) for the conterminous United States (CONUS) and from the 2021 NSHM for Hawaii (HI). The parameter β is determined from previous literature. Then, inversion of ground motion models for Central and Eastern United States (CEUS) is used to determine Δσ for CEUS events, and published information is used to determine Δσ for Western United States (WUS) events and HI events. Then, the definition of the corner period is used to determine T_L. The results yield a generally more conservative (or longer) estimation of T_L than the estimation that is currently used in engineering design standards.