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The Interplay Between the 2018 USGS National Seismic Hazard Model and Forthcoming Updates to U.S. Building Codes

Session: Recent Engineering Uses of National Seismic Hazard Models II

Type: Oral

Date: 4/22/2021

Presentation Time: 05:00 PM Pacific

Description: 

The U.S. Geological Survey (USGS) computes design ground motions by combining hazard results from the USGS National Seismic Hazard Models (NSHMs) with the site-specific design code procedures of the NEHRP Provisions. Prior to the 2020 NEHRP, these design code procedures were applied to the USGS ground motions at three spectral periods (0, 0.2, and 1s) at a reference site class (BC). These procedures included risk-targeted calculations, multiplication by maximum-direction factors, and deterministic capping. To obtain design ground motions for other site classes, multiplication by NEHRP site amplification factors was required. Design ground motions for other spectral periods were then derived from a Two-Period-based Response Spectrum (TPRS) shape. As a result, the updates of the USGS NSHM have traditionally given much attention to the behavior of ground motion models (GMMs) at the three mentioned periods and one site class. However, the more recent USGS models such as the 2014 NSHM also included hazard results at additional periods, which motivated the Multi-Period Response Spectrum (MPRS) design procedures in the 2020 NEHRP Provisions.

For the 2020 NEHRP Provisions, the design code procedures have been updated to use MPRS in order to overcome the recognized shortcomings of the TPRS and site amplification factors. As a result, the site amplification factors are eliminated from the provisions, which now rely on the USGS to provide hazard results for more periods (22) and site classes (8). We discuss how the MPRS affects the selection and modification of GMMs in the 2018 and future updates of USGS NSHMs. The 2018 NSHM update, which is used to compute the design ground motions of the forthcoming U.S. building codes, selected and modified GMMs such that they would be applicable for all periods and site classes of interest, and in some regions considered sedimentary basin effects. Outside of the conterminous U.S., generic spectral shapes are used to estimate MPRS.

Presenting Author: Sanaz Rezaeian

Student Presenter: No

Authors