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A Framework for Incorporating Epistemic Uncertainty in Site Effects in National Building Codes

Description: 

Probabilistic seismic hazard analysis (PSHA) is the basis for the development of the National Seismic Hazard Maps (NSHM) produced by the USGS. These maps are adopted by building codes for the design of structures. This research intends to provide a framework by which the NSHM can be postulated in terms of a new paradigm: partially non-ergodic PSHA. Within this approach, PSHA analyses are conducted using the so-called single-station sigma, which is the aleatory variability that results after accounting for repeatable site effects at every station. In recent years, seismic hazard analyses that have been conducted for critical facilities, such as Nuclear Power Plants, have been performed within the framework of partially non-ergodic PSHA; the proposed research aims to introduce these advancements into the NSHM and national building codes. The proposed framework for implementing non-ergodic PSHA in building codes builds on the current framework by introducing a correction factor called ‘ergodicity factor’ that accounts for the inherent ergodic assumptions in PSHA. These factors are postulated as a function of the level of knowledge on site properties reflecting the fact that uncertainty in site effects is considered as epistemic in partially non-ergodic PSHA. The ergodicity factors are computed for NEHRP site classes for varying levels of site characterization. It was observed that the factors increase with increase in uncertainty on the estimates of shear wave velocity at a site, indicating increase in design motions with increase in epistemic uncertainty. Given that lower uncertainty leads to lower design values, the proposed framework incentivizes practitioners for investing more in site characterization in expectation of lower design seismic loads. The resulting reduction in hazard, however, is relatively minor.

Session: USGS National Seismic Hazard Models: 2023 and Beyond

Type: Oral

Date: 4/18/2023

Presentation Time: 02:30 PM (local time)

Presenting Author: Balakumar Anbazhagan

Student Presenter: Yes

Invited Presentation: 

Authors