Seismic Response Analysis and In-Structure Floor Spectra (Task 5)
Task Overview
Objective
The objective of this task is to derive the best-estimate (or median-centered) seismic responses and their variability in the form of structural loads or floor response spectra. The loads and floor response spectra define the demand for which SSCs are evaluated.
Purpose
Derivation of these best-estimate loads and floor response spectra and their variabilities are obtained through simulation probabilistic response analysis, by new deterministic analysis with estimated variability, or by scaling of the SSE responses and assigning variability.
Guidance
The best-estimate seismic demand is determined at the mounting point of each item on the SEL to support fragility calculations.
Seismic demands are calculated by analyzing the dynamic response of structures and equipment to seismic input. Typically, seismic structure response analysis is first conducted using the input ground motion. Then, structures are evaluated using structure forces and displacements. The equipment and systems within the structures are evaluated using in-structure seismic response as input. Depending on the fragility approach, either realistic or somewhat conservative seismic demands can be developed from the structure and equipment response analyses.
For the separation of variabilities (SOV) fragility approach, median (realistic) seismic demands are calculated using median centered models and analysis parameters. For the hybrid fragility approach, the response factors and variabilities are used to inform the development of 84% non-exceedance probability (NEP) responses.
To account for conservatism or non-conservatism in the dynamic analysis methods, structure and equipment response factors are applied to the fragility as lognormally distributed random variables. The median values of these random variables are used to calculate the median ground acceleration capacity. Variabilities in the response factors are propagated through the seismic capacity analysis to calculate logarithmic standard deviations for fragilities.
The applicable variables used to generate floor spectra are as follows:
- Spectral shape
- Ground motion incoherence
- Damping
- Modeling
- Mode combination (if mode superposition time history is used)
- Soil–structure interaction, including reduction with depth of seismic input
Supplemental Guidance
Related Element of ASME/PRA Standard
Part 5, Seismic Fragility Analysis (SFR)
EPRI Guidance
Seismic Fragility and Seismic Margin Guidance for Seismic Probabilistic Risk Assessments (3002012994) provides extensive guidance on conducting seismic response analysis.
Other Guidance
(none noted)