The nuclear industry has been a pioneer in recognizing the importance of Soil-Structure Interaction (SSI) and its effects on the dynamic response of structures and components during an earthquake. Equivalent linear analysis in frequency domain (ELFD) is the current state of practice for performing seismic SSI analysis of nuclear facilities. While the ELFD method produces reliable results for small to moderate intensity ground motions, it may not be suitable to analyze structures under large seismic events. Nonlinear analysis in time domain (NLTD) is more suitable for a wide range of seismic events by allowing explicit modeling of material nonlinearities, including damage and failure, as well as soil-structure interface nonlinearities.

It is recognized that the key to building industry-wide confidence in the time domain approaches is to verify them by making the modeling assumptions equivalent to those of the ELFD approach. In other words, there is a need for a successful equivalent linear analysis in time domain (ELTD). In this study, a typical partially-embedded Pressurized Water Reactor (PWR) nuclear containment building under a seismic event, scaled to various intensities, is analyzed with various approaches. A novel treatment of damping is used for ELTD to achieve nearly frequency-independence and thus allow direct comparison with ELFD. Material and interface nonlinearities are then introduced into the time domain model to perform the NLTD analysis and compare the results to equivalent linear analyses. The time domain response history analyses are performed in LS-DYNA while the ELFD analyses are performed in SC-SASSI using the direct method of substructuring. The study is “blind” in the sense that no post-analysis model iteration was performed to calibrate or refine the response alignment.

It is shown that the obtained seismic responses, presented as in-structure response spectra (ISRS) at different locations within the containment building, are equivalent for both ELTD and ELFD analyses, while the seismic response from NLTD deviates for large seismic events. Linear analyses generally over-predict the response versus nonlinear analysis, but the observed differences are dependent on the frequency range of interest, the site soil and structural properties of the SSI system, as well as the characteristics of the ground motion under study.

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