Abstract
Dynamic site response is usually implemented using an equivalent-linear approach through analyses based on time-histories or random vibration theory (RVT). In the RVT approach the input motion is characterized in the frequency domain by means of Fourier Amplitude Spectra (FAS) or power spectral densities so that the need for selecting/developing multiple suitable time-histories is avoided. Nevertheless, past studies have found that RVT may produce results that differ significantly from empirically determined site amplification functions and from the time history approach. This work is aimed to further understand the potential differences in the results from RVT and time-histories based approaches by performing a comprehensive numerical evaluation that takes into account the effect of the input intensity level, input spectral shape, site conditions, and the methodology used to produce the input FAS. The results obtained corroborate that RVT over-predictions occur mainly at the site fundamental frequencies and are larger for relatively soft soil deposits with significant impedance contrast at the soil/rock interface. However, in the soft site evaluated the magnitude of the over-prediction was rather insensitive to the increase in the inelastic demand, conversely, the over-prediction in the stiffer site increased as the site softened due to the rising inelastic demand.
Highlights
In the seismic design and/or assessment of many critical structures, like nuclear power plants (NPPs), the seismic input is defined from a probabilistic seismic hazard analysis (PSHA) as a uniform hazard response spectrum (UHRS) at the rock outcrop
When using the random vibration theory (RVT) method, this increase in duration is not taken into account, only the input ground motion duration is used
Over the last years, the nuclear industry has been switching to the RVT-based approach to equivalent linear (EQL) site response analysis as an alternative to the time-histories method
Summary
In the seismic design and/or assessment of many critical structures, like nuclear power plants (NPPs), the seismic input is defined from a probabilistic seismic hazard analysis (PSHA) as a uniform hazard response spectrum (UHRS) at the rock outcrop. The dynamic site response analysis is usually implemented using an equivalent-linear (EQL) approach through methodologies based either on time-histories or, more recently, random vibration theory (RVT). Both approaches are currently accepted by the US Nuclear Regulatory Commission The RVT approach can be used to avoid the selection, scaling and matching of time-histories input motions. This approach uses a single input motion defined in the frequency domain as either a Fourier Amplitude Spectrum (FAS) or a Power Spectral Density (PSD) compatible with the design spectrum. It has been noticed that sites with low fundamental frequency and settled on hard rock produce the largest over-predictions [3, 4]
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