Abstract
One important aspect of applying seismic isolation to Nuclear Power Plants (NPPs) is that the complex interactions of flexible soil, flexible isolators, and stiff structures require careful investigation. In this paper, a NPP model was used to investigate the effects of SSI and the effects of changing soil and isolator properties on seismic response of an isolated NPP. The following aspects are considered in the study: horizontal excitation and vertical excitation; linear and equivalent-linear models of the isolators; scaling of the shear modulus of the soil profile model; and scaling of the horizontal equivalent stiffness of the isolators. It was found that Pseudospectral Acceleration (PSA) in the nuclear structure at the frequencies near the natural frequency of the structure increase with elevation, and the difference between the in-structure response spectral acceleration with and without SSI effects is concentrated at the frequencies near the natural frequencies of the superstructure. It is also found that the linear SSI analysis underestimates the in-structure response of the nuclear structures compared to the equivalent-linear SSI analyses, and the soil profile properties directly affect the effectiveness of the isolation system.
Highlights
Seismic isolation has been used effectively in a number of critical applications to protect important civil infrastructure [1] and seismically isolated structures have performed as expected in major earthquakes in Japan, the US, and elsewhere [2, 3]
In order to evaluate the effect of changing the shear modulus of soil on the seismic response, the S-wave velocity and P-wave velocity of the original soil profile model were first scaled upwards and downwards; the shear modulus of soil was taken as half or double that of the original modulus, while other portions of the model were unchanged
(1) For the horizontal excitation, a substantial deamplification of the horizontal floor spectra over a broad frequency range was expected in terms of the effectiveness of isolation for Nuclear Power Plants (NPPs)
Summary
Seismic isolation has been used effectively in a number of critical applications to protect important civil infrastructure [1] and seismically isolated structures have performed as expected in major earthquakes in Japan, the US, and elsewhere [2, 3]. Seismic design of isolated buildings is often performed assuming a rigid base and, the effect of SSI is ignored This is considered reasonable by some investigators who show that SSI has some influence on response of the structures this influence is much smaller on an isolated structure than on a nonisolated structure [10, 11]. Song and Ding [12] carried out a finite element study of an isolated 9-story shear wall building They assumed the soil was elastic but discovered that while the SSI had only modest effect on the story drifts, the isolator displacement could be somewhat bigger or smaller than predicted for a rigid base isolated building depending on the characteristics of input motion. SASSI [23, 24] was used to study the effects of SSI and the effects of changing soil and isolator properties on seismic response of an isolated NPP
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