Power spectral density (PSD) analysis is a widely used method for studying the responses of structures subjected to a random input. It is a statistical measure defined as the limiting root-mean square (RMS) value of a structure’s response. In this analysis approach, the magnitudes of the response can be specified only by probability distribution functions that show the probability of the magnitude taking a particular value. It is assumed that the random input has a zero mean and that its values are Gaussian distributed.
The problem presented here uses PSD analysis to simulate the response of a nuclear island (NI) component of a nuclear power plant (NPP) to a seismic event, taking into account the partial correlation between ground motions occurring at various locations of the basemat. This effect is known as ground motion incoherency (GMI), and is of particular significance in cases where nearby faults generate short-duration, high-frequency waves.
GMI consists of spatial variation of both horizontal and vertical ground motions. The horizontal spatial variation of seismic ground motion is the result of the combination of three phenomena:
Wave-passage effect, which is the difference in the arrival times of seismic waves at different locations.
The incoherence effect, resulting from reflection and refraction of waves through the soil during their propagation, as well as the superposition of waves arriving from an extended source at various locations.
The local effect, due to local soil conditions at each location.
Ground motions in recorded earthquake events exhibit spatial incoherency in high-frequency contents.[1][2] It is therefore important to account for these effects to accurately predict the response of the structure. By considering rock-like soil condition, the spatial variation due to both the wave-passage effect and the incoherency effect can be observed.
The method applied here could also be used to estimate structure-borne sound radiated into buildings such as concert halls when located within the immediate vicinity of railway lines. More generally, the same method can be applied to any simulation involving an incident field and/or wave with known statistical properties (involving frequency and space), such as an automotive windshield or a rocket nozzle.
For more information, see the following topics in the Mechanical APDL Theory Reference: