Document: NUREG-0800
Document ID: c165ebfe-8431-4d56-8bc9-97c2227d45bb
Document Type: srp
Title: SEISMIC SYSTEM ANALYSIS
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0706/ML070640311.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
tion methods. The mesh size should be adequate for representing the static stress distribution under the foundation and transmitting the frequency content of interest. The following limitations should be observed for deep soil sites: - The model depth, generally, should be at least twice the base dimension below the foundation level, which should be verified by parametric studies. - The fundamental frequency of the soil (or backfill) stratum should be well below the structural frequencies of interest. - All structural modes of significance should be included. 3.7.2-14 Revision 3 - March 2007 ! Half Space or Substructure Solution Technique The half space or substructure approach generally comprises the following steps: (1) Determine the motion of the massless foundation, including both translational and rotational components. (2) Determine the foundation stiffness in terms of frequency- dependent impedance functions. (3) Perform SSI analysis. The procedures, modeling assumptions and analytical bases adopted for performing the half space or substructure analysis, including use of frequency- independent soil spring parameters, and the spring and damping coefficients, will be reviewed on a case-by-case basis. ! There are advanced analytical methods that are being considered by the nuclear industry (e.g., the effects of incoherent ground motion) to reduce the potential effects of high frequency ground motion input. These might be used when a site acceptability determination is performed as discussed in subsection II.4 of SRP Section 3.7.1. If incoherency is used to reduce the high frequency response, the potential effects of increasing other responses (e.g., overturning and torsional responses) shall be considered. When approved for use by the NRC, via issuance of interim staff guidance, it should be noted that the effects of incoherent ground motion may be considered either at the Design Certification stage, or at the site-specific application stage, but not both. If