Document: NUREG-0800
Document ID: 78905d69-1945-4638-99b9-2db68eb3da77
Document Type: srp
Title: SEISMIC SYSTEM ANALYSIS
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070318.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
properties of the soil profile. For profiles consisting of competent soil or rock, with relatively uniform variation of properties with depth, the control motion should be located at the soil surface at the top of the finished grade. For profiles consisting of one or more thin soil layers overlaying competent material, the control motion should be located at an outcrop (real or hypothetical) at the top of the competent material in the vicinity of the site. Variation of amplitude and frequency content with depth may be considered for partially embedded structures. The spectral amplitude of the acceleration response spectra (horizontal component of motion) in the free field at the foundation depth shall be not less than 60 percent of the corresponding design response spectra at the finished grade in the free field (Reference. 511 ). 38 When variation in soil properties are considered (as required by the Specific Guidelines for SSI Analysis below), the 60 percent limitation may be satisfied using an envelope of the three spectra corresponding to the three soil properties. If the accompanying rotational components of motion are ignored, no reduction is permitted in the horizontal component at the foundation level. Specific Guidelines for SSI Analysis The following specific guidelines are provided here to facilitate the review and draw the attention of reviewers to some important aspects of the SSI analysis. These guidelines are not necessarily requirements for the acceptance of any methodologies or an SSI analysis. The behavior of soil, though recognized to be nonlinear, can often be approximated by linear techniques. Truly nonlinear analysis is not required unless the comparison of results from large-scale tests or actual earthquakes and analytical results indicate deficiencies that cannot be accounted for in any other manner. The nonlinear soil behavior may be accounted for by the following: - Using equivalent linear soil material properties typically determined from