Document: NUREG-0800
Document ID: 1f17483f-9659-41c2-9805-b715adfba8eb
Document Type: srp
Title: -
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1319/ML13198A223.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
Revision 4 – September 2013 dead, and other applicable gravity loads (per SRP Section 3.8.5 II.4.D uplift evaluations), the effect of the foundation uplift should be evaluated. The staff reviews the calculation of the ground contact ratio to ensure the linear SSI analysis remains valid. The ground contact ratio is defined as the minimum ratio of the area of the foundation in contact with the soil to the total area of the foundation, computed in each time step throughout the SSI analysis. Uplift for non-symmetric structures may be more affected by the phasing between the three directions of input motions. Therefore, technical justification should be provided if the effect of different phasing of the input motions is not considered in the calculation of the foundation uplift. If the non-symmetric conditions need to be addressed, then the effect of in-phase and out-of-phase input motions can be considered in the SSI analyses by using plus and minus 1.0 times the magnitude of the input motions. Linear SSI analysis methods are acceptable if the ground contact ratio is equal to or greater than 80 percent. The ground contact ratio can be calculated from the linear SSI analysis using the minimum basemat area that remains in compression with the soil. If the ratio is less than 80 percent, then the effect of the nonlinearity due to the foundation uplift should be evaluated. If the uplift effect on structural responses (e.g., in-structure response spectra, member forces, soil bearing pressure, and building displacements, etc.) is found to be significant (e.g., an increase in response of more than 10 percent), then the uplift effect should be accounted for in the seismic design, which is reviewed on a case-by-case basis. The acceptance criteria for the constituent parts of the entire SSI system are summarized as follows: A. Modeling of Structure. The acceptance criteria given under Subsection II.3 of this SRP section are applicable. B. Modeling of Supporting Soil. The effect