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
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CFR Title: 

Content:
encies 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 an iterative linear analysis of the free-field soil deposit. This accounts for the primary nonlinearity, or - Performing an iterative linear analysis of the coupled soil-structure system. This accounts for the primary and secondary nonlinearities. In the event the nonlinear analysis is chosen, the results of the nonlinear analysis should be judged on the basis of the linear or equivalent linear analysis (Reference. 410 ). 39 Superposition of horizontal and vertical response as determined from separate analyses is acceptable (assuming nonlinear effects are not important) considering the simple material models now available. DRAFT Rev. 3 - April 1996 3.7.2-12 The strain-dependent soil properties (e.g., shear modulus, damping) estimated from analysis of the seismic motion in the free field shall be consistent with the geotechnical information reviewed in SRP Section 2.5.4. Reports on recent earthquakes (e.g., Coalinga) seem to show that there may not be a decrease in shear modulus or increase in damping under high strains. Unless the site is well investigated, the variation in soil properties should be considered by performing SSI analyses using three sets of values (defined in terms of shear moduli and soil hysteretic damping ratio). These three analyses should be performed using the average (or best estimate) value, twice the average value and half the average value of the low strain shear modulus (G defined at max 10 percent peak shear strain). The same shear modulus degradation (G/G ) -4 max and hysteretic damping (D) curves as function of peak shear strain can be used for each of these three analyses. Final values of shear modulus and damping ratio used for each of the analyses are to be compatible with the strain levels expected in the free field consistent with