Document: NUREG-0800
Document ID: 29f5ac9f-4962-4021-913a-6375076e03fd
Document Type: srp
Title: SEISMIC SYSTEM ANALYSIS
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0523/ML052340566.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
e mechanism of wave motion at the site. The foundation motion may be determined by a number of techniques, including: 3.7.2-12 Rev. 2 - August 1989 - Analytic functions - Boundary integral equations - Finite element and difference methods. In calculating the foundation motion by one of these methods, the foundation mat is usually assumed to be rigid and bonded to the soil. However, this is not a necessary assumption because additional degrees of freedom may be specified for the foundation. Again, it must be emphasized that, in general, a translation specified on the surface of the soil produces a translation and rotation of the massless foundation. Stiffness characteristics of the soil, required in Step (2), may also be determined by analytic functions, boundary integral equations, and finite element and difference methods. When calculating the soil stiffness, variations in soil characteristics with excitation level should be accounted for. Typically, the SSI analysis of Step (3) is done using frequency- domain methods. That the frequency dependence of soil impedances be accounted for is believed to be important. For the case where time history analyses are performed using frequency- independent soil spring parameters, the specific values of damping coefficients tend to be unrealistically large. Therefore, the spring and damping coefficients will be reviewed on a case-by-case basis. 5. Development of Floor Response Spectra To be acceptable, the floor response spectra should be developed taking into consideration the three components of the earthquake motion. The individual floor response spectral values for each frequency are obtained for one vertical and two mutually perpendicular horizontal earthquake motions and are combined according to the "square root of the sum of the squares" (SRSS) method to predict the total floor response spectrum for that particular frequency (Ref. 7). If the three components of the motion are applied simultaneously (also see subsection