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:
e time history option, procedures used to account for uncertainties (by variation of parameters) and to develop design responses, including justification for the statistical relationship between input design response spectra and output responses. (For example, if the average response spectra generated from the multiple design time histories are used to envelop the design response spectra, then the average responses generated from the multiple analyses are used in design.) 3. Procedures Used for Analytical Modeling A nuclear power plant facility consists of very complex structural systems. To be acceptable, the stiffness, mass, and damping characteristics of the structural systems should be adequately incorporated into the analytical models. Specifically, the following items should be considered in analytical modeling: a. Designation of Systems Versus Subsystems Major Category I structures that are considered in conjunction with the foundation and its supporting media are defined as "seismic systems." Other Category I structures, systems, and componentsSSCs that are not designated as 33 "seismic systems" should be considered as "seismic subsystems." b. Decoupling Criteria for Subsystems It can be shown, in general, that frequencies of systems and subsystems have a negligible effect on the error due to decoupling. It can be shown that the mass ratio, R , and the frequency ratio, R , govern the results where R and R are m f m f defined as: Total mass of the supported subsystem R = S)))))))))))))))))))))))))))))))))))Q m Total mass of the supporting system Fundamental frequency of the supported subsystem R = S))))))))))))))))))))))))))))))))))))))))))))))Q f Dominant frequency of the support motion The following criteria are acceptable: (i) If R < 0.01, decoupling can be done for any R . m f (ii) If 0.01 R 0.1, decoupling can be done if m 0.8 R 1.25. f (iii) If R > 0.1, a subsystem model should be included in the primary system m model. If the subsystem is rigid