Document: NUREG-0800
Document ID: c165ebfe-8431-4d56-8bc9-97c2227d45bb
Document Type: srp
Title: SEISMIC SYSTEM ANALYSIS
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0706/ML070640311.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.7.2
CFR Part: 
CFR Title: 

Content:
esign 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. Category I structures that are considered in conjunction with the foundation and its supporting media are defined as "seismic systems." Other Category I SSCs that are not designated as "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, Rm, and the frequency ratio, Rf, govern the results where Rm and Rf are defined as: Total mass of the supported subsystem Rm = S))))))))))))))))))))))))))))) Total mass of the supporting system Fundamental frequency of the supported subsystem Rf = S))))))))))))))))))))))))))))))))))))))) Dominant frequency of the support motion The following criteria are acceptable: i. If Rm < 0.01, decoupling can be done for any Rf. ii. If 0.01 # Rm # 0.1, decoupling can be done if 0.8 $ Rf $ 1.25. iii. If Rm > 0.1, a subsystem model should be included in the primary system model. If the subsystem is rigid compared to the supporting system, and also is rigidly connected to the supporting system, it is sufficient to include only the mass of the subsystem at the support point in the primary system model. On the other hand, in case of a subsystem supported by very flexible connections, e.g., pipe supported by hangers, the subsystem need not be included in the primary model. In most cases, the