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
CFR Part: 
CFR Title: 

Content:
be used. The use of the equivalent static load method is also acceptable if the method can be justified. To be acceptable, dynamic analyses should consider the following items: (i) Use of appropriate methods of analysis (e.g., time history, response spectrum, frequency domain) accounting for effects of soil-structure interaction. (ii) Consideration of the torsional, rocking, and translational responses of the structures and their foundations. (iii) Use of an adequate number of masses or degrees of freedom in dynamic modeling to determine the response of all Category I and applicable non- Category I structures and plant equipment. (Caution should be exercised in reducing large static models to fewer degrees of freedom models (Reference. 39 ) for dynamic analysis.) The number is considered 29 adequate when additional degrees of freedom do not result in more than a 10 percent increase in responses. Alternatively, the number of degrees of freedom may be taken equal to twice the number of modes. The adequacy of the number of modes is discussed below. (iv) Investigation of a sufficient number of modes to ensure participation of all significant modes. The criterion for sufficiency is that the inclusion of additional modes does not result in more than a 10 percent increase in responses. Responses associated with high-frequency modes may be 3.7.2-7 DRAFT Rev. 3 - April 1996 important in some cases (where significant modes have frequencies greater than the frequency at which spectral accelerations return to the zero period acceleration; for example, 33 cycles per secondHz in the 30 case of structures, equipment, and componentsSSCs excited directly by 31 Regulatory Guide 1.60 design spectra). Therefore, a demonstration that adequate consideration is given to the high-frequency modes is required. (See Appendix A for acceptable methods to account for high-frequency modes.) (v) Consideration of maximum relative displacements among supports of Category I structures, systems, and