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:
meter variations are provided in subsection I1.5 of this SRP section. 10. Use of Equivalent Vertical Static Factors The use of equivalent static load factors as vertical response loads for the seismic design of all Category I structures, systems, and components in lieu of the use of a vertical seismic system dynamic analysis is acceptable only if it can be justified that the structure is rigid in the vertical direction. The criterion for rigidity is that the lowest frequency in the vertical direction is more than 33 cps. 11. Methods Used to Account for Torsional Effects An acceptable method of treating the torsional effects in the seismic analysis of Category I structures is to carry out a dynamic analysis that 3.7.2-15 Rev. 2 - August 1989 . . . 4 . incorporates the torsional degrees of freedom. An acceptable alternative, if properly justified, is the use of static factors to account for torsional accelerations in the seismic design of Category I structures in lieu of the use of a combined vertical, horizontal, and torsional system dynamic analysis. To account for accidental torsion, an additional eccentricity of ± 5 percent of the maximum building dimension at the level under consideration shall be assumed for both directions. 12. Comparison of Responses The responses obtained from both response spectrum and time history modal analyses at selected points in typical Category I structures should be compared to demonstrate approximate equivalency between the two methods. 13. Analysis Procedure for Damping Either the composite modal damping approach or the modal synthesis technique can be used to account for element-associated damping. Use of composite modal damping for computing the response of systems with nonclassical modes may lead to unconservative results (Ref. 10). Therefore, the composite modal damping approach is acceptable provided the composite modal damping is limited to 20 percent. One of the other methods mentioned below is generally applicable if the