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:
Spectra Method When the response spectra method is adopted for seismic analysis, the maximum structural responses due to each of the three components of earthquake motion should be combined by taking the square root of the sum of the squares of the maximum codirectional responses caused by each of the three components of earthquake motion at a particular point of the structure or of the mathematical model. b. Time History Analysis Method When the time history analysis method is employed for seismic analy- sis, two types of analysis are generally performed depending on the complexity of the problem: (1) To obtain maximum responses due to each of the three components of the earthquake motion, the method for combining the three-dimensional effects is identical to that described in item 6.a except that the maximum responses are calculated using the time history method instead of the response spectrum method. (2) To obtain time history responses from each of the three com- ponents of the earthquake motion and combine them at each time step algebraically, the maximum response can be obtained from the combined time solution. When this method is used, the components of earth- quake motions specified in the three different directions should be statistically independent. 7. Combination of Modal Responses When the response spectrum method of analysis is used to determine the dynamic response of damped linear systems, in general, the most probable response is obtained as the square root of the sum of the squares of the responses from individual modes. Thus, the most probable system response, R, is given by: N 1/2 R = (£ R2) (1) k = 1 3.7.2-14 Rev. 2 - August 1989 : sX - . | where R is the response for the kth mode and N is the number of significant modes considered in the modal response combination. When modes with closely spaced modal frequencies exist (two modes having frequencies within 10 percent of each other), the methods delineated in Reference 8 are acceptable. Use of