Document: NRC Regulatory Guide
Document ID: db0cf2ba-a5df-49f8-9003-005c7f8bd08a
Document Type: regulatory_guide
Title: Combining Modal Responses and Spatial Components in Seismic Response Analysis (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1814/ML18142B201.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.92
CFR Part: 
CFR Title: 

Content:
tion 1.1 of Revision 3 remain acceptable. Based on recent licensing experience, staff also identified three other technical issues that need further confirmatory research to address issues potentially affecting RG 1.92. They are: (A) The first issue involves the spatial combination of the effects of the three directional (3D) components of an earthquake motion. RG 1.92 Revision 3 provides guidance for the spatial combination of the 3D effects for individual response parameters but does not explicitly include guidance for multiple response parameters, such as column axial force and moment that occur in the same interacting design equations. The lack of such guidance is not an issue when the square-root-sum-of-the-squares (SRSS) method is used for spatial combination. However, it has been an issue in the past licensing reviews when the 100-40-40 rule was used. There are relevant criteria to address this in American Society of Civil Engineers Standard ASCE 4-98, “Seismic Analysis of Safety-Related Nuclear Structures and Commentary,” published in 1999, which is referenced in RG 1.92 since Revision 2. ASCE 4-16 was published in 2017 and is an extensive update of ASCE 4-98, but it remains about the same in the area of spatial combination. However, some believe that one of the ASCE 4 methods that has been widely used for spatial combination is overly conservative and an alternative implementation of the 100-40-40 rule may be acceptable. Therefore, there is a need for confirmatory research to compare the results of the 100-40-40 rule with time history analyses using multiple time histories and develop an explicit regulatory positon in RG 1.92 on spatial combination of multiple interacting response parameters. (B) The current seismic response spectrum analysis (RSA) methods in RG 1.92 Revision 3 were established based on the typical RG 1.60 spectral shape, which is characterized by several structural response regions: periodic, transition from periodic to rigid,