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:
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, transition from rigid to max acceleration, and max acceleration. Different combination rules were developed to combine periodic modes and rigid modes. In other words, the methods in RG 1.92 Revision 3 rely on the existence of a zero-period-acceleration (ZPA) frequency in the input response spectrum to identify in-phase modes. However, the hard-rock high frequency (HRHF) response spectra evaluated in recent license applications often do not exhibit a clear ZPA frequency. Therefore, whether this structural response characterization can be extended to the HRHF spectral shapes should be investigated by benchmarking RSA against time history analyses using the HRHF input, in order to develop appropriate technical positions. (C) There is a need for a more robust benchmark of the RSA methods than that documented in NUREG/CR-6645. Based on the random vibration theory, the RSA methods produce the average peak responses of a structure subjected to an input response spectrum, which is a key representation of the stochastic input motion. NUREG/CR-6645 compared the RSA methods to the time history methods using only one input time history. One time history is only one realization of the underlying stochastic process, and does not always provide sufficient information on the effects of the stochastic process. Therefore, a more rigorous benchmark of the RSA methods is needed to use multiple input time histories and to consider varied fundamental frequencies of the structure relative to the input response spectrum. This will require confirmatory research to address the issue. The resultant insights from this benchmark will inform the need to update certain aspects of the regulatory positions in RG 1.92. 2. What is the impact on internal and external stakeholders of not updating the RG