Document: NUREG-0800
Document ID: c3ea80e5-e862-496d-babf-f2404035419d
Document Type: srp
Title: 4.76
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1419/ML14198A460.pdf
Revision Date: 2023-06
Chapter: 3
Section ID: 3.05
CFR Part: 
CFR Title: 

Content:
, and story drift, Δ, are permitted to be taken as the average of the QEi, Ψi, and Δi values determined from the analyses. If fewer than seven ground motions are analyzed, the design member forces, QE, design member inelastic deformations, Ψ, and the design story drift, Δ, shall be taken as the maximum value of the QEi, Ψi, and Δi values determined from the analyses.” The Commentary to IBC 2003 indicates that the code’s earthquake load requirements are based on ASCE 7 and the National Earthquake Hazards Reduction Program’s (NEHRP) Recommended Provisions for the Development of Seismic Regulations for New Buildings (FEMA 368). This FEMA Standard has been superseded by the 2009 edition of FEMA P-750, entitled NEHRP Recommended Seismic Provisions for New Buildings and Other Structures. The Commentary to FEMA P-750 discusses the number of ground motions to be used in a nonlinear time history analysis and the use of the average values or maximum values of deformations in a similar manner to the ASCE 7 Standard. In addition, the Commentary to FEMA P-750 indicates that “…It is very important to note, however, that assessment of deformations in this manner should not be done without careful inspection of the story displacement histories of each analysis. It is possible that the maximum displacement or drift may be completely dominated by the response from one ground motion, and such dominance, when due to ratcheting (increasing deformations in one direction resulting in a high residual deformation), may be a sign of imminent dynamic instability. Where these kinds of dynamic instabilities are present, the analyst should attempt to determine the system characteristics that produce such effects. The ground motion that produces dynamic instability should not be replaced with one that does not.” 5. Deleted reference to Regulatory Guide 1.165 because it was withdrawn. 6. Revised Appendix B to provide detailed guidance for determining target PSD’s for