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:
ion, N is the number of modes included in Step 1, ni is the nth natural mode of the system, and cn is the participation factor given by: 3.7.2-23 Rev. 0 - August 1989 I: toT { n n T [on) [ml 4dn Next, determine the fraction of DOF mass not included in the summation of these modes: ei = di - 6i. where 6.. is the Kronecker delta, which is one if DOF i is in the directilA of the earthquake motion and zero if DOF i is a rotation or not in the direction of the earthquake input motion. If, for any DOF i, the absolute value of this fraction e. exceeds 0.1, one should include the response from higher modes with t~ose included in Step 1. Step 3. Higher modes can be assumed to respond in phase with the ZPA and, thus, with each other; hence, these modes are combined algebraically, which is equivalent to pseudostatic response to the inertial forces from these higher modes excited at the ZPA. The pseudostatic inertial forces associated with the summation of all higher modes for each DOF i are given by: Pi = ZPA x Hi x ei where P. is the force or moment to be applied at DOF i. MNi is the mass or mass moment of inertia associated with DOF i. The structure is then statically analyzed for this set of pseudo- static inertial forces applied to all of the degrees of freedom to determine the maximum responses associated with high-frequency modes not included in Step 1. Step 4. The total combined response to high-frequency modes (Step 3) are combined by the square-root-of-sum-of-squares method with the total combined response from lower-frequency modes (Step 1) to determine the overall structural peak responses. This procedure requires the computation of individual modal responses only for lower-frequency modes (below 33 Hz for the Regulatory Guide 1.60 response spectrum). Thus, the more difficult higher-frequency modes need not be determined. The procedure ensures inclusion of all modes of the structural model and proper representation of DOF masses. An acceptable alternative to this