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# Reevaluation of Regulatory Guidance on Modal Response Combination Methods for Seismic Response Spectrum Analysis (NUREG/CR-6645, BNL-NUREG-52576)

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## Publication Information

**Manuscript Completed:** September 1999**Date Published:** December 1999

**Prepared by:** R. Morante, Y. Wang

Brookhaven National Laboratory Upton, NY 11973-5000

R. M. Kenneally, NRC Project Manager

**Prepared for:** Division of Engineering Technology Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington DC 20555-0001 NRC Job Code W6521

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## Abstract

Regulatory Guide 1.92, "Combining Modal Responses and Spatial Components in Seismic Response Analysis," was last revised in 1976. The objectives of this project were to re-evaluate the current regulatory guidance for combining modal responses in response spectrum analysis; evaluate recent technical developments; and recommend revisions to the regulatory guidance. In addition, Standard Review Plan Section 3.7.2, "Seismic System Analysis," was reviewed to identify related sections which may need to be revised. The objectives were addressed through a literature review of past studies, supplemented by analysis of a piping system model previously utilized in NUREG/CR5627, "Alternate Modal Combination Methods in Response Spectrum Analysis."

This project evaluated (1) methods for separation of the in-phase and out-of-phase modal response components; (2) methods for combination of the out-of-phase modal response components; (3) the contribution of"missing mass"; and (4) the combination of the three elements of response to produce the total response. Numerical results from response spectrum analyses were compared to corresponding time history analysis results to assess the accuracy of the various combination methods tested.

During the course of the project, several insights relating to potential improvements in the methodology for seismic analysis were identified and documented. These include (1) improvements in correlation between mode superposition time history and direct integration time history; (2) use of response spectrum generation single degree of freedom oscillator responses to define the frequency above which modal responses are in-phase with the input time history; and (3) evaluation of the effects of potential differences in mass distribution used in static and dynamic analyses of a piping system.

The conclusions of the project are (1) several of the modal response combination methods evaluated should be included as acceptable methods in the next revision to Regulatory Guide 1.92; (2) deletion of several of the modal response combination methods currently in the Regulatory Guide should be considered; (3) the Regulatory Guide should be revised to specifically define acceptable procedures for constructing the total response spectrum analysis solution; and (4) several variations of modal response combination methods, which were examined to try to improve accuracy, showed promising results and warrant further study.

This project did not specifically analyze the modal response combination methods applied to highly flexible, low frequency systems. A separate evaluation for these systems should be considered.

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