Effect of Missing Mass and Rigid Motion on Seismic Response Spectrum Analysis

Abstract

In using Response Spectrum Analysis (RSA), most standard/codes require more than 90% mass participation. The Regulatory Guide of the United States considers this rule is not conservative and should not be used for nuclear facilities, and the missing-mass method should be used in all RSAs to improve the accuracy of computations. This paper provides the research results on how the structural response is affected by the truncated higher modes and the phase effect due to rigid motion. The characteristics of modal mass participation factor and missing-mass effect are discussed. Case studies are performed to demonstrate the influence of these effects on structural response. Results indicate that the 90% rule is applicable to stick modeling, but not to complex three-dimensional finite element modeling. Applying the missing-mass method with rigid motion effect and proper cut-off frequency shall be considered to ensure the RSA accuracy in design of nuclear facilities.