Securing an adequate seismic margin has been important in safety reviews regarding the seismic design of equipment and piping systems in nuclear power plants, and there exists an increasing need for a more exact method for evaluating seismic margins. To this end, it is reasonable to take into account the reduction of seismic responses resulting from elastoplastic deformation. The authors, therefore, launched a research program to develop an approach to seismic design that uses elastoplastic dynamic analysis for equipment and piping systems. The allowable limit is one of the essential parameters, especially for our approach of using elastoplastic analysis, and was focused on in the program. We studied this approach by utilizing the conventional allowable limit and other potential limits such as the ductility factor. The applicability of the proposed approach was investigated by comparison with the conventional design method. For the investigation, nonlinear time-history analyses producing elastoplastic responses were conducted, and the results were compared with those of the conventional elastic analysis to quantify the response reduction leading to the seismic margin. For the comparison, the authors used three models that simulated a cantilever beam, tank, and core shroud. In this paper, the beam was constructed and applied to the analysis herein. In the next report, the authors will discuss the applicability of the three models. The cantilever beam is the simplest structure among the three models, and it might be useful for obtaining suggestive results from the analysis. The discussion on the beam, therefore, was conducted prior to the other two models, and, in addition, the sensitivity of model parameters such as yielding stress and secant stiffness will be examined in a parametric study using the model. In this paper, we outline the research program and present a scheme for developing the design approach of using elastoplastic analysis. Moreover, calculated analysis results for the cantilever beam are partly reported, and the applicability of the design approach of using elastoplastic analysis is discussed.

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