Determination of Fourier expansion number for elasto-plastic analysis of piping systems using flexibility factor

The finite element analysis (FEA) model of piping systems is commonly used to evaluate structural integrity under seismic conditions. However, after the Great East Japan Earthquake, it became necessary to confirm the strain response under the beyond design basis earthquake (BDBE) condition. In general, solid and shell elements are used to confirm the strain value; however, unlike in the case of the conventional FEA model for confirming elastic response, using these elements can incur additional modeling costs. In this study, an elasto-plastic analysis using a beam element-based model was primarily investigated. Furthermore, to accurately calculate the strain response, the elbow element was used, and the appropriate degree of freedom of sectional deformation was determined using the Fourier expansion number. To determine the appropriate Fourier expansion number in turn, a determination method was used by introducing the flexibility factor, and this was validated by applying the derived value to the actual nuclear power plant surgeline piping system FEA model. As a result, it was concluded that the appropriate Fourier expansion number determined using the flexibility factor is effective in terms of conservatism and accuracy.