Modal analysis in seismic qualification of nuclear safety related systems and components

Summary form only given. In most cases of seismic qualification of nuclear systems and components, the cutoff frequency or the seismic zero period accelerations (ZPAs) are determined to be at 33 Hz. The residual mass not included with the modes above the cutoff frequency is multiplied by the highest spectral acceleration beyond the cutoff frequency and applied conservatively as the rigid body forces to the structure. For modal-combination purposes, this residual response is considered as an additional mode having a frequency equal to the cutoff frequency or the ZPA frequency, whichever is higher. An alternative method is used at Ellis and Watts Company for qualifying its products using ANSYS finite element analysis software. Using the previously stated approach, two key factors, the master degree of freedom and total frequency modes in ANSYS input data files, determine the accuracy of the modal analysis