Simplified finite element mesh for dynamic analysis of a nuclear reactor building

The design of nuclear facilities requires dynamic analysis to evaluate their performance during an earthquake. In modal analysis, a method recommended for dynamic analysis by the Nuclear Regulatory Commission, the natural frequencies and mode shapes of the structure are commonly calculated using a stiffness matrix computed using the finite element method. A primary goal of this study was to design a finite element mesh, capable of modelling a reactor structure, with a minimum number of nodes and elements. This is necessary since analysis using a mesh fine enough to include all of the structural elements is not feasible because of exorbitant computer time and storage requirements. It was found that for lateral motion, a representative reactor structure could be accurately modelled using a simplified mesh with only one isoparametric quadratic quadrilateral element per floor. In this mesh essentially infinite springs must be placed between the masses at each floor level to prevent internal movements of the lumped masses within the quadratic quadrilateral elements. It was also found that a structure with rigid heavy bottom floors and a light top floor can be subjected to large displacements in the top floor during earthquakes. It was concluded that the properties of that light top floor (especially shear area) controls the value of the fundamental frequency of the building.