Component Mode Synthesis Based SSI Analysis of Complex Structural Systems Using SASSI

Abstract

Three-dimensional seismic soil-structure interaction (SSI) analysis of nuclear power plants (NPP) is often performed in frequency domain using the computer program SASSI [1]. This enables the analyst to properly a) address the effects of wave radiation in an unbounded soil media, b) incorporate strain-compatible soil shear modulus and damping properties and c) specify input motion in the free field using de-convolution method and/or spatially variable ground motions. For large, complex structural systems with multi-million degrees of freedom (DOF) and large foundation impedance matrices associated with deeply embedded foundations, the conventional sub-structuring analysis approach employed in SASSI often results in a coefficient matrix that is too large to solve with currently available computer resources. To address this problem, the method of component mode synthesis (CMS) is employed in the SSI analysis. This involves partitioning the structure into several interconnected components, calculating the reduced-order model of each component, and then assembling the reduced-order component models into a global model of the total SSI system. This technique has been implemented in MTR/SASSI® [2] utilizing the super-element capability. After the component boundary and foundation motions of the synthesized SSI model are determined, these motions are substituted back into the structural component model to compute the response of the structure. This paper presents the formulation of component mode models, and their implementation into the global SSI model. To check out this procedure, an example of seismic SSI analysis of a simplified NPP model on flexible basemat subject to horizontal and vertical excitations is considered. The total SSI system is first analyzed with SASSI using the conventional approach to compute the baseline (target) solution. The structure is then partitioned from the basemat and analyzed separately using the ANSYS® [3] program to compute the component mode properties that are used to form the boundary super-elements. These super-elements are input into the foundation/soil model and analyzed by MTR/SASSI® to calculate the basemat response. The response of the flexible basemat is checked against the baseline solution. In the final step, the foundation basemat response that includes the SSI effects is imposed onto the structural component to calculate the response of the structure. Comparison of the responses show excellent agreement between the baseline solution and those obtained using CMS method.