Modeling and Analysis Considerations
NUREG-0800 SRP 3.7.2 provides general outline for the required analysis approach. In general, elastic structural finite element analysis is used for containment analysis. Global models are developed as main tools to perform analysis as needed. Since the analysis also needs to include the soil structure interaction (SSI) effects, it generally requires two separate global analyses, one for static loading (non seismic loads) using commercial software programs like ANSYS and a separate analysis involving SSI using, for example, using SASSI program [31-33]. In order to minimize differences between the two global models, a SSI model is usually developed based on the static model and correlation study for fixed base boundary conditions is performed to ensure that two models have the same global and dynamic characteristics. The SSI analysis is then conducted for a suite of site specific ground motions representing various soil conditions that may be present. The results of the SSI seismic analyses are then combined with analysis of the structure for other non-seismic loading using appropriate load combinations given in design codes.
Nonlinear finite element model may be developed using software program such as BOSOR-5 , for example, to perform buckling analysis. The nonlinear buckling analysis has been further discussed in Section 3.2.4.
A lumped-mass stick model may be used as a valuable tool in the early stages of the seismic analysis. But with the advent of computer technology and finite element methods, three-dimensional (3-D) model utilizing shell or solid elements are now generally carried out to meet the current expectations. Due to the homogeneous material characteristic of steel containments, shell elements are generally adequate for the analysis and design purpose. If solid elements are used, nonlinear through thickness stress may result. In this case, such stresses may be linearized in order to be compared with the code allowables. The concrete basemat which provide anchorage for the steel containment is modeled through several layers of brick or solid elements while the containment may be modeled using shell elements. The containment shell elements need to be appropriately connected to solid basemat elements at the interface.
Besides the global model, local models with geometry details and refined mesh may be developed separately in order to evaluate effects of discontinuities or large penetrations such as an equipment hatch or a personnel access. Boundaries of a local model need to be established at adequate distance from the subject “geometry disturbs” so that disturbing effects on the boundary are negligible. The results from the analysis using the global model are applied as the boundary conditions for the local model.
The detailed finite element analysis results produce a significant amount of element output results that need to be processed for appropriate combinations and subsequent design. Therefore, a robust post-processing approach is required to handle the data in order to automate the process and eliminate human error. But such post processer becomes part of the analysis and design process and needs to be independently verified and appropriately documented.