Performance of eigensolvers in modal analyses of nuclear plant building structures

The modal time history analysis of nuclear plant building structures subjected to dynamic excitations requires the computation of free vibration modes and frequencies. The models are extremely complex and are characterized by a large number of equations, a large half-bandwidth and a large number of modes and frequencies to be computed. Several improved eigensolution routines have been offered on the software market to address this situation. They are determinant search methods, subspace iteration methods with shifting, the Lanczos and the Householder OR methods. The large size of the models as well as the large number of modes and frequencies requires a thorough investigation of the performance characteristics of these eigensolution routines before a choice is made of the most suitable one. While their performance with carefully selected benchmark problems may be impressive their application to building models with unexpectedly clustered free vibration characteristics may pose a number of problems. These include lack of convergence, skipping of modes, exceeding demands on secondary storage capacities, exceeding allotted run times and excessive time the free vibration analysis spends being resident in the computer (wall clock time). Computations are carried out with different eigensolution routines on a number of large size nuclear plant structural models. A number of performance characteristics are recorded to offer some basis of comparison for the user of such algorithms. (orig./HP)