Q: How does MOSES do modal analysis?
REV 6.02

A: MOSES actually does not perform modal analysis. MOSES does extract modes and their corresponding natural frequencies. To perform this task, the method of subspace iteration is used as outlined in Bathe's book, Finite Element Procedures in Engineering Analysis (ISBN 0-13-317305-4). This is a robust method which deals with the complete stiffness and mass matrices and does not require a "Guyan Reduction" for efficiency. With most other programs, the modes are the solution; in MOSES, they are the beginning. Since the mass and stiffness matrices will normally change as a simulation progresses, MOSES uses the modes as additional degrees of freedom during a simulation. In this way, the natural frequencies for the current situation are used instead of those at some nominal position.