A combined method of the Lanczos process and a substructure analysis technique for calculating natural frequencies and mode shapes of large structural systems is developed. The method does not require generation and storage of stiffness and mass matrices of the entire structure. That is, the method uses only the stiffness and mass matrices of each substructure. No approximating assumptions other than the usual assumption of linear elastic system modelled by finite elements, are made. Thus, natural frequencies and mode shapes for the finite element model employed are the same with or without the substructuring algorithm. This is demonstrated by computing first ten natural frequencies and the corresponding mode shapes for an open truss helicopter tail-boom structure.
