The analysis of the natural frequencies and modes of structure is restricted by cost and utter size of the computer analysis. When it has rotationally periodicity, efficient analysis is possible using wave propagation characteristics(Floquet theorem) in periodic structure. They constraint one substructure at its boundary with following substructure in a manner to impose phase difference. In the analysis described here, a finite element modelling is used not a whole structure modelling, but small angle of it. So small size of computer memory is required and rapid convergence of eigenvalue problem is shown. This is demonstrated by computing natural frequencies of free undamped vibrations of annular plates, usual assumption of linear elastic system is made.
