A method to detect structural damages by the modal perturbation method is presented. Also a practical interpretation of structural stiffness changes and an approximate method to calculate the modal participation factors in the perturbation equation are suggested. It is assumed that only a limited number of natural frequencies and mode shapes at several modes can be measured. To solve the first order perturbation equation, an optimization technique with constraints on each variable is used. Numerical Analysis are carried out on idealized structures with multi-degree of freedom. The numerical results indicate that the structural damages can not always be detected exactly. However, in most cases, the damaged members can be reasonably identified.
