Most of the diagnostic techniques via modal analysis are based on an optimality criterion where the stiffness distribution of a chosen reference configuration of the structural system is updated so that the first few natural frequencies closely match with the measured ones at a certain level of deterioration. Despite the common use of these techniques in damage identification, basic questions such as how accurate the reference configuration has to be or which a priori hypotheses are needed to get around the not well-posedness of the diagnostic problem are unclear and still open. With an eye to these questions and to practical use of vibration monitoring on field measurements, in this paper a diagnostic strategy for identifying a localized damage in a multi-storey steel frame is discussed. It was found that the dynamic characterization of the frame at a certain stage of its service is indispensable to defining an accurate analytical model of the structure, viz. to defining a reference configuration of the frame which is useful for identifying possible incremental damages. Moreover, some working hypotheses commonly used in structural diagnostics were found to be crucial in reducing the indeterminacy of the identification problem. The obtained results essentially confirm a more careful resolution on the localization of the damage as its severeness increases.
