FREQUENCY RESPONSE FUNCTION VERSUS OUTPUT-ONLY MODAL TESTING IDENTIFICATION

The identification of modal parameters from frequency and time domains have now reached a high degree of maturity. Several issues have been considered on their for and against, on the possibility of using ambient excitation for large (civil) structures, on the difficulty of identifying scaled modes from output-only data, on the quality of identified parameters when using data derived from either domain. However, a strict comparison between these two techniques was not completely performed, especially when one has the choice of using both of them, as it is typical when testing lab structures. Moreover the identification of coincident modes is a task that cannot be fulfilled by SIMO methods: actually only polyreference or MIMO methods are recognized of this possibility and it is important to state whether only-output tests can provide some information on the subject. In this paper a comparison between FRF and output-only techniques, when applied to a lab structure, is performed: scaled modes are provided for both methods and an estimate on the quality of results assessed.