Experimental Validation of Damage Indices Based on Complex Modes for Damage Detection in Vibrating Structures

Commonly, the damage detection techniques for structures prone to earthquakes are based on the changes of modal parameters between different structural states, namely before and after a seismic event. Recently, in this context, damage indices based on measures of the imaginary part of complex mode shapes turned out to be an interesting alternative as compared to the direct use of the modal parameters. In fact, numerical simulations proved that the increase in mode shapes complexity comes from the energy dissipated along with damage occurrence. In literature, several indices have been suggested on purpose. However, such indices, for being successful in damage detection, should possess at least two properties with respect to the damage severity: monotony, to guarantee solution uniqueness, and sensitivity, to cope with incipient damage. The present work aims to investigate experimentally the behavior of one of these indices that has been proved the most effective by previous numerical studies. To this end, the results provided by a laboratory physical model subjected to base motion have been used. The damage severity was graded by stepping the amplitude of the base motion so that quasi-linear conditions of the overall structure response can be preserved. The model response was processed via the joint use of the Empirical Mode Decomposition and the Complex Plane Representation methods. The resulting damage index was used to validate the numerical results of previous studies; it was found that numerical and experimental values of the damage index are well comparable.