Procedures and results are presented relative to the modal identification of an old masonry building from two sets of experimentally determined frequency response functions. Both tests were performed by exciting the structure with low intensity forces produced by a vibrodyne, before and after high intensity shakes comparable to a destructive earthquake, in order to detect and quantify the damage suffered by the masonry. In spite of nonlinearities appearing in the frequency response functions, a significant modal model is obtained in both cases, provided that a non-proportional damping model, yielding complex modes, is adopted. The decreasing of natural frequencies, in addition to the increasing of damping factors between the two low intensity tests, indicate remarkable structural damage. In order to obtain useful quantities for the identification of a finite element model, which is the most suited for damage localization and quantification, real modes have been extracted by filtering out the damping effects from the complex modes derived by appropriately curve-fitting the frequency response functions.
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