Damage Identification and Localisation Using Changes in Modal Parameters for Civil Engineering Structures

In this paper dynamic testing of some civil engineering structures is presented. The investigated objects are prestressed and non-prestressed industrially produced concrete slab elements under laboratory conditions, as well as an in situ prestressed concrete bridge. In order to achieve damage assessment, different analyses based on modal parameters changes are done. To this end, the structures are artificially damaged by cutting locally their passive or prestressed steel reinforcement. Moreover, to cause cracks, the structures are loaded with a varying experimental mass. For modal analysis all objects are excited sinusoidally by an electrodynamic shaker in the laboratory and by special exciters in the case of the bridge. In this article, a set of modal parameters, i.e. eigenfrequencies and modes shapes are determined and compared according the damage states. It results that the shift of the eigenfrequencies is a valuable damage indicator whereas the modeshapes variations are small and more difficult to interpret. Further, the modal parameters are used to calculate the flexibility, i.e. the inverse stiffness matrix and their changes revealing clear differences according to the damage at the correct locations. In addition, the mode shape slope and curvature, as well as the bending and strain fractional energy method for the bridge are presented, indicating the loss of stiffness and hence the damage.