A nondestructive technique to assess structural health after natural and man-made hazards under development at the University of Arizona is experimentally verified. The procedure is essentially a linear time-domain system identification technique. A structure is represented by finite elements. The procedure tracks changes in the stiffness property of all the elements in a structure. Since the input excitation information is not available in most cases, the proposed technique identifies a structure and thus assesses its health in the absence of such information. Experimental verification is emphasized in this paper. A two-dimensional three-story one-bay steel frame was built in the laboratory. The defect-free frame was identified using computer generated and laboratory measured response information. Several defects with various amount of severity were introduced. Two less severe defects, (i) reducing the cross sectional area over a finite length and (ii) presence of a crack, are specifically addressed in the paper. The frame was represented by three sets of finite element and the location of defect with respect to a node is then studied. The results indicate that the accuracy of the proposed method is much better than the other currently available methods even when the input excitation information was used for identification purposes. The defect identification process improves if a node is selected close to the defect spot. It is established, using both theoretical and laboratory investigations, that the proposed method can be used for structural health assessment after natural and man-made hazards.
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