Diagnostic studies of steel structures through vibrational signature analysis

This dissertation is concerned with diagnostic studies of steel structures using modal analysis. First, a model structure is built under a US-China joint research project on structural dynamic research. The dynamic properties of the model are obtained by experimental system identification. Secondly, a damage diagnosis theory applicable to civil engineering structures is presented. It is found that a new vibrational signature can be sensitive to the presence of structural damage, providing a means of damage detection.; A 0.4 scale model of a prototype single-bay and five-story high steel building is designed and manufactured to simulate the dynamic properties of the prototype. Component tests, such as coupon tests and residual stress tests, are used to determine the material and member properties of the model. Dynamic structural tests for system identification are conducted to investigate the overall structural behavior of the model. The results show that the relationship between the prototype and model has been established satisfactorily. Experimental techniques in modal analysis to determine vibrational signatures of the structure are also described.; A new vibrational signature called Strain Mode Shape (SMS) is obtained from the system identification results and presented for damage diagnosis. The theory and physical meaning of SMS is mathematically derived and explained. Two experiments are designed to verify the sensitivity of the proposed diagnostic approach. One is tested on the five-story model, the other is on a simply supported beam. On the former, several types of geometric damage such as reduction in the member cross section and loosening of bolts are introduced. New structural signatures, SMS 's, experimentally obtained for different degrees of structural integrity are compared to the baseline values. The changes of SMS reflects the magnitude of force redistribution and can indicate the location of damage. On the latter, a bridge component with fatigue cracks is simulated. The SMS change obtained shows the stress concentration effect and flexural strength reduction of a beam with flange cracks, although it fails to detect effects of web cracks.