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This paper investigates the use of non-contact laser vibrometers for continuous or periodic bridge monitoring and damage detection through the detection of changes in the bridge's natural frequencies. For this purpose, the vibrations of a full-depth precast-prestressed voided slab bridge and a steel girder bridge were measured under normal traffic using a laser vibrometer. Detailed finite element (FE) models were developed to study the bridge dynamic behavior for various damage scenarios, including longitudinal joint damages in a concrete bridge deck and a full-depth girder fracture in a steel girder bridge. In the concrete bridge deck, longitudinal reflective cracking on the deck surface and leakage had hinted at some level of damage at the joints. The bridge dynamic analysis showed that modal parameters in bridges with precast deck panels are sufficiently sensitive to changes in the state of the panel joints and, therefore, can be used for identifying joint damage. The results clearly confirmed the visible evidence of the damage. For the steel-girder bridge, the results of the dynamic analysis also indicated a noticeable frequency change in the bridge's natural frequencies after a fracture at a level that can be effectively detected with bridge frequency monitoring. Moreover, the proposed approach proved to be more cost-effective and less disruptive than standard monitoring approaches and, at the same time, capable of predicting damages from relatively small vibration amplitudes. This approach can be used to warn of potential damages between planned inspection intervals and to detect damages otherwise hidden from the eye during routine inspections.
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