Dynamic strain monitoring of a highway steel bridge subject to fatigue damage

In the paper, the results of a Structural Health Monitoring system of dynamic strain on a steel highway bridge subject to fatigue damage are presented and discussed. Orthotropic steel decks, directly subjected to traffic loads, are very sensitive to fatigue: in most cases, fatigue defects appear as cracks in the top plates, longitudinal ribs and the cross-beams of the deck. The case study of a 20 years old box girder bridge affected by fatigue problems is here presented: the bridge, located in one of the busiest Italian highways, characterized by high volumes of heavy truck loads, has an orthotropic steel deck with three spans whose total length is 152m. After the cracks started on the longitudinal welding of the ribs, a dynamic identification campaign was carried out using piezoelectric accelerometers, and afterwards a SHM system was installed by positioning 56 resistive strain-gauges along one of the steel decks of the bridge. Operational Modal Analysis was carried out by using not only the accelerometers' dynamic data but also the dynamic strain emerging from the strain gages. An automated system identification algorithm by strain data was developed and compared with conventional techniques. The outcomes of the experimental campaign and SHM here presented are currently being used for assessing the stress redistribution in steel elements due to the crack initiation and propagation with comparison to reference FE models, and subsequently for "observing" in real time the effects of the definitive strengthening intervention which will be applied in a near future.