Monitoring of orthotropic steel decks for experimental evaluation of residual fatigue life

Orthotropic steel decks, directly subjected to traffic loads, are very sensitive to fatigue: defects mainly appear as cracks, which affect the top plates, longitudinal ribs and cross-beams of the deck. In this kind of bridges, structural health monitoring can be effectively adopted in characterising the safety threshold of the ongoing fatigue damage. This paper estimates the residual fatigue life of an existing orthotropic box-girder bridge, using experimental data from long-term monitoring of real traffic-induced effects. The bridge in question, less than 25 years old, is located on one of Italy’s busiest toll highways and evidenced fatigue damage due to deficiencies in original design and detailing. After preliminary visual inspections, the dynamic identification of the bridge was investigated by operational modal analysis, and a long-term monitoring campaign of strain measurements was subsequently carried out, involving series of strain gauges located on the most critical bridge sections. According to stress cycles recorded at these points, unlimited life verification was carried out according to EN 1993-1-9, and estimates of the residual fatigue life of the various elements were then derived by extrapolation of the experimental data. The proposed method can effectively be applied to fatigued steel bridge decks so that residual fatigue life and the real urgency of retrofitting interventions can be evaluated in operational conditions.