Vibration-based early warning strategies for prestressing tendon failure in a critical box-girder bridge

This contribution presents the results of a vibration-monitoring campaign implemented on a critical box-girder viaduct affected by the recent failure of unbonded prestressing tendons. The bridge, completed in 2005, features a hybrid prestressing system that combines bonded internal tendons with unbonded external 27-strand tendons. Following the rupture of 4 out of the 12 external tendons in one of the two independent carriageways, a continuous monitoring system was deployed on the undamaged carriageway to support risk mitigation and structural safety management. A network of sixteen high-sensitivity uniaxial accelerometers was installed directly on the external tendons, with the purpose of detecting vibration signatures potentially indicative of strand failure. The paper describes the acquisition strategy, post-processing procedures, and criteria adopted to interpret the measured acceleration data. Particular attention is given to the identification of unexpected increases in vibration levels that may correspond to the sudden failure of individual strands within an external tendon. The monitored data are critically assessed to distinguish between operational or environmental effects and anomalies associated with damage events. On this basis, acceleration thresholds are defined to establish a robust early-warning framework aimed at promptly informing the infrastructure owner of possible prestressing failures. The proposed approach provides a practical tool for structural health monitoring of externally prestressed bridges in a real case study. The results highlight the potential of tendon-mounted accelerometers to support timely maintenance actions and to enhance safety across strategic road networks.