Distributed Vibration Monitoring of Bridges with Fiber Optic Sensing Systems

Distributed fiber optic sensing (DFOS) is a well-established technique for monitoring slow strain changes of structures. However, until recently such methods were not suitable for the dynamic monitoring of bridges due to their low measurement rate of 0.1 Hz or less. New advances like Distributed Acoustic Sensing (DAS) enable the measurement of dynamic strain changes with measurement rates up to several kHz and over long distances of tens of kilometers. Hence, DAS is a very promising method for vibration-based assessment of civil engineering structures such as bridges. This article focuses on a real-world application of DAS for the ambient vibration monitoring of a highway bridge. Two spans of a steel-concrete composite bridge were equipped with a continuous sensing cable. Strain rate changes were measured over several timespans with a DAS interrogation unit. Contrary to conventional point-wise sensors, the DAS measurements enable the determination of the frequency response seamlessly along the entire spans. In order to assess the accuracy of the novel measurement approach, additional acceleration sensors were attached to the steel structure. Furthermore, dynamic displacements were directly observed with a tracking total station. The comparative analysis highlights the potential of DAS for civil structural health monitoring (CSHM).