Fiber Bragg Grating–Differential Settlement Measurement System for Bridge Displacement Monitoring: Case Study

Vertical displacements are one of the crucial parameters defining, for example, the load-carrying capacity of a bridge deck in short- and long-term monitoring. Bridge managers are always looking for an easy way to measure vertical displacements of bridges. However, such measurements are difficult to perform. With the advancement of fiber-optic technologies, fiber Bragg grating (FBG) sensors are more commonly used in structural health monitoring due to their outstanding advantages, including multiplexing capability over long distances as well as high resolution and accuracy. In this study, a set of FBG-differential settlement measurement (DSM) sensors, connected by a hydrostatic leveling system of communicating vessels, and previously adopted for displacement monitoring along a steel member, were used for displacement measurements along a large-scale prestressed concrete I (PCI) beam. Specifically, the PCI beam was subjected to a set of three-point bending tests in the laboratory. The measured displacements matched well with the corresponding experimental values recorded by LVDTs. In addition, in situ experiments on Bridge No. 24 of Highway No. 86 in Taiwan indicated that the FBG-DSM system can be effectively employed to measure vertical displacements along span bridges. In conclusion, the proposed FBG-DSM system can be used for field bridge applications referring to an absolute reference and without any external physical reference or a requirement for accessibility or particular environmental conditions.