Long-term deflection monitoring of a box girder bridge with an optical-fiber, liquid-level system

In this study, a fiber Bragg grating (FBG)–differential settlement measurement (DSM) system was used to conduct 2-year monitoring of the vertical displacement of a prestressed concrete box girder bridge that had previously been damaged in an earthquake. This system is based on the principles of the hydrostatic liquid leveling of connected vessels, buoyancy, the equilibrium of forces, and the photoelasticity of FBGs. Specifically, a series of vertical displacements were measured at 42 observation points along the aforementioned bridge and used to determine its load-carrying capacity. Static vehicle loading tests were also conducted, and the displacement measurements of the FBG–DSM system were compared with those of conventional sensors and the displacement values simulated through finite-element simulation. The results reveal that the FBG–DSM liquid-level system has high potential for the inexpensive, long-term monitoring of bridges over long distances without requiring suitable environmental conditions, in situ visibility and intensive labor work.