This paper is about the design and application of optical-fiber differential settlement measurement (DSM) sensor for bridges. By using the law of connected vessels, bouncy principle, forces equilibrium condition, and the photo elasticity of Fiber Bragg Grating (FBG), the DSM sensor is easily manufactured and sensitive to the level change. Besides the design principle, this study illustrates the application of the DSM sensors on underpass bridges. From the field case, it is shown that the proposed DSM sensors are stable and precise to monitor the behavior of multiple-span underpass bridges. In this paper, to share the optical fiber DSM field-experience is the aim of this study. Based on the field experience, enhancement design for the DSM sensor will be discussed for the future application
