# Fiber Monitoring System Applied to Railway Bridge Structures in a Near-Fault Region
###### Abstract  
Bridges are widely used for train transportation. Some bridges must be constructed close to geologic faults or across them due to the constraints of travel route alignment and the geographical environment. Taiwan is located at the junction of the Eurasian Plate and the Philippine Plate, where geological joints are present and earthquakes are frequent. In Taiwan, the monitoring and early warning of structural displacements is increasingly important, especially in the mutual control and monitoring of bridges and railways. This study utilizes fiber as a continuous sensor to monitor the safety of railway bridges in a near-fault region. This research builds upon the theory of Brillouin frequency shift (BFS) and applies it to a practical scenario of a fault-crossing railway bridge. BFS is related to the strain and temperature change in a single-mode fiber. Distributed fiber optic sensing (DFOS) systems enable us to detect shifts in frequency on the sensing fiber. A systemic approach to installing DFOS systems will be discussed. Data from a DFOS system are collected, and through data processing, they are converted into strain with regard to the deformations (bending, tension, compression) of a box girder bridge. Changes in the geometric structure of the box girder bridge throughout the year are measured and processed into graphical data. This system can be effectively applied to the structural safety monitoring of railway bridges. Through this research, several functions have been achieved, including continuous displacement, automatic monitoring, and real-time automatic alarm functions, without the need for human intervention. b 2024 Bridge inspection vehicles, which rely on human visual inspections, are traditional inspection technique for bridges [1]. Bridge safety has been managed by regular visual inspection, and in most cases, only a simplified physical model is used to assess the structure due to the lack of accurate data. This often leads to the underestimation of bridge health. Even while using settlement marks or inclinometer monitoring of the structure, continuous displacement data measurement and real-time alarms are still not achievable. With the continual rise in human resource costs and higher safety standards, a more effective technology for monitoring bridges is urgently needed. Various studies have demonstrated that strain and temperature changes in fibers can be detected through the theory of Brillouin frequency shift (BFS) [2; 3; 4; 5; 6; 7; 8; 9; 10]. Other environmental factors like humidity and air pressure are not taken into consideration for various reasons. Humidity does not contribute much to the changes in BFS [11; 12], and the overall change in humidity in Taiwan is around 75-77% according to the Taiwan Central Weather Administration. For this research, we calculate the difference in different parallel fibers on a box girder. This means that if we assume the nearby fibers experience the same humidity and air pressure, these factors would be deducted. Through this sensing technology, structures could be monitored continuously. This research applies this technique and is able to gather continuous data for the geometric structure of the box girder bridge. A systematic approach to installing fiber sensors that accurately detect the conditions across the entire bridge is explained in this research. The installed fiber sensor provides us with accurate data that reflect the condition of bridges. Data accuracy and reliability are taken into account during the assessment of fiber installation. Through experimentation, the data sampling density is increased to obtain continuous measurements that are much more refined. The distributed fiber optic sensing (DFOS) technique has been proven to be of significant value in structural monitoring [13; 14; 15]. Data collected through the Brillouin DFOS system are transferred to a server for further data processing.