# Cable Force Health Monitoring of Tongwamen Bridge Based on Fiber Bragg Grating
## 1 Introduction  
Large suspension bridges are common in river-crossing and gulf-crossing transportation projects. Magnitude and distribution of cable tension force can reflect the stress state of a bridge, and a reasonable cable force state is extremely important for the safe operation of a bridge. With the rapid development of China's transportation system, heavy load and overloaded vehicles are common in transportation. As a result, operation loads of the bridge often exceed the design load, resulting in damage and failure to the bridge structure. Therefore, the long-term real-time cable force state and parameter measurements are critical to bridge engineering safety monitoring during the operation of suspension bridges [1; 2]. Fiber Bragg grating (FBG) sensors have received considerable attention in research publications and a wide range of applications have been implemented due to their advantages, such as immunity to electromagnetic noise, small size, light weight, ease of incorporation into a sensor network, and long signal transmission distance [3; 4; 5; 6]. Although use of fiber grating sensing technology in civil engineering applications such as bridges and tunnels has spanned over several decades, fiber Bragg grating based structural health monitoring technology has undergone rapid growth in recent years, and research and applications in various engineering fields have been reported consistently by numerousresearch groups at home and abroad [7; 8; 9; 10]. [PERSON] [9] proposed a smart stay cable for bridge health monitoring. This cable is a glass fiber with high durability and high intensity, which has an embedded FBG and is coated by steel wires. [PERSON] and [PERSON] [11; 12; 13] have reported numerous articles on FBG-based health monitoring of aerospace composite structures. Here, FBG arrays were used to obtain the dynamic strain response of an aerospace structure under vibration excitation. Tongwamen bridge, located in Xiangshan County, Ningbo City, Zhejiang Province, is a sea-crossing bridge connecting Shipu Town and Dongmen Island. It is a boom arch bridge, 292 m in length from east to west. There are 19 suspenders on both north and south sides. Each suspender has a diameter of 64 mm and a height ranging from 8.919 m to 35.268 m. The distance between adjacent suspenders is 10 m. Since 2002, when public traffic was first allowed on the bridge, traffic flow has seen a significant increase each year, exceeding the design traffic load. In addition, some adverse factors including overloading of vehicles, traffic jams, and typhoons have also adversely affected the health of the bridge. During maintenance, the cable forces of the 38 suspenders were found to be abnormal. This motivates the need for real-time monitoring of cable forces. Compared with bridges under construction, which can be embedded with sensors for structural health monitoring, bridges already in operation are not suitable for direct measurement by modifying their anchor cable structure largely, since it will destroy their original structure. For example, an FBG force ring is usually mounted at the end of a cable during bridge construction. However, when the construction is completed, it would be inappropriate to disconnect cables to install a force ring [14]. Suspension cable measurement technology based on the theory of string vibration can be used for bridge cable force measurement without making any structural changes to the main structure of the cable and bridge. As a result, this method has strong engineering practicality [15; 16; 17]. Considering cable sag and bending stiffness, [PERSON] and [PERSON] [15; 16] further developed the taut string theory. [PERSON] [17] reported an FBG vibration sensor-based real-time monitoring system for cable force. But a complicated signal processing system reduces its engineering practicability. Based on the vibration frequency method, [PERSON] [18] investigated a wireless monitoring system for bridge cable tension force monitoring.