This paper mainly studies the application of fiber Bragg grating (FBG) in strain monitoring of civil engineering structure. The principle of FBG was illuminated. Static tests of a steel truss instrumented with FBG sensors were done, in order to study whose distinct sensing character and monitor strains of the truss. Then, FBG sensors were instrumented in a cable stayed bridge named Songhua River Bridge located in the Harbin city of China to monitor strains of key structure sections. A number of meaningful results were concluded.
It is still a practical problem how to effectively install FBG sensors on bridge cabes. In this paper, a simple and effective solution is introduced to develop smart bridge cables using FRP-OFBG bars developed in HIT (Harbin Institute of Technology). Here, the FRP-OFBG bar acts as one component of the cable and shows force resistance and well-protected sensors in service. The installation techniques and the sensing properties of FBGs in three kinds of cables, FRP cables, common steel-wire cable and extruded-anchor cable, are introduced and tested under dead load. Moreover, the preliminary introduction of a practical field application based on this solution has been also given. The experimental results show that the deformability of FRP-OFBG bars in the smart cables can reach the terminal and show wonderful accuracy, which shows that such kind of smart cable is practical in field application.
Binzhou yellow river Highway Bridge with 300 meter span and 768 meter length is located in the Shandong province of China and is the first cable stayed bridge with three towers along the yellow river, one of the biggest rivers in China. In order to monitoring the strain and temperature of the bridge and evaluate the health condition, one fiber Bragg grating sensing network consists of about one hundred and thirty FBG sensors mounted in 31 monitoring sections respectively, had been built during three years time. Signal cables of sensors were led to central control room located near the main tower. One four-channel FBG interrogator was used to read the wavelengths from all the sensors, associated with four computer-controlled optic switches connected to each channel. One program was written to control the interrogator and optic switches simultaneously, and ensure signal input precisely. The progress of the monitoring can be controlled through the internet. The sensors embedded were mainly used to monitor the strain and temperature of the steel cable and reinforced concrete beam. PE jacket opening embedding technique of steel cable had been developed to embed FBG sensors safely, and ensure the reliability of the steel cable opened at the same time. Data obtained during the load test can show the strain and temperature status of elements were in good condition. The data obtained via internet since the bridge's opening to traffic shown the bridge under various load such as traffic load, wind load were in good condition.
The feasibility and advantages of smart bridge cable based on FBGs are discussed. And the sensing properties of FBGs installed in the cables under dead load are tested. The experimental results and practical applications show that the smart bridge cable is proper to be used in bridge cables transfer, construction control and long-term health monitoring.
Sensing properties of the bare FBG have theoretically and experimentally been studied in this study, and then a temperature compensation method for FBG strain sensors was proposed. Due to the fragility of bare FBG, a technique of encapsulating bare FBG in a capillary steel tube was developed and the strain and temperature sensing properties of the encapsulated FBG were furthermore studied. With the consideration of the practical application in civil infrastructures, the technique of FBG to adhere or embed on steel and in reinforced concrete structures was studied. With successfully affixing and embedding bare FBGs and encapsulated FBGs sensors on a steel truss and in a reinforced concrete beam, respectively, the experiment of using the above FBG sensors to monitor the structural strain and deformation process. In order to verify the quasi-distribution sensing ability of FBG, the test that 3 bare FBGs were connected one by one and adhered on a steel truss was done and the independently sensing strain ability was found. Finally, FBGs strain sensors were successfully embedded in a reinforced concrete bridge to monitor the strain history and distribution under construction and in service. The measuring signals indicated that the embedded FBGs could precisely and long-term monitor the strain history, therefore, they can be employed to evaluate the fatigue life for the bridge.