Study And Application About Bridge Cable Force Testing Methods Based On Fiber Optical Grating Sensing Principle

In recent years, sensing networks based on fiber optical grating have been widely used in a great deal of large structures satety monitoring. Long span bridges health monitoring is one of the most active domains for the application of sensing networks based on fiber optical grating. Embedding sensing networks system that based on fiber optical grating in structures can form smart structures. The fiber optical grating sensor which can be used to measure stress, strain, temperature, vibration and other structural parameters has high test precision and good dynamic response and its application perspective is so flourishing in fiber optical sensors for smart material systems and structures. For long-span bridge structures, cable force monitoring is always the most important component of bridge structural health monitoring system. For the prestressed concrete bridge structures which have been widely used, the direct monitoring means for prestressed cable is still vacancy at present. Therefore, tearing the mechanical condition of steel wires inside the cable and the security of structural prestressing system usually become the common attention focus of design engineer, construction personnel and scientific research departments.In addition to some advantages possessed by common fiber optical sensor, namely, light weight, corrosion-resistant, anti-electromagnetic interference, safety and reliable in use and so on, fiber optical bragg grating (FBG) sensor has some unique advantages, namely, small probe size, wavelength modulation, good anti-interference ability, integration of sense and transmission, good multiplexed ability, easy to constitute a sensor network, wide range testing parameters and easy to implement many parameters simultaneous monitoring. Although a good many research work about cable force real-time monitoring have been carried out in the world at present, generally speaking, the radical technical difficulties about cable force testing haven't been solved and the practical engineering application reports were also rare. For the structures which bear high stress, especially such as prestressed reinforcement, external cable in bridge and steel wires in bridge cable, the direct strain testing means is still a challenge. In order to further exert the advantages of FBG sensing technology, study new testing methods which based on FBG principle (in the following text , "FBG-based" was used as the abbreviation for "based on FBG principle") for cable, achieve the FBG-based prestressing loss monitoring, implement the FBG-based distributed condition monitoring for steel wires stress values and search an effective data processing method for extracting meaningful monitoring information from the continuous collecting cable force data, this paper carried out the following research work:(i) Contrast study on existing bridge cable force measure methods based on FBG1. The status application of smart tendon that encapsuled by FRP which lacked the real engineering support was pointed out. More matured cable force sensing monitoring systems based on fiber optical grating involving frequency measure principle, pressure sensing principle and differential sensing principle were introduced. Also the corresponding technology trait, structure form and engineering application effects were presented.2. From the angle of engineering application, some index relative to the aforementioned cable force testing methods, namely, adaptation object, unit price, measure manner, temperature compensation, alterability and testing signal, were summarized and compared. Aiming at a real engineering and proposed relatively index about existing cable force sensor based on fiber optical grating that mentioned in this paper and, a more objective method for type decision-making of sensor was elicited by using fuzzy consistent matrix.(ii) Study the prestressing loss monitoring method based on FBG1. According to the analysis about the technical difficulties of prestressing loss monitoring based on FBG, the high strain monitoring strategy and the FBG-based monitoring ideas for prestressed cables in newly-built bridges and exsiting old bridges strengthening were elicited.2. The FBG sensor prototype which can be applied for prestressing loss monitoring in newly-built bridges was designed. Experiment analysis results verified the validity of proposed design thought and related problems in practical application which should be further taken into accout were pointed out.3. Based on a concrete bridge external cable strengthen engineering example, the FBG-based monitoring means for prestressing value which was adapt to construction and operation stage was proposed.(iii) Study the smart stay cable structure based on FBG1. Combining the fabricating arts and crafts flow of parallel wires cable of cable-stayed bridges with several conclusions derived from the parallel wires model tension experiment, the configuration scheme of smart stay cable was designed.2. Aiming at the high stress work condition of stay cable, some FBG pre-relaxed methods for sensor were put forward. A new micro strain sensor was designed, which ensured the validity to transfer steel wire stress and its mensurability as stay cable force testing means. According to the equivalent simulation of strain sensor's high stress work condition, temperature compensation of the strain sensor was solved.3. Based on the character of fabricating arts and crafts of parallel wire stay cable, protection measures, possible disabled reasons and avoidance strategies of sensor structure used in the smart stay cable were analyzed.(iv) Study the broken wires monitoring based on FBG cable force testing methods1. The fatalness of broken wires inside the stay cable was illuminated. By using a certain numerical model, structural deformation, displacement, cable force and stress distribution were analyzed under different cases which composed by various of broken wires number and damage extent. The character and monitoring means of different broken wires inspection methods were compared. 2. The strategy for broken wires monitoring which based on FBG cable force testing methods was proposed. Associating the whole model of stay cable with the local model based on steel wires, several broken wires samples were established, which using stay cable force (average cable stress), steel wires stress derived from smart stay cable as input parameters. Using the BP pattern recognition method, the mentioned monitoring strategy can recognize the broken wires location and meet the 2% degree requirement for monitoring broken wires, which still can exert its function under small noise disturbance.(v) Study the trend extraction of cable force data based on EMDThe function of extracting average trend and high amplitude pulse from the original continuous acquisition cable force data based on IMFs which were decomposed by using EMD method was elicited.