Research And Application Of A New Type Of Cable Force Measurement Method

Cable force measurement and monitoring of cable structure is the key technology to ensure the safety of construction works and normal operation of the project. But there’re still many problems in cable force measurement. In this paper, a new method of measurement named method of the fiber bragg grating base fixture is presented based on the sensing principle of fiber gratings and the structural characteristics of the cable. Firstly, the structural characteristics of the new cable force measuring device and the principle of cable force measurement were introduced. Then through the tensile test for the fiber bragg grating base fixture, the applicability of measurement method of the fiber bragg grating base fixture was proved and some parameters influencing on the measurement through the fiber bragg grating base fixture such as the initial bending of the cable, the slippage of the base fixture and the torsion of the cable were studied. Then the FBG base fixture method was used in scaling model test of cable structures, it has proved that the FBG base fixture method has higher reliability and stronger applicability by comparing the FBG base fixture method with pressure transducer method and resistance strain method. At last the FBG base fixture method was applied in monitoring of the cable forces for the stay cables of the Kunshan Yufeng bridge, the effects of some factors such as the reliability of monitoring of the cable forces for operation engineering with the FBG base fixture method and temperature variation on cable force are investigated. The main conclusions of this paper are summarized as follows:(1)In this new method, firstly, base fixture is used to make the fiber bragg grating sensor and the cable parallel fixed; then the cable force is introduced by measuring the cable strain between the base. The method is simple and practical, and it is suitable for the bare wire without cover, because it does not damage the anticorrosion protection on the cable surface.(2)By combining the effective metal section and elastic modulus of the cable, the theoretical calculated formula reasonably utilized to the new method of the cable force measurement is deduced based on the Hooke’s law in the mechanics of material.(3)In order to eliminate the influence of the initial bending of the cable, the cable should be tensioned as a straight line before the installation of the base fixture and the fiberbragg grating sensor. And cable force is gotten by reading oil pressure gauge at the stage of pre-tension.(4)From the tensile test, the measurement method of the fiber bragg grating base fixture can be effectively used in the cable force measurement, and cable force test range can cover the tensioning construction stage and the whole service stage of cable type curtain wall project.(5)According to the Monitoring test of cable structure model, pressure sensor comparision method and the measured values of electric-resistance strain method, it can be verified that this method can be applied to the cable force measurements under complex stress conditions with extensive practical. At the same time, this method can measure both positive cable strain and negative strain, which can meet engineering cable tension monitoring requirement.(6)By combining fiber Bragg grating base fixture measurement method with vibration frequency method. It verify the already operating prestressed bridge in the cable force for the feasibility of real-time monitoring, the monitoring results show that the optical fiber grating sensor has a good stability, connection is firm between base clamp and cable. And the testing data is reliable.