Study On Automatic Monitoring System For Cable Tension Of External Prestress Based On Vibration-frequency Measurement

As a reliable and effective means of reinforcement, external prestressingtechnology can greatly improve force caused by diseases structure, and it has a goodeffect on improving structure strength, crack resistance and structural bearing capacity.The technology became more and more important in strengthening old bridge,especially in long-span continuous steel-structure bridge. Subsequently, externalprestressing technology has been widely used in engineering application. However,there might be some different levels of abrasion during its operation which mayinfluence its force and operation state and the reinforcement ability of the bridge. Thus,there is a must in the real-time monitoring of changes in external prestressing cableforce.Nowdays there are several primary ways of measuring cable force: Oil PressureGauge Reading Method, Pressure Sensor Measurement, the Oscillating FrequencyTechnique, and Magnetic Flux Method and so on. Obviously none is suitable for themeasurement of external prestressing cable force except the Oscillating FrequencyTechnique. The Oscillating Frequency Technique is based on either the ideal onedimension wave equation or correction formula established on the influence of flexuralrigidity and sag of the cable. According to some relevant papers, the OscillatingFrequency Technique is generally applied in cable force measurement of cable-stayedor suspension types of bridge but seldom applied in long-time on-line monitoring ofcontinuous steel-structure bridge strengthened with external prestressing cable force.The cable length, material, applied environment and vibration characteristics ofcable-stayed bridge is different from that of the continuous steel-structure bridgestrengthened with external prestressing. Thus we can’t simply applied the system ofmonitoring cable-stayed bridge’s cable force into monitoring continuous steel-structurebridge strengthened with external prestressing’s. So it’s a necessary to researching anddeveloping a set of system which fits for the monitoring of cable force of continuoussteel-structure bridge strengthened with external prestressing.So based on the Oscillating Frequency Technique, we designed hardware andsoftware system of automatic monitoring continuous steel-structure bridgestrengthened with external prestressing cable force. Hardware is combined by MCUand Accelerometer. By collecting the vibration signal of external prestressing cable, Accelerometer temporarily store the signal into MCU and then deliver the data to hostcomputer once received the deliver order from the host computer. Software primarilystore and handle those collected primitive vibration signal.To prove automatic monitoring system of external prestressing cable force’smeasurement consequence’s accuracy and steady towards vibration signal, we havedesigned contrast experiment by using dynamic test set and automatic monitoringsystem of external prestressing cable force and finally analyzed the contrast dataconsequences。At last, we used Jiangjin Changjiang Bridge as a pilot project by installing a setof automatic monitoring external prestressing cable force system at the fifth and sixthcable of both upstream and downstream. Through the analysis of the dataconsequence of the long-time on-line monitoring, it shows out automatic monitoringexternal prestressing cable force system can accurately and steadily achieve goal ofthe real-time on-line monitoring of the changes in several external prestressing cableforces.