Research On Bridge's Vertical Displacement Monitoring Method Using Liquid-air Linkage Differential Pressure Connecting Pipe

This study has been carried out with the support of the National Basic Research Program of China(2012CB723305),under the research subject of Evolutionary Theory of Bridge Structural Behaviours and Safety Monitoring Methods.And it is also part of the program of Research and Development of Liquid-air Coupled Pressure Differential Flexometer used to bridges,funded by China Railway Major Bridge Engineering Group Co.,LTD(K2009-35).In light of the problems in bridge displacement measuring based on the liquid level sensing system,the improvements to the measuring system technical options are discussed,and the related experimental study and theoretical analysis have been done.The major works include the followings.(1)Several kinds of liquid level catching methods of using different connecting pipes were used to examine the applicability of the improvements.Based on the geometrical,physical and balancing conditions of the liquid inside the pipe,the mechanical behaviors of the liquid inside the liquid level connecting pipe were studied,taking into account of the viscosity of the liquid as well as the inclining angles of the end vertical pipes.The influential law of these factors on the displacement measuring results was figured out.(2)The displacement measuring method based on liquid level connecting pipes has its drawbacks,such as the energy friction loss caused by liquid flowing,being sensitive to the inclining angles at the measuring locations and the dimension effect of measuring set ups as well as poor environmental suitability.To resolve these problems,it is suggested that a certain volume of air be enclosed to the ends of the vertical pipes to transform the system to the liquid-air linkage differential pressure connecting pipes system.A series of theoretical studies have been carried out to examine the differential pressuredisplacement transferring principle and the displacement measuring applicability under different arrangements of the connecting pipes.The software and hardware of the system have been designed,the experimental studies concerning the feasibility and applicability have been done,and the reasonability of the method has been verified.(3)An innovative dynamic and static performance testing platform of the displacement measuring system using liquid-air linkage differential pressure connecting pipes was prepared.Combined with the finite amplitude shallow water gravity wave theory and fluid mechanics theory,the influential laws of three-directional vibration of the main girder of a real bridge on the static and dynamic response properties of the displacement monitoring system were studied,and the responding treatments were proposed.(4)The influential features of the air bubbles in the liquid inside the horizontal pipes,the altitude and latitude of the measuring site,the liquid density,system temperature effect and the natural wind site on the monitoring results were analyzed using both theoretical methods and actual testing,and the responding treatments are proposed.(5)To resolve the problem of initial liquid level difference caused by man-made factors,it is suggested to set outlets that can be opened and closed for liquid escape at the certain height of the vertical pipes,so as to control the liquid level.To resolve the problem of automatic liquid level compensation induced by liquid evaporation during the longterm monitoring,an automatic liquid level compensation method was proposed.To realize the goal of high-precision automatic monitoring of structures with great height difference and small deformation,the measuring method of using symmetrical closed liquid-air linkage differential pressure connecting pipes was put forward.And the related theoretical studies and experimental verification have been done.(6)The improved vertical displacement monitoring system based on the liquid-air linkage differential pressure connecting pipes has been used to the short-term vertical displacement measuring and long-term deflection monitoring of several long-span bridges,and the results were compared with those obtained by the existing methods and set ups.Engineering practices have proved that the displacement measuring system proposed has reliable precision and good environmental suitability.