Experimental Research On Vortex Induced Vibration Of Non-hangers Side Span Main Cable On A Stayed Cable And Suspension Hybrid Bridge

With the increasingly growth of span,bridge,especially cable-bearing bridge,is more and more sensitive to wind load.In recent years,the phenomenon of high order vortex induced vibration of cables has been observed on cable-stayed Bridges such as Sutong Yangtze River Bridge and Jingyue Yangtze River Bridge.High order vortex induced vibration caused the damage of the facilities such as sleeves and dampers,which caused great economic loss.This phenomenon exceeds the understanding of people about the vortex-induced vibration of bridge cable structures.The Xihoumen Road-Rail Bridge on Yongzhou Railway under construction in China(‘Xihoumen Bridge’for short)is a stayed cable and suspension hybrid bridge.The side span cable is 727m.It may occur the phenomenon of vortex induced vibration with obvious amplitude or even high order vortex vibration.In this paper,a three dimensional elastic test model of the side span main cable of the long span cable stayed cooperative cable system was designed,and the vortex induced vibration response characteristics and vibration suppression measures of the bridge were studied by wind tunnel test.The main content and results of this paper are as follows:(1)Using ANSYS finite element software to analyze the dynamic characteristics of the bridge and its side span main cable.The results show that the first transverse bending frequency of the full bridge is 0.0893 Hz,the first antisymmetric vertical bending frequency is 0.1291 Hz,the first positive symmetric vertical bending frequency is 0.1418 Hz,and the first torsion frequency is 0.2645 Hz.The first in plane and out of plane frequencies of the side span main cable are 0.202 Hz and 0.184 Hz,respectively,and the first five order in plane frequencies are higher than the out of plane frequencies.(2)Considering the influence of sag effect,a new design method of three dimensional elastic model for hangers is extended to inclined cables.Based on the background of Xihoumen side span main cable,the influence of vertical span ratio on the dynamic characteristics of side span main cable is studied.The vortex-vibration responses of the prototype structure and the model are compared by theoretical analysis method.The applicable conditions of extending the model design method to inclined cable are explored.Based on this,the geometric scaling ratio of the model is determined,and the three dimensional elastic test model of the main cable is designed and made.The results show that the sag ratio of the short model increases with the increase of the geometric scale ratio,and is close to the prototype structure at the geometric scale ratio of 1:20.The frequency calculation results of the short model are higher than those of the long model.The short model has no obvious vibration when the geometric scaling ratio is higher than 1:20,and the response of the long model is close to that of the geometric scaling ratio is greater than or equal to 1:20.The aeroelastic model in this paper uses 1:20 geometric scale ratio.(3)Wind tunnel test of three-dimensional elastic test model is carried out to study the wind induced response characteristics of the main cable without hangers of Xihoumen bridge.The influence of wind attack angle and wind speed on the vortex induced response of side span main cable is studied.The results show that the wind angles of the main cable without hangers are0°,+10°,-10°,+50°,-50°,+60°and-60°.The maximum response appears at-50°,the acceleration root mean square response is 5.2m/s~2,and the amplitude is 0.8mm,which corresponds to the actual bridge amplitude of 16mm.When the wind angle is 0°and the wind speed ranges from 4.8 m/s to 6.6m/s,the vortex-induced resonance mode is as high as the 16th order.(4)Wind tunnel test method of three-dimensional elastic test model is used to study the vibration reduction and suppression measures of vortex induced vibration of the main cable.The effects of the damping ratio and the helix on the vortex-induced response of the main cable were studied under the wind angle of-50°.The results show that when the damping ratio of the model reaches 0.25%,the vortex induced response of the main cable is effectively suppressed.The vortex-induced response of the main cable can also be effectively suppressed when the distance between the helix is 12D.