Wind-induced Vibration Characteristics And Vibration Control During Cable-stayed Bridge Construction Phase Under Downburst Wind

Downburst is a non-stationary,sudden and destructive strong wind that spreads along the ground or near the ground caused by a strong subsidence airflow.At present,there are few reports on the impact of douwburst on bridges.Current Wind-resistance Design Specification for Highway Bridges in China(JTG/T3360-01-2018)also does not reflect the effect of local strong wind fields such as downburst.This paper takes the maximum single cantilever state of a large-span cable-stayed bridge as the engineering background,and uses numerical methods to compare and analyze the wind-vibration response characteristics of the single cantilever of the cable-stayed bridge under the action of downburst and normal wind.Based on the CFD simulation test on the dynamic and energy dissipation characteristics of the the water in the TLD,the control effect of the TLD on the maximum single cantilever construction state of the cable-stayed bridge is analyzed.The research work is given as follows:1)The deterministic–stochastic hybrid model is used to simulate downburst wind field of a long-span cable-stayed bridge.The modified Fourier spectrum method is used to simulate the fluctuating wind speed of downburst and A-type ABL wind field respectively.Finally,By Davenport quasi-steady buffeting force model and considering the wind azimuth’s continuous change,a horizontal dynamic force model of moving downburst acting on the bridge was proposed.2)The nonlinear buffeting response under the action of moving downburst and A-type atmospheric boundary layer(ABL)wind field is compared and analyzed numerically.The results show that the response of the cantilever end of the main girder,the top of the bridge tower and the cable under the moving downburst is greater than that under the action of the A-type ABL wind field.The maximum lateral,vertical and torsional fluctuation displacement of the beam end can reach 2.22,2.29 and 1.78 times of that of the A-type ABL wind field.The maximum lateral,vertical and torsional fluctuation acceleration of the beam end can reach1.75,1.62 and 1.59 times of that of A-type ABL wind field.The maximum fluctuation displacement along the bridge,across the bridge and torsion of the tower top can reach 2.28,2.23 and 1.33 times of that of A-type ABL wind field.The maximum fluctuation accelaration along the bridge,across the bridge and torsion of the tower top can reach 1.46,2.31 and 1.37 times of that of A-type ABL wind field.The maximum,time-varying average maximum and time-varying RMS of the stress response of the cantilever cable are 1.14,1.18 and 1.97 under A-type ABL wind field,respectively.The response participation modes of the cantilever state of the cable-stayed bridge under the action of downburst wind and normal wind are basically the same,which are mainly manifested as first step vertical bending of main girder,first step side bending of main girder and first step side bending of bridge tower.3)The feasibility and accuracy of using Fluent software to analyze the water sloshing problem are verified by three examples,namely,the numerical simulation of the water state of the constant acceleration linear motion water tank,the simulation calculation of the frequency characteristics of the rectangular water tank and the simulation test of the forced vibration with harmonic excitation of the water tank.4)Fluent software was used to conduct CFD simulation of the water tank with internal pillars to study its frequency characteristics and energy dissipation characteristics.The results show that the pillars in the tank can slow down the flow speed and reduce the natural frequency of water sloshing.Similarly,the internal water sloshing shows strong linear characteristics,because the internal pillars inhibit the non-linear fluctuations of the water.In addition,the non-dimensional energy consumption value of the built-in column water tank per cycle of water sloshing increases first and then decreases with the increase of the excitation frequency ratio.The energy consumption of the water tank is best when the excitation frequency is close to the natural frequency of the water sloshing.5)Taking the wind vibration control of a large-span cable-stayed bridge under the construction condition of maximum single cantilever as an example,the TLD system of water tank is designed according to the calculation formula of the recommended optimal parameters in the code and the actual situation.The TLD damping system is converted into an equivalent TMD damping system by using the method of parameter equivalence.Thereby the numerical model is established in the finite software to realize the simulation calculation of the TLD structural system.The preliminary analysis of the TLD damping effect is completed.The results show that the TLD at the beam end reduces the vibration response peak value of the main girder lateral displacement of the cable-stayed bridge under the maximum single cantilever construction by 42%.However,the TLD at the top of the tower is not ideal in reducing the vibration along the bridge.