Study On The Vortex-induced Vibration Safety And Flutter Stability Of A Low-tower Cable-stayed Bridge

In recent years,the construction momentum of low-tower cable-stayed bridges has developed rapidly in China,but there has been a lack of special design specifications,and many key technical problems need to be solved urgently.The planned Hening Bridge is a single-cable low-tower cable-stayed bridge with the span of 80+138+80m and a box-shaped main beam with the width of 26 m.The bridge is in an offshore environment and is affected by severe weather such as typhoons,special studies must be conducted on wind resistance issues.In this paper,the computational fluid dynamics method is used to analyze the vortex-induced vibration(VIV)safety of the completed state and the most unfavorable construction state(the most double cantilever state).The three-dimensional flutter derivative are also obtained in Fluent,and based on the coupled analysis theory,the flutter stability of the bridge is analyzed in ANSYS.The main conclusions are as follows:The finite element model of the bridge at completed state and the maximum double cantilever state are established in ANSYS.The first-order vertical bending frequency of the completed state is 0.570 Hz,which is the first-order mode;the first-order torsion frequency is 3.139 Hz,corresponding to the 11th-order mode;the maximum double-cantilever construction state has a first-order vertical bending frequency of 0.309 Hz,which is the first-order mode;and its first-order torsional frequency is 3.202 Hz,which corresponds to the eighth-order mode.By estimating and comparing the natural vibration frequency and vortex shedding frequency of the cables,it is found that the cables are not easy to generate vortex-induced resonance,so the cable does not need special vibration-damping treatment.In the computational fluid dynamics analysis software Fluent,the Newmark-β method is embedded in the UDF program to solve the dynamic equations of vertical and torsional degrees of freedoms.Combined with the dynamic mesh technique,the VIV safety of the completed state and the construction state are analyzed.The results show that there are not VIV for the construction state under 0° and ±3° wind angles of attack,and for the completed state at 0° and-3°.However at the completed state,the torsional VIV occurs at +3° wind angle of attack.The locking-in range is 25~31m/s,the peak response is 0.314°,which is lower than the allowable amplitude 3.139°.Therefore,the VIV safety of the bridge meets the requirements.The vertical bending and torsional forced vibration of the main beam model are conducted in Fluent through the UDF program,combined with the dynamic mesh technology.Based on the separate free vibration identification flutter derivative theory,the three-dimensional dither derivative of the main beam with the initial wind angle of attack of 0° and ±3° are identified in the range of 0~16 reduction wind speeds.The purpose is to provide key parameters for subsequent three-dimensional nonlinear flutter stability analysis.Based on the multi-modal composite three-dimensional coupled flutter frequency domain theory,the flutter stability analysis is carried out under the initial wind angle of attack of 0° and ±3° for the bridge at completed state and the maximum double cantilever state.The main vertical bending and torsional modes of the structure are selected for analyzes.According to the logarithmic decay rate of each modality and the variation curve of frequency with wind speed,it is found that the critical wind speed of the bridge under the state of completed bridge and maximum double cantilever are higher than 200m/s,which are much higher than the respective tremor.Therefore,the bridge has sufficient flutter safety.