Investigation Of Vortex-induced Vibrations Of Twin-box Girder Of Long-Span Suspension Bridges

Super long-span bridge is a sign of science and technology of human being in constructing bridges. Breakthrough of the limitation of span width of bridge is one of the goals that mankind is seeking for. As the bridge span increases, the flutter stability becomes more and more important. To improve the flutter stability for super-long span cable-supported bridges, twin or multi-box girders have been extensively adopted in actual bridges. Although the type of bridge can reach very high critical flutter wind speed, the characteristics of flow around box girder become more complicated due to the gap between the separated box girders. For example, vortex-induced vibrations (VIVs) become obvious, due to the effects of the vortices forming in the gap. In the paper, systemic experimental investigations of VIVs of a twin-box girder were performed in the wind tunnel. Moreover, long-term full-scale filed monitoring was carried out on a long-span suspension bridge.Firstly, static and dynamic section model of twin-box girder with gap ratio L/D=1.710tests were carried out in Wind Tunnel and Water Flume of HIT. Wind pressure distributions on the surface of model, vortex shedding characteristics and vortex structures of stationary twin-box girder were investigated deeply. And the principle of flow motion around the twin-box girder in low Reynolds number range was revealed. Through analyzing the VIV characteristics and the flow characteristics around moving twin-box girder, the relationship between amplitude of VIV and reduced wind velocity, and the evolution of flow pattern during the VIV process were obtained. Moreover, a large eddy simulation combining with wind tunnel test results was carried out to investigate the VIV of twin-box girder with L/D=1.710. For the method, the measured VIV displacements obtained by wind tunnel testing were used as the inputting boundary conditions. Through the innovation approach, the evolution of vortex cores around twin-box girder in the VIV process were obtained, and the principle of energy transform between flow and twin-box girder motion was revealed.Secondly, the gap effects on the flow characteristics around twin-box girder and VIV were investigated in detail. Through analyzing wind pressure distributions on the surface of model, vortex shedding characteristics and vortex structures of static twin-box girder at various gap ratios, it was found that the flow pattern around the gap suddenly changed as the gap ratio increasing, and the critical gap ratio has Reynolds number dependence. According to the changes of vortex shedding pattern, three flow patterns were defined. Analyzing the vibration characteristics at various gap ratios, the relationship between the types of VIV and gap ratio was obtained.Thirdly, the Reynolds number effects on the flow characteristics around section model and VIV of twin-box girder with gap ratio L/D=1.710were studied in detail. Wind pressure distributions on the surface of model, vortex shedding characteristics and vortex structures of static twin-box girder at various Reynolds numbers were analyzing deeply. The relationships of drag coefficient and vortex shedding frequency in the wake of downstream box girder with leading bubble length on the lower surface of upstream box girder were created, and the mechanism of Reynolds number effect on the twin box girder was revealed. The investigation of Reynolds number effects on the VIV of twin-box girder showed that the maximum amplitude of VIV increases with the increase in Reynolds number.Fourthly, the attachment structures of the surface effects on the flow characteristics and VIV of twin-box girder were discussed. Through comparing the characteristics of wind pressure distributions and vortex shedding characters between twin-box girders with and without attachment structures, the mechanisms of disturbances of attachment structures are obtained. The VIV characteristics of twin-box girders with and without attachment structures were carefully compared.Finally, a perfect long-term wind and wind effects monitoring system was set up on a long-span suspension bridge, which includes four monitoring modules：free wind field monitoring system, wind pressures on the lower surface of twin-box girder monitoring system, wind-induced vibration monitoring system and field flow visualization system. Based on large amount monitoring data, the wind filed characteristics, the wind pressure distributions on the surface of the girder, the vortex-shedding characters around twin-box girder, the flow patterns and VIV of field prototype bridge were deeply analyzed. It was found that the wind filed along the span-wise of bridge was imhomogenous, which had significant effect on the amplitude of VIVs. The flow characteristics around prototype bridge and wind conditions inducing VIVs of field prototype bridge were obtained. And finally, the validation for1:25scale section model test is conducted using the field monitoring results.