Research On Vortex-induced Vibration Performance For Three Tower Cable-stayed Bridge With Slotted Twin-box Girder

The slotted twin-box girder has been more and more widely employed in long-span cablesupported bridges due to its superior flutter stability.However,the characteristics of flow around box girder become more and more complicated due to the central slot,which is prone to vortex-induced vibrations(VIVs).Based on the sectional model wind tunnel tests and CFD simulation,this paper takes a large span three towers cable-stayed bridge as an example and studies the VIV performances,VIV suppression measures and the flow field characteristics around the sections.Firstly,the VIV characteristics of twin-box girders with 7 various slot sizes were investigated by the sectional model wind tunnel test in a scale ratio of 1:70,and the relationship between the VIV performance of twin-box girder and slot size was summarized: The maximum vertical VIV amplitudes decrease with the increasing of the bottom slab width,and the vertical VIV responses can be effectively decreased by increasing the internal position web height.The relationship between the maximum torsional VIV amplitudes and the bottom slab width is depended on the internal position web height,and the torsional VIV performance of all sections in all vortex intervals will be not improved effectively by increasing the internal position web height.Then,the section with better VIV behavior is determined.Then,the control effectiveness of 4 aerodynamic countermeasures including closuring maintaining roadway railings,installing grid plates,installing internal position guide vanes,and installing the horizontal flow-isolating plates were separately compared based on the better section.The results show that the closuring maintaining roadway railings has a significant amplification effect on the vertical VIV responses,however,the control effect of the torsional VIV responses depends on the closure of the railings.Installing the grid plates have obvious effect on suppressing the vertical VIV amplitudes of the girders.Installing the internal position guide vanes can only suppress the torsional VIV responses at high wind speed vortex intervals;The horizontal flow-isolating plates are the better optimal one at 4 countermeasures,which can effectively suppress the VIV responses of the girder both vertical and torsional.Finally,the flow field characteristics for the better section and the section with horizontal flow-isolating plate were analyzed by numerical simulation method,then,revealed the causes leading to the VIV of the slotted twin-box girder and the suppression mechanism of the horizontal flow-isolating plates.The calculation results show that the large-scale vortices in the central vent were considered to have significant influence on the VIV responses of twin-box girder.When the horizontal flow-isolating plates are installed,the interaction between the upper and bottom vortices is weakened,the scale of the upper vortices at the central slot is obviously reduced.In addition,the vortex on the upper surface of the leeward side girder also become significantly weakened and consequently,the vortex-excited forces at the box girders will be decreased,while the occurrence of the VIV is effectively suppressed.