Influence Of Beam Spacing And Wind Attacking Angle On Vortex-induced Vibration Performance And Flow Field Characteristics Of Double-layer Box Girder

With the development of economy,the increasing traffic volume puts forward higher requirements on the capacity of the bridge.Overly wide single-layer box girder section will lead to excessive transverse bending moment of the main beam,and serious problems of distortion and shear lag.In short,there are many problems in the practical project.Double-layer box girder can double the capacity of the bridge without increasing the width of the girder,but the aerodynamic interference between the upper and lower beams and the increase of the wind resistance area of the girder makes the double-layer box girder’s vortex performance and surrounding flow field characteristics different from the single-layer box girder.In order to prevent the double-layer box girder bridge from exceeding the limit of vortex-induced vibration to ensure the stability of double-layer box girder bridge,and provide a reference for the wind resistance design and subsequent research of double-layer box girder bridges,it is particularly important to study the vortex vibration performance and surrounding flow field characteristics of double-layer box girder.In this paper,single-layer box girder and double-layer box girder are taken as the research objects.Through the segment model vibration measurement and pressure measurement,the vortex characteristics of them are studied,including the vortex amplitude and lock-in wind speed range,the average wind pressure coefficient and pulsating wind pressure coefficient of the measurement points,the correlation between the aerodynamic force at the measuring point and the total aerodynamic force,and the vortex vibration contribution coefficient of the measuring point aerodynamic force.Besides,the surrounding flow field characteristics of double-layer box girder were analyzed by using computational fluid dynamics(CFD).The main conclusions are as follows:(1)The vortex performance of single-layer box girder and double-layer box girder under negative wind attacking angle is better than that under positive wind attacking angle.As the wind attacking angle increases,the vortex performance of single-layer box girder and doublelayer box girder gradually deteriorates.(2)when the wind attacking angle is 0°,the vortex performance of the double-layer box girder gradually becomes better as the beam spacing increases.when the wind attacking angle is +3° or +5°,as the beam spacing changes,the vertical bending vortex vibration performance and torsional vortex vibration performance of the single-layer and double-layer box girder show different changes.(3)With the change of beam spacing and wind attacking angle,the average wind pressure coefficient at different area of the double-layer box girder shows different change rules.The simultaneous fluctuation of the pulsating wind pressure coefficient and the aerodynamic correlation at the measurement points on the upper surface of the upper beam causes the change of vortex vibration contribution coefficient of measurement point aerodynamics,which affects the vortex vibration performance of the double-layer box girder.(4)With the change of beam spacing and wind attacking angle,the flow separation and vortex development of the upper surface’s front of the upper beam and the edge of the diagonal web in the windward zone of the lower beam have changed.As the beam spacing decreases,the lee side diagonal web of the upper beam which disturbs the flow field downstream of the upper surface of the lower beam causes the rear vortex area of the lower beam to be amplified.(5)The optimal beam spacing of double-layer box girder can be determined by wind tunnel tests and computational fluid dynamics.As far as this study is concerned,the optimal net spacing of the double-layer girder is 1.5h～2.5h(H is the height of the single-layer box girder),which can get better vortex performance.