Experimental Study On The Influence Of Railing Structure On Vortex-induced Vibration Performance Of Streamlined Box Girder

Vortex-induced vibration(VIV)is a kind of wind-induced self-limiting vibration with both forced and self-excited characteristics,which is easy to occur in long-span bridges at low wind speeds.The wind-induced vibration of long-span bridges under strong wind has been widely concerned.The subsidiary structure has a significant influence on the separation and reattachment of air flow over the streamlined box girder.Variation of flow field such as separation and reattachment will lead to change of aerodynamic performance of box girder.Especially,the vortex-induced vibration performance is obviously affected by the railings.In this thesis,Ningbo Xiangshangang Bridge is taken as the engineering background,and the streamlined box girder is taken as the research object.The wind tunnel pressure and vibration test methods of the section model are used.The response of vortex-induced vibration of streamlined box girder in construction state and bridge state,the evolution process of vortex-induced vibration and the evolution law of wind pressure in bridge state are studied,and the systematic research work is carried out on the optimization design of railing structure shape.The influence of the three parameters of the inclination angle of the railing,the height of the railing and the ventilation rate of the railing on the vortex vibration response of the streamlined box girder is studied.The average and fluctuating wind pressure distribution on the upper and lower surfaces of the main beam,the correlation and contribution coefficient of local lift and vortexinduced force,and the variation law of phase difference are analyzed.The influence mechanism of different parameters on vortex-induced vibration is explained,and the railing shape parameters that effectively suppress the vortex-induced vibration of streamlined box girder are found.It provides a theoretical basis for the analysis of the vortex vibration performance of the actual bridge and a reference for the wind resistance design of long-span bridges.The main conclusions are as follows:(1)In the evolution process of vortex-induced vibration of streamlined box girder,the vibration frequency of the main girder is complex before and after vortex-induced vibration,the vibration time history curve is disordered and non-harmonic vibration,the fluctuating wind pressure on the surface of the main girder is low,and the change trend is ’ one ’.In the starting vibration,when the vortex amplitude is the largest,the vortex vibration of the main beam has a single dominant frequency in the vibration reduction,and the vibration time history fluctuation is stable and simple harmonic vibration.The fluctuating wind pressure value is the largest when the vortex is in the starting vibration,the fluctuating wind pressure value is the second when the vortex amplitude value is the largest,and the fluctuating wind pressure value is the smallest when the vortex is withdrawn.(2)When the sidewalk railing is tilted inward by 20° and the railing is tilted outward by 10°,there is a significant vibration suppression effect,which can basically completely suppress the occurrence of vortex vibration.When the railing is tilted outward by 20°,it has a certain damping effect,but it cannot completely suppress the occurrence of vortex vibration,and will also make the wind speed locking interval move forward.Considering the engineering practice,it is suggested to use the railing with 10° outward inclination of the railing,which is of great significance to the wind resistance design of long-span bridges.(3)When the sidewalk railing is inclined inward,the larger the inclination angle is,the more obvious the vibration suppression effect is.When the railing is inclined inward by 20°,the maximum amplitude of vortex vibration decreases by 90.82%.When the railing is inclined outward,the vibration suppression effect of the main beam with an outward inclination of 10° is better than that with an outward inclination of 20°.Although the railing has a certain vibration suppression effect when it is tilted outward20°,it will make the wind speed lock interval move forward.Considering the engineering practice,it is of great significance for the wind resistance design of longspan bridges to adopt the railing with 10° outward tilt.(4)The change of the height of the railing outside the sidewalk will significantly affect the vortex-induced vibration performance of the streamlined box girder.When the height of the railing is 0.45 times the beam height,the vortex amplitude of the main beam is the largest.On this basis,appropriately reducing or increasing the height of the railing has a good suppression effect on the vortex vibration of the main beam,and the effect of reducing the height of the railing is better.Therefore,in engineering applications,the functional requirements and vortex-induced vibration performance can be comprehensively considered to select the optimal railing height.(5)After installing the railings,the average wind pressure coefficient on the surface of the main girder increases,and the fluctuating wind pressure coefficient changes complicatedly.The excellent frequency of pulsating wind pressure is basically the same as the self-resonance frequency of the model.The contribution of local lift to vortex vibration increases,which aggravates the vortex vibration of the main girder.The change of the railing height has no effect on the average wind pressure coefficient of the main beam surface,but has a great influence on the distribution law of the fluctuating wind pressure coefficient and the amplitude of the fluctuating pressure power spectrum.The change of the railing height makes the contribution of the local lift force in the front and rear areas of the upper surface of the main beam to the vortex vibration show a significant difference.When the contribution value increases,the vortex vibration response of the main beam increases.(6)By adjusting the blocking height at the bottom of the railing,the ventilation rate of the railing is changed.The research shows that within a certain range of the air permeability rate of the railing,although the change of the air permeability rate of the railing cannot completely suppress the vortex vibration of the main beam,it is beneficial to reduce the amplitude of the vortex vibration of the main beam when the air permeability rate of the railing is reduced,and the effect is better when the air permeability rate of the railing is low.