| In order to cross valleys,straits,rivers and lakes and other steep terrain,cable-stayed bridges have become the preferred type of long-span bridges,and π-type composite beam section is widely used in cable-stayed bridge engineering because of its light weight and convenient construction.However,due to the semi-open section of π-type girder,its torsional stiffness is low and its aerodynamic performance is poor,so vortex-induced vibration(VIV)may occur under frequent wind speed.Although VIV does not directly affect the aerodynamic stability and wind safety of the structure,it may bring hidden dangers to the operation comfort and driving safety of the bridge,and accelerate the fatigue failure of bridge components.Therefore,it is of great significance and practical engineering value to study the VIV characteristics of π-type composite beam section,put forward feasible vibration control measures and reveal its flow mechanism.In this thesis,the vortex vibration performance of the π-type composite beam section is studied by wind tunnel test of segmental model,and the vibration suppression effect of different aerodynamic measures is analyzed.On this basis,the computational fluid dynamics(CFD)numerical simulation method is used.The excitation mechanism of VIV and the flow mechanism of aerodynamic control of the lower stabilizer are analyzed,including the following aspects:(1)This thesis briefly reviews the development history of π-type composite girder cablestayed bridge,and introduces the VIV of the bridge from the aspects of basic theory,research methods and influencing factors.In addition,the typical cases of VIV of bridge structures at home and abroad are listed,and the research status of VIV of π-type composite beam is summarized.(2)Taking two typical long-span π-type composite girder cable-stayed bridges as the research objects,the wind tunnel tests of segmental models are carried out,and the definitions of allowable value of vortex vibration amplitude in domestic and foreign standards are compared and analyzed.Finally,the limit value of vortex vibration amplitude is determined according to the acceleration standard of " Wind-resistant Design Specification for Highway Bridges".The test results show that the two bridges have obvious vertical VIV at wind attack angles of 0°,-3°,and +3°.Among them,the vortex vibration interval of bridge 1 is about11～15m/s and 20～ 30m/s,the vortex vibration interval of bridge 2 is 7～10m/s,10～13m/s and16～29m/s,both of them have the maximum vertical vortex vibration amplitude when the wind attack angle is-3°.They are 174 mm and 309 mm,far exceeding the allowable value of the specification.(3)Aiming at the vertical VIV phenomenon of π-type composite beam section,the vibration control effects of various vibration control measures,such as lower stabilizer plate,flow suppression plate,guide plate and changing the structure form of railings were compared.The results show that the installation of the lower stabilizer plate can effectively reduce the vertical VIV of the main beam without changing the vortex vibration range.When the wind attack angle is 0°,the vibration control efficiency of two lower stabilizers is 76%,and that of three lower stabilizers is 83%.However,the improvement effect of other single aerodynamic measures on the vertical VIV of the main beam is poor,and the vibration control efficiency is less than 30%.Although the combined measures can effectively reduce the vertical VIV of the main beam,the engineering benefit is not high.(4)Based on the CFD software Fluent platform,UDF(User-Defined-Function)programming is used to numerically simulate the vertical vortex-induced vibration of the π-type composite beam section.Through the analysis of the flow field characteristics,pressure distribution characteristics and aerodynamic characteristics around the section,the vortex excitation principle and the vibration suppression mechanism of the lower stabilizer plate are studied.The results show that the vertical VIV of π-type composite beam is caused by the combined action of vortices in the upper surface,beam bottom and wake region.The vortex at the beam bottom is the main vortex structure with concentrated energy,and the unsteady aerodynamic force formed by the main vortex structure drifting backward along the lower surface is the main cause of VIV.After installing two lower stabilizing plates at the bottom of the beam,the air flow will be more stable and smoother when passing through the section,and a stable return flow is formed at the bottom of the beam without vortex drift,so that the pressure pulsation value of the lower surface is significantly reduced,thereby achieving an effective vibration suppression effect. |
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