Study About Wind (Rain) Effects And Vibration Control Of Flexible Structural Members In Long-Span Bridge

With the continuous improvement of construction technology and application of new engineering materials, bridge structures develop towards large span and light weight. This also makes the bridge girders and its structural members tend to be flexible and more sensitive to the dynamic actions of wind. At the same time, the strong wind are usually companied by sunstantial rain. Consequntly the combined effects of rain and wind may cause notaible effects on structures. However, due to the complex coupling effect of wind, rain and structure, at present, there is little research about the effect of wind driving rain to the structure.In this paper, the wind driving rain effect and the vibration control method of the typical bridge’s slender flexible members are investigated. Based on the comprehensive review and analysis of the existing literature on the above topic, this paper makes an intensive study. The detailed work and outcome from this study are as follows:(1) a detailed summary of the characteristics of the natural rainfall was induced, including the rainfall intensity、diameter distribution of raindrops、the end point velocity and so on. A theoretical analysis of the effect of the wind driving rain on the main girder was carried out from the angle of the change of the air density、the impact force of rain drops and the angle of the rain film. Analysis shows that the horizontal force of wind driving rain is mainly reflected by the horizontal impact force, the vertical force is mainly reflected by the vertical impact force and the weight of the additional water, air density changes caused by rainfall can be ignored. The wind driving rain load to a double side drainage rectangular main girder whose size is close to actual main grider was analyzed, when the wind speed is 10m/s~20m/s, the impact load in heavy rain condition can increase 10%, compared with the drag force caused by wind only, but the effect is reduced in the high wind speed. According to the analysis of the rainfall’s acting on the main grider, the addition force caused by rainfall can mitigate the main grider’s dynamic response caused by wind.(2) A simplified calculation method of the impact force of raindrops based on the equivalent of the drop diameter was adopted. The comparison result between this method and theoretical calculation indicates that the error of the vertical and horizontal impact force is +8% and-4%, respectively, this method is very accuracy. Based on this method, the complex rain intensity distribution can be described by a uniform diameter and can be applied to analyze the effect of rainfall rapidly.(3) Aiming at the shortcomings of the existing rainfall scaling laws, rainfall intensity’s simplified similarity of main girder between wind tunnel test and actual wind driving rain condition was established from the aspects of the impact of rainfall, water thickness and quality and appearance, these scaling laws can become the experimental foundation for the wind driving rain experiment.(4) The experimental platform of wind driving rain was built in HD-2 wind tunnel of Hunan university, by use of this wind tunnel, wind driving rain experiments of several typical bridge sections were implemented, the rainfall’s influence rules about the steady-state aerodynamic cofficients、vortex induced vibration and flutter of the Girder section were gotten. The experimental results show that the impact of rainfall and surface water are the key factors to influence the static and dynamic response of structure, the measured value of the impact force is close to that of theoretical analysis, the correctness of the calculation model used for the impact force of raindrops was verified according to the experiment. The rain drops usually slightly increase the total damping of the structure, which is beneficial to reduce the response of the vortex vibration, but effect to the flutter stability of the system is negligible.(5) Aiming at the vortex induced vibration and galloping which can occur in the flexible and bluff hangers of the long span steel arch bridge easily, a new type of eddy current TMD(tuned mass damper) in the form of cantilever beam is developed, which can be used to suppress the hanger’s vibration. Its outstanding characteristic is that the cantilever beam is used as the elastic element and the non contact eddy current is used as the damping element. The fatigue property of the beam can be ensured by limiting the stress of the beam’s root. TMD’s vibration reduction principle and parameter optimization method were elaborated, the key point of the research is amplitude estimation of the mass after a single TMD divided into multiple TMD, the research can provide design parameters for the fatigue design of elastic element.(6) Vibration reduction test of eddy current tuned mass dampers was implemented, the dampers was installed in the hangers of the steel arch bridge in Xiasheng railway. The field test results show that the first order vibration modal damping ratio of the hanger is about 3%, which is more than 3 times of the same structures. In addition, the vibration frequency of the hanger calculated by full bridge model is more close to the measured value of hanger’s vibration frequency than the value calculated by single hanger model.(7) Aiming at the complex wind-induced vibration problems which can occur in the hoist cable(consist of multiple wireropes) in long span suspension bridge, a method for reducing hoist cable’s vibration was proposed, that is the damper on the joint of the separator, vibration reduction mechanism and parameter optimization method of this type of measure was elaborated. According to the hoist cable’s complete aeroelastic model test、damper’s shaking table test、numerical analysis and field measurement, this type of reduction method was successfully applied to the actual bridge engineering, the application can verify that the method is effective.