Effects Of Sectional Details On The Aerodynamic Performances Of A Streamlined-like Box Girder

Wide-body flat steel box girders, which are considered to be streamlined-like, have been widely applied to long-span suspension bridges. Due to their advantages in easing traffic pressures, this kind of griders have become a new trend in the development of Highway Bridges. Founded by the national natural science fund project “Aerodynamic characteristics and wind-induced vibration(VIV) characteristics of wide flat steel box girder bridge across theYangtze River in Three Gorges Reservoir Area”(51578098), this paper has studied the coefficients of static loadings and vortex-induced vibration performances of a flat box girder with the slenderness ratio 12/1.Firstly, stationary wind tunnel tests(1:60) were conducted to mearsure static forces and moment at different attack angles ranging from-12°~+12°. Representative cases were calculated by two-dimensional(2D) computational fluid dynamics(CFD) in Fluent 15.0 code, streamline and stream-wise mean pressure distributions around the deck were compared to figure out the influence mechanisms of sectional details on static forces and moment.Moreover, dynamic wind tunnel tests were conducted to determine whether vortexinduced vibrations tack place in the designed cases or not. The VIV critical wind speed, lock-in range and oscillation amplitude were observed and analyzed. Then, a User Defined Function(UDF) code was developed based on the Newmark-? equation. Combining the UDF code with dynamic meshing method, 2-degrees(both vertical and torsional) responses of the deck under designed conditions were solved in Fluent 15.0. Besides, the oscillation amplitude histories were transferred by Fast Fourier method(FFT) to obtain their harmonics.The vortex shedding patterns based on the numerical results and harmonics based on FFT were analyzed to figure out the influence mechanisms of sectional details on VIV performences.Main research contents and results are as follows:(1) The effects of attack angles on static forces and moment mainly attribute to the differences in range and intensity of the flow separations. While the effects of attack angles on VIV response should be ascribed to the dominating of fundamental frequency(both vertical and torsional), as well as the vortex shedding patterns.(2) The effects of guardrail ventilation rate on static loadings and VIV response were analyzed. Flow separations and reattachments, as well as positive and negative pressure zones, vary with the ventilation rate changes, which result in the differences of static forces and moment. On the other hand, both vertical and torsional VIV response were amplified as the ventilation rate decreased. The former one resulted from the gradually formed “2S” pattern of wake vortex sheddings, the latter one mainly attributed to the rapidly developed vortexs in the windward middle of both upper and lower surfaces(3) The effects of guardrail location on VIV response were studied. It indicted that, the vertical VIV response was decreased while the torsional response was amplified as the guardrails moved inward, which should be respectively ascribed to the degraded wake vortex shedding(ie. “2S” pattern to “S” pattern) and the rapidly developed vortexs on the windward upper surface.(4) The effects of inspection rails location on static loadings and VIV response were investigated. As the inspection rails moved inward, both static forces and moment were decreased in the case of positive attack angles. However, in the case of negative attack angles, neither static forces nor moment has a significant fluctuation. On the other hand, both vertical and torsional VIV response were reduced as the the inspection rails moved inward, which should be respectively ascribed to the degraded of wake vortex shedding and the windward lower surface vortex shedding.(5) The effects of deck surface roughness on static loadings and VIV response were investigated. The effects on static forces and moment lied in the differences of flow separations and surface pressures, and an oppsite trend was found in cases of positive and negative attack angles. Besides, as deck surface roughness increased, both vertical and torsional fundermental frequencies have showed a tiny fluctuation. Hence, both vertical and torsional responses were decreased at positive attack angles, while they were amplified at negative attack angles.