Flat Box Girder Research Articles

Influences of Attack Angles on Aerodynamic Derivatives and Flutter Characteristics of Flat Box Girders

The flutter response of Nanjing Fourth Bridge flat box girder under different wind attack angles was tested in detail through sectional model test. The evolution of unsteady and steady critical amplitude with wind speed was discussed. Based on the amplitude envelope of the flutter response and the Hilbert transform, the amplitude- dependent modal damping of the system is identified, and the mechanism of the flutter mode change in the wind angle of attack is initially explained. Secondly, the modal parameters of the system under different wind attack angles were extracted. Based on the bimodal coupled flutter analysis method, the nonlinear flutter derivatives of the section under different wind attack angles were identified, and the change law of the amplitude dependence of the key flutter derivatives on the wind attack angle and the potential influence on the flutter morphology and characteristics of the section were studied. Finally, the effect of wind attack angle on uncoupled and coupled aerodynamic damping is analyzed by deconstructing the modal damping one by one, and the dynamic mechanism of the differential flutter performance caused by fractional damping is illustrated.

A Quick Assessment and Optimization Method for a Flutter Aerodynamic Measure of a Typical Flat Box Girder

Flutter is one of the most serious wind-induced vibration phenomena for long-span bridges and may cause the collapse of a bridge (e.g., the Old Tacoma Bridge, 1940). The selection and optimization of flutter aerodynamic measures are difficult in wind tunnel tests. It usually takes a long time and consumes more experimental materials. This paper presents a quick assessment and design optimization method for the flutter stability of a typical flat box girder of the long-span bridges. Numerical analysis could provide a reference for wind tunnel tests and improve the efficiency of the test process. Based on the modal energy exchange in the flutter microvibration process, the global energy input and local energy input are analyzed to investigate the vibration suppression mechanism of a flat steel box girder with an upper central stabilizer. Based on the comparison between the experimental and numerical data, a quick assessment method for the optimization work is proposed. It is practical to predict the effects of flutter suppression measures by numerical analysis. Thus, a wind tunnel test procedure for flutter aerodynamic measures is proposed which could save time and experimental materials.

The Aerodynamic Characteristics of Road Vehicles Overtaking on Bridge Deck under Crosswinds

The aerodynamic characteristics of road vehicles in windy environments are the prerequisites for the evaluation and prediction of the driving safety and stability. To investigate the aerodynamic characteristics of the overtaking vehicles on the bridge deck under the effects of crosswinds, models of a cable‐stayed bridge with a typical flat box girder and road vehicles involving articulated lorry and commercial van with a scale of 1 : 40 were tested in the wind tunnel laboratory. A series of tests figured out the variation of the aerodynamic forces of road vehicles during the overtaking process after considering the aerodynamic interference between lorry and lorry, van and van, and lorry and van. Additionally, the influence of the lateral overtaking distance between the overtaking vehicles was regarded as well. The result reveals the upstream vehicle has a significant influence on the aerodynamic coefficients of the downstream vehicle, which have experienced dramatic fluctuations during the overtaking process, and the various shapes of the aerodynamic coefficients are highly dependent on it.

Effects of geometrical parameters on the aerodynamic characteristics of a streamlined flat box girder

The effects of aspect ratio (B/D, width to height ratio), wind fairing angle (θ) and relative height of wind nose (H/h, h and H are the distances from the wind nose to the upper and lower surfaces of the girder, respectively) on the aerodynamic characteristics of a streamlined flat box girder were systematically investigated through wind tunnel experiments. A total of 21 cases were tested with B/D = 6, 9 and 12, θ = 45°, 55°, 65° and 75°, and H/h = 1, 2, 3 and 4, which covered most of the typical large span bridges in use. The Reynolds number based on the freestream oncoming velocity U∞ and model height D was 3.3 × 104. The experimental results show that the effects of B/D, θ and H/h are negligible at small wind attack angle (α) with |α| < 4°. The aerodynamic performance of the girder is very sensitive to these geometric parameters at |α| > 4°, which is associated with the states of the flow around the girder. With the increases of |α|, the flow around the girder may transfer from trailing edge vortex shedding (TEVS) to impinging leading edge vortex (ILEV) and leading edge vortex shedding (LEVS). Generally, the girder with H/h = 2–3 and larger B/D and θ corresponds to larger range of α with TEVS, which is favorable to the wind resistance stabilization of the girder.

Investigation and Control of VIVs with Multi-Lock-in Regions on Wide Flat Box Girders

On the preliminary designing of a wide flat box girder with the slenderness ratio 12, vertical and torsional vortex-induced vibrations (VIV) are observed in wind tunnel tests. More than one lock-in region, which are defined as“multi-lock-in regions,”are recorded. Therefore, suspicions should be aroused regarding the viewpoint that wide box girders are aerodynamic friendly. As the three nascent vortexes originating at the pedestrian guardrails and inspection rails shed to near-wake through different pathways with different frequencies, the mechanisms of VIVs and multi-lock-in regions are analyzed to be determined by the inappropriate subsidiary structures. A hybrid method combiningLarge Eddy Simulation(LES) with experimental results is introduced to study the flow-structure interactions (FSI) when undergoing VIVs; the vortex mode of torsional VIV on wide flat box girders is defined as “4/2S,” which is different from any other known ones. Based on the mechanism of VIV, a new approach by increasing ventilation rate of the pedestrian guardrails is proved to be effective in suppressing vertical and torsional VIVs, and it is more feasible than other control schemes. Then, the control mechanisms are deeper investigated by analyzing the evolution of vortex mode and FSI using Hybrid-LES method.

Aerodynamic Admittance Research on Wide-body Flat Steel Box Girder

Aerodynamic admittance is a key parameter affecting the analysis accuracy of the bridge buffeting response. However, few articles have covered the issue of the wide-body flat box girder with much smaller depth-width ratio (such as 1/12 studied in this paper). Therefore, to explore the real buffeting force of the wide-body flat box girder, experiments of the static force coefficients and aerodynamic admittance are carried out in wind tunnel. Wooden segmental model has a scale ratio of 1/60 to an actual 42 m wide suspension bridge girder. Using the high-frequency-force-balance (HFFB) equipment fixed with the segmental model under the conditions of different wind speeds and different wind attack angles, wind power spectrums of the buffeting force are measured and the variation of aerodynamic admittance parameters is analyzed. The results show that the aerodynamic admittance of the box girder measured in the experiment with this so much smaller depth-width ratio differs from the corresponding classical Sears expression. Some inspiration can be presented for the future study of the buffeting response for this kind of much wider bridge with the similar ratio in this paper.

The Static Wind Load Research of a Flat Box Girder Based on the Numerical Wind Tunnel

In this paper, the wind field around the flat box girder of a long-span bridge under 0o attack angle was investigated by the numerical wind tunnel technique, which can not only get the distributions of the pressure, velocity and vortex in the flow field, but also obtain the various aerodynamic parameters of the bridges. The velocity profiles were obtained, and the coefficient of tri-component from the numerical simulations was in good agreement with that from the wind tunnel test, which demonstrated that it was reliable and feasible to utilize the numerical wind tunnel technique to simulate the wind field and certificate the coefficient of tri- component of the bridge.

Large Eddy Simulation of Aerodynamics of a Flat Box Girder on Long-span Bridges

The wind field around a box girder of a large span cable-stayed bridge, under Reynolds number of Re=1.27×105 in uniform flow, is computed by using three-dimensional large eddy simulation (LES). The LES employs the Smagorinsky subgrid turbulent model. The averaged drag, lift and moment coefficients, as well as the Strouhal number at various angles of attack are obtained. The research shows that the vortex shedding period changes with time, and vortices do not shed synchronously in span-wise direction, while the vortex shedding energy concentrates on a narrow band of frequency. The distribution of mean and root-mean-square value of pressure around the girder surface is also provided, as to represent the flow separation and reattachment pattern on the girder surface. The present results are compared with results from wind tunnel test. It is confirmed that the LES is capable of investigating the aerodynamics of bridge girders and flow mechanism.

Aerodynamic simulation by CFD on flat box girder of super-long-span suspension bridge

The aerodynamic behavior of two self-oscillating sections are predicted by nonlinear k− ε model. One is the parallel rectangular cross-section ( B/ D=15; 2 d= B′; B: total width; D: height; B′: single width; 2 d: space of the sections) and another is the flat box girder cross-section. The section has the special feature that the box girder is separated and the vertical and the horizontal stabilizers are installed. At first, in order to investigate the applicability of the numerical method, the calculated results of aerodynamic behavior were compared with the experimental data for the parallel rectangular cross-section such as the priority test. Secondly, the method was applied to the flat box girder cross-section, and the aerodynamic instability mechanism on the stabilizer was examined from the calculated results of the flow pattern and the unsteady pressure characteristics.