Strong crosswinds may cause safety problems, such as shaking, derailment and overturning of travelling trains. Apart from the limitation on the operating speed and the cancellation of train trips in strong-wind scenarios, wind barriers have also been used to mitigate the negative effects of crosswind on trains, and are considered an effective and economical way to improve the safety of travelling trains. However, for the bridges, this approach may in return cause additional wind loads associated with the aerodynamic effects acting on the wind barriers, which are transferred to the bridge. In order to ensure the safety of both travelling trains and the bridge, it may be required for the wind barrier to allow more or less wind to pass depending on the wind pressure level such that the wind load transferred to the structure can be reduced.
Wind tunnel tests and numerical modelling have been conducted to investigate the associated parameters to understand the aerodynamic performance of wind barriers and trains. For example, the effects of airflow speed, train speed and wind directions on the aerodynamic performance of a train with a rigid wind barrier were investigated through wind tunnel tests. It was concluded that a scenario with a static train is more critical than those with moving trains in terms of the mechanical loads on the train caused by the crosswind; meanwhile, the effect of airflow speeds on the aerodynamic coefficients of the train is insignificant [6]. In order to investigate the effect of the relative angle between the crosswind and the train (i.e., the yaw angle) on the mechanical responses of the train–barrier–bridge system, a numerical model of a railway system supported by a steel truss bridge was established using a computational fluid dynamics (CFD) method, where the simulation results were compared with those from wind tunnel experiments. It was suggested that airflow with a yaw angle of 90° (i.e., perpendicular to the travelling direction of the train) would lead to the maximum side force and overturning moment on the train [13].
According to the results from wind tunnel experiments on a train–barrier–bridge system [14,15], a taller wind barrier with a lower porosity may provide a greater reduction in the mechanical load on the train caused by the crosswind, and therefore provide better shielding effects; however, the side force and overturning moment transferred to the bridge from the wind barrier are increased. Therefore, the design of wind barriers (e.g., porosity and height) can introduce different effects on the aerodynamic performance of trains and bridges. In strong wind conditions, where train trips may be cancelled, or even during the time there are no trains travelling on the bridge, such shielding effects become unfavourable since adverse wind loads and the associated aerodynamic effects would be acting on the bridge. Several railways are supported on bridges [8]; therefore, the safety consideration for both travelling trains and supporting bridges with wind barriers needs to be addressed. Recently, louver-type wind barriers with adjustable blades were introduced [10,16], where the porosity of the wind barriers can be adjusted by changing the incline angle of the blades. For example, during regular conditions (i.e., without a strong crosswind), the wind barriers can be given a low porosity (achieved by putting the blades in closed mode) and better shielding effects can be provided to mitigate the wind effects on the train; meanwhile, during the presence of a strong crosswind, where the train trips are cancelled or during their intermission, the wind barriers can be adjusted to have high porosity (i.e., by opening the blades) and therefore cause a lower load to be transferred to the bridge. Obviously, such louver-type wind barriers require adjustment of the blades, where this would become time-consuming and hard to operate if completed manually, or would have a high cost and may not be reliable in harsh weather conditions (such as a strong wind) if completed automatically.
In this study, an innovative concept of a wind barrier structure to achieve such an adjustment that allows wind to pass was proposed using an adaptive method. This was done by using a wind barrier with an appropriate bending stiffness. When subjected to a strong crosswind, the wind barrier may deform as bending due to the transverse wind load, where the deformed wind barrier will naturally let wind pass through the deformed shape. Traditional wind barriers made from steel and concrete are associated with a high bending stiffness and the deformation when subjected to transverse wind is minor. In this study, glass-fibre-reinforced polymer (GFRP) composites were proposed for such wind barriers, where their elastic moduli ranged from 5 to 35 GPa [17,18], resulting in much lower structural stiffness than steel and reinforced concrete. Such GFRP composites have been introduced for the construction of railway noise barriers [19] and bridge decks [20]. However, the concept of a wind barrier that can allow wind to pass in an adaptive manner using deformation has not been examined yet, especially because such deformation caused by a wind load is associated with complex aerodynamic behaviour under different wind conditions.
Experimental and numerical studies were conducted on the aerodynamic responses of a train–barrier–bridge system under a crosswind, where GFRP composites were used as the material for the wind barrier. In the wind tunnel experiments, a scale ratio of 1:40 was used for the models of the bridge and the train, as was used in [16] for a better comparison between the results of the proposed wind barrier and existing barrier types. A lateral crosswind at different speed levels was applied perpendicularly to the train–barrier–bridge system. The barrier heights ranged from 0 cm (no barrier) to 13.5 cm. The influences of the barrier height, airflow speed and location of the train on the aerodynamic responses of the system were studied. Based on the parameters of the airflow in the wind tunnel and the dimensions of the experimental models and the wind tunnel, both the reduced- and full-scale finite element (FE) models of the train–barrier–bridge system were established using finite element approaches, and the results were compared with the experimental results. The variations in the results from reduced- and full-scale models were also evaluated to validate the effectiveness of the reduced-scale experiments for predicting the aerodynamic behaviours of full-scale applications.
2. Material Properties of GFRP
GFRP materials have been used in bridge superstructures [21] and building structures [22,23,24] for their superior resistance to corrosion, high strength-to-weight ratio and competitive cost [17]. In this study, the wind barriers were made of GFRP plates with a thickness of 0.6 mm, which was chosen by considering the reduced scale and the bending stiffness requirements. The aerodynamic performance of the adaptive wind barrier is closely related to the elastic modulus of the plates, thus tensile tests on the GFRP plates were conducted.
The GFRP plate was prepared through a wet layup approach using epoxy resin and two layers of woven GFRP fabrics, resulting in a total thickness of 0.6 mm. Five sample specimens with dimensions of 250 × 25 mm2 were cut from the same GFRP plate that was used for the wind barrier in the wind tunnel tests. Aluminium alloy plates with a length of 50 mm each were adhesively bonded to both ends of each GFRP specimen to protect the clamping area at both ends. As shown in Figure 1, two perpendicular strain gauges were installed at the centre of each GFRP specimen to obtain the strains in the longitudinal and transverse directions under loading. Tensile tests were conducted according to ASTM D3039 [25] for the five specimens. The load was applied using an MTS machine (MTS, Eden Prairie, MN, USA) at a loading rate of 2 mm/min until the failure of the specimens. The tensile loads and strain results were continuously recorded at 2 Hz during the loading process.
