## 3 Wind Tunnel Test Setup  
The wind characteristics at the bridge site under different oncoming wind speeds at 0\({}^{\circ}\), 30\({}^{\circ}\), and 180\({}^{\circ}\) wind directions were comprehensively investigated in this study. Additionally, the influence of different wind field conditions, including uniform and turbulent wind fields, on the wind characteristics at the bridge site was also examined. When the oncoming wind is uniform, the oncoming wind speeds \(U_{0}\) at each wind direction are 6 m/s, 8 m/s, 10 m/s, 12 m/s, and 14 m/s, respectively. However, the oncoming flow cannot always be uniform in practice, as buildings and vegetation in the actual terrain are considered to affect wind speed. To render the research results more relevant to engineering, roughness elements are also placed in the wind tunnel to conduct a comparative study of the wind characteristics at the bridge site under turbulent atmospheric boundary layer (ABL) wind field conditions. The wind profile and turbulence intensity profile of the ABL inflow in the empty wind tunnel (taking the case of \(U_{0}=10\) m/s as an example) is shown in Figure 3.  
The wind tunnel test was conducted within the low-speed test section of the STU-1 wind tunnel laboratory. The test section is 4.4 m wide, 3.0 m high, and 24.0 m long, with a turntable diameter of 3.0 m. The wind speed range in the test section is 0 to 30 m/s, with a turbulence intensity of less than 0.4%. A circular area with a diameter of 2.4 km around the midspan of Jinqi Bridge was selected as the terrain range. To prevent the formation of artificial cliffs, a sloped terrain transition section was used, which provided a good transitional effect on the wind characteristics, allowing the wind to maintain essentially the same properties as the oncoming wind when it reaches the terrain [23; 24]. The transition section is 600 m long, and the total diameter of the model is 3.6 km. A scale ratio of 1:1000 was chosen to seat the terrain model  
Figure 2: Terrain and wind direction schematic diagram. onto the turntable. A circular plywood board with a diameter of 3 m was crafted to serve as the base of the model. Foam boards with a thickness of 2.5 cm were used to layer and cut the terrain until the highest location within the terrain range is reached. A layer of gypsum was evenly applied to the surface of the completed model and painted green. The wind speed data were monitored by a Series 100 J-Cobra probe, which was manufactured by Turbulent Flow Instrumentation Pty Ltd., an Australian company. This Cobra probe has a wind speed measurement range of 2 to 100 m/s, a measurement accuracy of \(\pm\)0.5 m/s, a data collection frequency of 10,000 Hz, an output frequency (\(f_{s}\)) of 625 Hz, and a data collection time of 60 s. The experimental equipment is capable of meeting the requirements for the measurement of wind speed range and the necessary precision. Two J-Cobra turbulence meters were employed in the experiment. The simultaneous measurement of wind speeds at two points in each set of conditions is allowed, which enables a better understanding of the coherence between the two sets of wind speed data. A movable measuring frame was used to suspend the J-Cobra turbulence meters. The movable measuring frame was installed within the test section, which primarily consists of two rigid supports on either side and a central horizontal bar. Bolt and sleeve connections are present at the joints, allowing for the vertical and horizontal movement of the J-Cobra turbulence meter, thereby obtaining wind speed data at different monitoring points on Jinqi Bridge. When the oncoming wind direction is 0\({}^{\circ}\), the final model installations for uniform and ABL oncoming wind are shown in Figure 4a,b, respectively. A total of 7 monitoring points were arranged along the main beam at locations \(x/L=0\), \(1/6\), \(1/3\), \(1/2\), \(2/3\), \(5/6\), and \(1\).