## 3 Wind Tunnel Tests  
### Experimental Details  
Wind tunnel testing is the most commonly employed method to investigate wind-induced vibration problems [39; 40]. This research experimented at the Experimental Center of Civil Engineering, Chongqing University, in a 15.0 m (long) \(\times\) 1.8 m (high) \(\times\) 2.4 m (wide) DC wind tunnel, as shown in Figure 4. The test section is equipped with a special device for dynamic testing of the bridge sectional model. Tests were performed in uniform flow with equal wind speeds across all locations in the wind tunnel. Wind speeds ranging from 0.50 to 35.00 m/s were continuously adjusted, with turbulence intensity less than 0.50% of the corresponding wind speed. Moreover, the airflow deflection angle, velocity field deviation, and velocity stability were less than 1.00%. To ensure that the test conditions are similar to the actual situation and to reduce the effect of friction on the incoming flow on the test wall of the wind tunnel, the tests were conducted at the middle height of the wind tunnel. To measure the vibration response of the sectional model, aerodynamic forces, and real-time wind speed in the wind tunnel, two laser displacement sensors [41], a high-frequency force balance [42], and a TFI series 100 cobra probe [43] were employed, respectively.  
The sectional model used in this investigation was selected from the middle section of the truss girder and was constructed to a scale of 1:55, with a length of 2.2 m, as shown in Figure 5. To ensure accurate experimental results, the sectional model was designed to match the actual aerodynamic shape of the bridge [44]. A spring-suspended sectional model (SSSM) system was developed to simulate vertical and torsional vibrations precisely, as depicted in Figure 6. The sectional model was suspended from the angle-of-attack turntable using eight springs through two steel brackets, with independent vertical and torsional displacements. Additionally, the aspect ratio of the sectional model was 4.0, and the end plates were not installed according to the Chinese design specification [45]. The  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Order Number** & **Vibration Mode \({}^{\star}\)** & **Frequency (Hz)** & **Vibration Mode (Front View)** \\ \hline
3 & 1-S-V & 0.211 & \\
4 & 1-AS-V & 0.237 & \\
8 & 2-S-V & 0.402 & \\
11 & 1-S-T & 0.478 & \\
20 & 2-AS-V & 0.571 & \\
25 & 1-AS-T & 0.706 & \\ \hline \hline \end{tabular} \({}^{\star}\)S: symmetric; AS: antisymmetric; V: vertical; T: torsional.  
\end{table}
Table 1: Primary vibration modes and frequencies of the suspension bridge.  
Figure 3: Three-dimensional finite element model.  
weight and distribution of the counterweight blocks were adjusted to achieve the required equivalent mass and mass moment of inertia. Adjusting the spring stiffness and spacing ensured the appropriate vertical and torsional vibration frequency and frequency ratio of the sectional model. Furthermore, the actual-to-experimental wind speed ratio was reduced by increasing the spring stiffness, which was 4.48 in this test. The wind speed for the force test was set to 10.0 m/s, while the wind speed for the vibration test was gradually increased from 0 to 10.0 m/s, with a wind loading range of 0.2 m/s. When VIV occurred, the wind loading range was reduced to 0.1 m/s. Table 2 shows the critical design parameters of the SSSM system.  
Figure 4: DC wind tunnel at Chongqing University.  
Figure 5: Construction and completion stage sectional model. (**a**) construction stage. (**b**) completion stage.  
Figure 6: The SSSM system used in the tests.