## 2 Experiment Setup
### Structural Dynamic Parameters  
The main purpose in the present study was to explore the potential mechanism of classical flutter and postflutter LCO, which was induced by the different aerodynamic configurations of leading and trailing edges. Considering the convenience of controlling variables and reduction of experiment complexity, a basic rectangular section with different bluff or sharped leading edges, not a real bridge section shape, was chosen as a sectional model. Therefore, the scaling conditions of the 2D sectional model were not followed strictly with the real bridge structure. As a substitute, the main dynamic parameters, including damping ratio and vertical-torsional frequency ratio were designed to be similar with the real bridge structures and make sure that the classical flutter and nonlinear flutter in the tests had similar characteristics with the real bridge structures.  
For each case, vertical and torsional free vibration tests were carried out to identify structural dynamic parameters. The natural frequency was obtained by analyzing the frequency spectrum of the free vibration displacement based on the Fast Fourier Transform method. The damping ratio of the dynamic system was calculated via the logarithmic attenuation of the free vibration amplitude. The results of the structural dynamic parameters are listed in Table 2.  
In Table 2, for all the cases, it can be seen that the vertical and torsional natural frequencies of the dynamic system were about 3.77-3.80 Hz and about 4.40-4.50 Hz, respectively, which indicated the balance weights worked well enough to make the inertial mass and moment to be basically consistent for all cases with different aerodynamic configurations. The vertical and torsional damping ratios of the dynamic system were 0.25-0.43% and 0.25-0.34%.  
It is remarkable that the postflutter LCO is also sensitive to the nonlinearities of the mechanical system. In previous research, the structural nonlinearity of a spring-suspended dynamic system has been paid a lot of attention. For the spring-suspended sectional model, the structural stiffness and damping of the spring-suspended system were expressed as a nonlinear function of the vibration amplitude based on the equivalent linearization approximation method, which were identified by the free vibration tests [28; 32]. In the present paper, the same method was used to obtain the nonlinear structural damping ratio and frequency and an example of the identified results (case: L30) was displayed in Figure 4. It can be seen the variation of the natural frequency via the vibration amplitude was very small, which means the stiffness nonlinearity was weak enough to be ignored. Nevertheless, the nonlinearity of the damping ratio has to be taken into consideration in the nonlinear flutter analysis.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Case**} & \(f_{\text{H0}}\) & \(\xi_{\text{H0}}\) & \(f_{\alpha 0}\) & \(\xi_{\alpha 0}\) & \multirow{2}{*}{**Case**} & \(f_{\text{H0}}\) & \(\xi_{\text{H0}}\) & \(f_{\alpha 0}\) & \(\xi_{\alpha 0}\) \\  & **(Hz)** & **(\%)** & **(Hz)** & **(\%)** & **(Hz)** & **(Hz)** & **(\%)** & **(Hz)** & **(\%)** \\ \hline L30