## 5 Test Results and Interpretation
### Modal Parameters of the Bridge Superstructure  
The modal parameters of the bridge are identified for all the performed AVTs (25 tests) and on the basis of the acceleration measurements, adopting the covariance-driven stochastic subspace identification (SSI-COV) technique [32]. The latter is an output-only identification method working in the time domain that assumes the excitation sources are characterized by a flat spectrum (i.e., the input is considered as a white noise). Each of the 25 performed identification procedures foresees the use of a stabilization diagram through \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Load Conf.**} & \multicolumn{2}{c}{**Longitudinal Displ. (Y) [mm]**} & \multicolumn{2}{c}{**Transverse Displ. (X) [mm]**} \\ \cline{2-5}  & **EXP** & **NUM** & **EXP** & **NUM** \\ \hline C1 & 3.1 & 4.0 & 0.1 & 2.9 \\ C2 & \(-\)11.8 & \(-\)17.4 & 3.2 & 1.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison between the experimental and numerical displacements of the isolator (I). Figure 14: Air and road surface temperature variations during the proof test procedures. which modes are identified once they are recognized to be stable. In this work, a vibration mode is assumed to be stable if the frequency variation (\(\Delta f\)) between two subsequent model orders is less than 1%, the damping ratio variation (\(\Delta\xi\)) is less than 2%, and the modal assurance criterion (MAC) is greater than 0.99. The stabilization diagram of the Dyn.1 test is reported in Figure 15; the first singular value (SV) is also reported for completeness and to highlight the peaks that characterize each vibration mode of the unloaded bridge. The results of the Dyn.1 test (relevant to the unloaded bridge) are taken as reference for the interpretation of the dynamic test outcomes. In Figure 16, the first six vibration modes that were identified for the unloaded bridge are reported, together with the relevant resonant frequencies and damping ratios. The first three modes represent the first bending, the first torsional, and the second bending mode of the deck, respectively. The fourth and fifth modes are both typical of the second deck torsional mode, even if the mode shapes are also characterized by a significant bending of the edge spans that, in turn, governs the response of the lateral spans more than the torsion. Finally, the sixth mode is the typical third bending deck mode. Figure 17 shows the evolutions of the frequency values and mode shapes (the latter expressed through the MAC indexes that were evaluated with respect to the unloaded configuration) for the first three vibration modes during the whole loading tests, which foresees a cyclic loading and unloading of the bridge. Moreover, in Table 5, the identified resonant frequencies and damping ratios that were relevant to dynamic tests of greater interest (from Dyn.1 to Dyn.5) are listed. Figure 15: Example of stabilization diagram that was obtained for the identification procedure of the Dyn.1 test. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mode**} & \multicolumn{2}{c}{**Dyn.1**} & \multicolumn{2}{c}{**Dyn.2**} & \multicolumn{2}{c}{**Dyn.3**} & \multicolumn{2}{c}{**Dyn.4**} & \multicolumn{2}{c}{**Dyn.5**} \\ \cline{2-10}  & \(f\) [Hz] & \(\xi\) [\%] & \(f\) [Hz] & \(\xi\) [\%] & \(f\) [Hz] & \(\xi\) [\%] & \(f\) [Hz] & \(\xi\) [\%] & \(f\) [Hz] & \(\xi\) [\%] \\ \hline
1 & 0.85 & 0.96 & 0.69 & 1.16 & 0.85 & 1.45 & 0.84 & 1.5  
The trends of modal parameters, reported in Figure 17, can be correlated with the bridge loading procedures to investigate the effects of loads (and hence masses) on the bridge dynamic response. The reference frequency value, relevant to the unloaded bridge, is identified for each mode and highlighted with a horizontal dashed line in Figure 17. By gradually loading the bridge up to the C1 loading configuration (reached around 10:00 a.m.), a decreasing frequency trend is evident.