## 5 Test Results and Interpretation
### Results of Impact and Braking Load Tests  
ILTs and BLTs were performed with the aim of characterizing the bridge dynamics under a higher level of vertical and horizontal excitations that were provided to the structure. Obviously, these tests are not used to identify the modal parameters of the bridge with conventional methods because the effects of the excitation that were provided to the structure quickly vanish and hence the recorded accelerations cannot be adopted neither into output-only algorithms (too short signals) nor into input-output procedures (input provided by the truck was not measured). However, they may be very useful tests for investigating the frequency dependency on the acceleration amplitude. To this purpose, the time-frequency analysis was performed in this work through the short time Fourier transform (STFT) algorithm. The methodology provides information on the variation of the frequency content of the analyzed signals with time. Figure 19 shows the acceleration time histories and the STFT obtained from the vertical accelerations registered by accelerometers that were placed at the mid-span (V1 and V2) and at the north edge span (V3 and V4) during the truck impacts in ILT 2. As for the STFT, a light color (yellow in this work) represents the maximum frequency content of the signal, i.e., a probable resonant frequency, while a dark color (blue in this work) identifies the minimum frequency content. From the time histories of Figure 19a, relevant to the truck passing slowly over the bump, there are four impacts that are clearly evident, corresponding to the four truck axes, and consequently, in the relevant STFTs, vertical yellow bands rapidly vanishing with time can be observed. Conversely, horizontal yellow bands are always visible in correspondence \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{3}{*}{**Mode**} & \multicolumn{6}{c}{**Frequency [Hz]**} \\ \cline{2-7}  & **U** & & **C1** & & **C2** \\ \cline{2-7}  & **EXP** & **NUM** & **EXP** & **NUM** & **EXP** & **NUM** \\ \hline
1 & 0.85 & 0.81 & 0.69 & 0.63 & 0.84 & 0.81 \\
2 & 1.15 & 1.12 & 0.91 & 1.06 & 1.15 & 1.12 \\
3 & 1.31 & 1.34 & 1.19 & 1.20 & 1.24 & 1.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison between experimental and numerical frequencies during loading phases. of the resonance frequencies of the bridge and remain constant over time (marked with dash-dot lines). The horizontal yellow lines (resonant frequencies) are evident considering the STFT that was obtained when the truck passes over the bump with a constant speed (Figure 19b); however, in this case, it is not easy to individuate the impact applications from the acceleration time histories. In all cases, the non-dependence of the bridge resonant frequencies on the vibration amplitude indicates a linear elastic behavior of the bridge in the observed amplitude range. This phenomenon was observed also for ILT 1. It is worth mentioning that the accelerations recorded during the ILTs ranged between about 1.5-0.3 \(\times\) 10\({}^{-2}\) g, values that are much greater than those that were measured during AVTs (that were of the order of about 10\({}^{-4}\)-10\({}^{-5}\) g). The accelerations that were recorded from BLTs can be analyzed in the same manner. In Figure 20 the time histories and the relevant STFTs for the accelerometers measuring in the longitudinal direction are reported. From the time histories, the effects of the two truck breaks, producing high oscillations of the bridge in the longitudinal direction for a time period of about 20 s, are clearly evident. The relevant STFTs reveal the typical horizontal  
Figure 19: Acceleration time histories and relevant STFTs during the ILT 2: (**a**) slow truck passage over the bump and (**b**) passage with constant speed. light-yellow lines as in the case of ILTs.