## 2 Experimental Tests for the Dynamic Identification of a Bridge
### OMA Test  
In this section, the experimental results of the identification of the structural dynamic parameters of the bridge \"Interchange 12\" are reported. As already mentioned, with the aim of validating the proposed approach, a preliminary traditional OMA identification was performed. Using various accelerometers directly positioned over the bridge, the main frequencies of the structure were obtained. Specifically, eight high-sensitivity piezoelectric mono-axial accelerometers (PCB 393A03), were chosen to record the response of the bridge acquiring the vertical acceleration at various points of the structure. Further, all of the sensors were connected to the QuantumX MX840 module from HBM, which acquired the signal with a sampling rate of 600 Hz. Then, four accelerometers were symmetrically positioned according to the bridge's structural configuration, in the main span, and the other two were placed on each secondary span (see Figure 3). Data from the eight accelerometers were acquired and processed in the MATLAB environment. In particular, several tests were performed with the bridge under ambient vibrations for a total duration of 31 min. In order to investigate the bridge's response under various load cases, which determines the different levels of structural excitation, the acquisitions were carried out in three different cases:  
* Case 1: regular vehicular traffic;
* Case 2: heavy vehicular traffic;  
Figure 1: “Interchange 12” bridge. Figure 2: Front elevation and section of Interchange 12 bridge. Specifically, in Case 1, two ordinary cars made repeated passages on the bridge during the acquisitions. In Case 2, two vans were used to simulate a heavier vehicular traffic condition. Furthermore, the speed bump adopted in Case 3 makes the increase of the stress level possible, thanks to the impulsive load generated through the numerous passages of heavy vehicles over it. It should be noticed that, in all the three cases, an additional noise component is always present, and is given by traffic on the motorway below the bridge. Once the signals were acquired, a band-pass filter was used, choosing a band of frequencies between 0.2 Hz and 40 Hz. Then, the respective PSD functions were determined, following the Welch's Method [35], by partitioning the signal recorded into 186 sub-signals of 10 s. Finally, a Hanning window was employed, with an overlap of 50% between the segments. The results are reported in Figures 4-6. They are organized by separately showing the PSDs obtained from the data acquired through the accelerometers 1-4, positioned on the left sidewalk, and those positioned on the other sidewalk, namely accelerometers 5-8. Figures 4-6, as well as Table 1, show that the frequencies obtained in the three cases are consistent with each other. Specifically, the first frequency, at 3.0 Hz, can clearly be detected in all cases, with the largest discrepancy of 0.07 Hz occurring between Case 3 and Case 2. Moreover, the second frequency \(f_{b2}\) is clearly identified only through the data acquired by the accelerometers placed on the right sidewalk. This difference may be due to the correspondence of a torsional mode at this frequency. Finally, another frequency, referred to as \(f_{b3}\), is detectable at 9.4 Hz. It is worth to notice that, in this case, the discrepancies related to all cases are negligible. Figure 4: PSDs obtained from the acquisitions in Case 1; data from accelerometers positioned in the left sidewalk (L) and in the right sidewalk (R). Figure 3: (**a**) PCB 393A03 accelerometer; (**b**) position of the accelerometers on the deck. Figure 5: PSDs obtained from the acquisitions in Case 2; data from accelerometers positioned in the left sidewalk (L) and in the right sidewalk (R). Figure 6: PSDs obtained from the acquisitions made with the accelerometers in the structure in Case 3; data from accelerometers positioned on the left-hand sidewalk (L) and the right-hand sidewalk (R).