## 5 Test Results and Interpretation
### Modal Parameters of the Bridge Superstructure
In particular, the first and second modes reached frequency values that were around 20% lower than those of the unloaded configuration, while the third frequency reduced about 9%. The frequency values remained stable up to 11.15 a.m. since the C1 load was maintained for more than 1 h. At 11.30 a.m. frequencies returned to the initial values after the removal of the C1 load, demonstrating the absence of permanent damage to the structure. A similar trend can be also noticed during C2 loading procedures and mostly for the third mode, while it was not so evident for the first and second ones. This may be explained by the different position of the added masses on the deck. Indeed, the masses in the central span contribute more to the bridge dynamics with respect to those on the lateral ones, as confirmed by mode shapes. At the end of the load tests, the frequencies of the first three modes were almost the same of those that were identified at the beginning, providing useful information on the health status of the structure after the tests. The MAC indexes remained almost always constant and close to one, demonstrating that mode shapes were not sensibly affected by the masses that were added on the deck. Only for the third mode (during C2 loading procedures) did the MAC indexes assume  
Figure 16: Experimental and numerical modal parameters of the bridge during the Dyn.1 test. lower values; however, they were always greater than 0.6, meaning that the mode shapes remained quite similar. The experimental results were compared with the numerical ones that were obtained by the FE model that was described in the previous section. At first, the unloaded bridge is considered, and the numerical modal parameters, that were obtained from a classical eigenvalue analysis, show a very good agreement with the experimental ones, as can be noted from the frequency and mode shape comparisons shown in Figure 16. In particular, the first three modes show a very good agreement both in terms of frequency and mode shapes; the latter are clearly visible in Figure 18 from the MAC indexes matrix. Finally, it is interesting to observe that the fundamental experimental frequency is slightly higher than the numerical one, revealing that the real bridge is slightly stiffer than expected, which is considered to be a positive outcome in the framework of proof testing. Figure 17: Frequencies and mode shapes of the first three vibration modes during the test day. Figure 18: MAC index matrices for the comparison between the experimental and numerical mode shapes. As for higher modes, it must be recognized that it was not possible to properly individuate numerically the fourth and fifth experimentally-identified modes. Indeed, they are both similar to the fourth numerical model, which represents the typical second torsional mode of the deck. The latter seems to be more similar to the fifth experimental one in terms of frequency, and to the fourth experimental one in terms of MAC index. According to above considerations, the fourth and fifth experimental modes are both compared with the fourth numerical mode, and further investigations on the reason of the presented issue will be discussed in the subsequent sections, addressing the soil-foundation-superstructure possible interactions. Finally, the sixth numerical mode is quite similar to the sixth experimental ones, especially in term of frequency (difference of about 5%), whereas the mode shapes are quite different to each other, with a MAC index rather low. The bridge numerical model was also used to interpret the frequencies that were obtained from AVTs that were performed on the loaded bridge. To this purpose, the FE model was loaded with masses that simulate the truck presence over the deck during C1 and C2 loading configurations. In these cases, the comparison between the experimental (EXP) and numerical (NUM) modal parameters was performed considering only the first three modes, which demonstrated to be the most robust ones.