PSI - Issue 78

Ana Avramova et al. / Procedia Structural Integrity 78 (2026) 1633–1640

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3. Reference modal parameters and monitoring results During the preliminary dynamic tests performed before the installation of the monitoring system (Garcia-Fernandez et al., 2024), 9 vibration modes were identified for the bridge at study in the frequency range 0 – 7 Hz. On the other hand, the normal traffic conditions existing at the beginning of continuous monitoring allowed us to successfully identify 14 vibration modes with good identification rate. In more details, 7 vertical bending modes and 7 torsion modes were identified: it should be noticed that the lower two series of four bending modes (B 1 -B 4 in Error! Reference source not found. ) and four torsion modes (T 1 -T 4 in Error! Reference source not found. ) are perfectly corresponding to theoretical expectations for a continuous beam with 4 spans. The mode shapes summarized in Error! Reference source not found. were obtained by applying the SSI-Cov method to the dataset collected on 08/06/2024, h 16:00 and were subsequently used as references in the MT process. The time evolution of natural frequencies in the investigated monitoring period − between May 20 th , 2024 and May 18 th , 2025 − is illustrated in Error! Reference source not found. a, whereas the corresponding changes of air temperature (approximately ranging between -3°C and +37°C) is shown in Error! Reference source not found. b. The inspection of Figures 4a and 4b clearly reveals that automatically identified natural frequencies exhibit clear daily and seasonal trends, both of which can mostly be attributed to temperature changes. (a) (b)

Figure 4. Time evolution during the first year of monitoring of (a) identified natural frequencies and (b) measured air temperature.

Table 1. Statistics of the identified modal parameters during the first year of monitoring (from 20/05/2024 to 18/05/2025). Mode f avg (Hz)  f (Hz) f min (Hz) f max (Hz) f 2 −T MAC avg  MAC MAC min B 1 1.412 0.010 1.390 1.451 0.663 0.9999 0.0002 0.9904 T 1 1.683 0.019 1.642 1.759 0.906 0.9912 0.0123 0.9015 B 2 1.863 0.013 1.837 1.916 0.772 0.9996 0.0004 0.9886 T 2 2.118 0.023 2.073 2.204 0.903 0.9937 0.0083 0.9023 B 3 2.409 0.017 2.376 2.473 0.878 0.9996 0.0005 0.9861 T 3 2.643 0.027 2.593 2.740 0.893 0.9823 0.0177 0.9012 B 4 2.737 0.016 2.701 2.801 0.812 0.9979 0.0032 0.9275 T 4 2.952 0.025 2.896 3.040 0.888 0.9955 0.0076 0.9018 B 5 4.825 0.025 4.767 4.948 0.874 0.9992 0.0008 0.9797 T 5 5.169 0.041 5.082 5.325 0.873 0.9828 0.0150 0.9028 B 6 5.528 0.031 5.451 5.671 0.862 0.9961 0.0031 0.9152 T 6 5.817 0.046 5.701 5.999 0.860 0.9760 0.0180 0.9002 B 7 6.355 0.057 6.230 6.604 0.850 0.9929 0.0062 0.9225