PSI - Issue 84

Francesco Nigro et al. / Procedia Structural Integrity 84 (2026) 183–190

188

Mode 1

Mode 2

Mode 3

Mode 4

Mode 5

Mode 6

Mode 7

Mode 8

Mode 9

Mode 10

Mode 11

Mode 12

Mode 13

MAC

Fig. 6. Reference mode shapes of the bridge identified through the accelerations acquired on April 14 th 2025 between 7:00 and 8:00 a.m. The colormap refers to the vertical displacements.

It should be observed that the first three modes represent global structural responses, as they involve the entire bridge decking and the piers. More specifically, mode 1 represents the fundamental global flexural mode, whereas modes 2 and 3 correspond to higher-order global flexural modes. As the modal order increases, a more complex bending pattern occurs, still maintaining a high degree of inter-span coupling. Conversely, a significant portion of the higher-order modes is characterized by vibrations confined to specific spans, while the remainder of the structure remains nearly motionless, as indicated by the deep blue regions in the Z displacement plots. In particular, modes 4, 5, 6, and 7 exhibit a clearly dominant torsional response localized within individual spans.Furthermore, modes 8, 10, and 12, as well as modes 9, 11, and 13, display combined higher-order flexural and torsional components localized near the saddles of the central spans. These modes are characterized by significant displacements concentrated in the drop-in spans, while still showing a non-negligible interaction with adjacent spans. This behaviour suggests a localized modal response that nonetheless induces partial participation of the surrounding structural elements, likely due to the specific boundary conditions and structural connectivity in these regions. Nevertheless, as reported in Table 1, the frequencies of similar local modes in spans 2 and 3 (modes 8–9 and 10– 11) differ by approximately 1 Hz. This discrepancy can plausibly be attributed to the presence of the retained saddle