PSI - Issue 84

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

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in the second span, which emphasizes the structural discontinuity introduced at this joint. Consequently, the local modes associated with span 2 exhibit lower natural frequencies. The integration of the stability analysis with long-term tracking, following established SHM protocols (Rainieri and Fabbrocino, 2014), implemented within the “P3P” software (García-Macías et al., 2022), confirms that the identified dynamic parameters are stable and physically consistent. 3.2. Frequency tracking To assess the structural integrity over time, a continuous frequency tracking procedure was implemented. To ensure reliability of the identified parameters and to filter out spurious modes, the procedure incorporates the following key stages: stabilization diagrams, to identify the “stable poles” (Magalhães and Cunha, 2011); the automated pole selection, based on a set of clustering criteria depending on frequency tolerances and the Modal Assurance Criterion (MAC) (Allemang, 2003); modal tracking, to monitor the structural evolution post-intervention. Figure 7 illustrates the temporal evolution of the natural frequencies in relation to the recorded ambient temperatures.

Fig. 7. Frequency tracking and temperature recordings between 13 th May and 13 th November 2025

As expected, Fig. 7 shows that the seasonal thermal variations result in a more negligible influence in terms of lower modal frequencies, while higher frequencies present more pronounced variations. The absence of irregular shifts in the tracked frequencies (once environmental effects are accounted for) confirms the reliability of the baseline modal model for future condition assessments. 4. Concluding remarks The present study reports on the first six-month dynamic monitoring results obtained for a Gerber-type bridge, whose original half joints were structurally fixed as a result of a major structural strengthening intervention.