PSI - Issue 84

Ana Avramova et al. / Procedia Structural Integrity 84 (2026) 560–568

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(a)

(b)

Fig. 8 Hotelling's T 2 control chart for: (a) VI03 (Olona East) viaduct and (b) Olona West (VI04) viaduct.

5. Structural Health Monitoring As previously stated, a classic SHM strategy based on natural frequencies is currently applied: to minimise the influence of EOVs, a subset of vibration modes with the highest identification rates was selected for PCA. Specifically: (a) to define the PCA-based regression, the first year of monitoring was assumed as training period; (b) to maximize the occurrence of damage-detection indicators, the PCA analysis is carried out adopting 11 (out of 12) identified modes for the VI03 viaduct and 12 (out of 14) for VI04; (c) only 1 Principal Score has been retained in the PCA-based regression model. The effectiveness of the minimisation and removal of EOVs can be proven by comparing the time evolution of the natural frequencies in Fig. 5 and the cleansed natural frequencies in Fig. 7. Since the cleansed frequencies (Fig. 7) still exhibit small scattering, the T² control chart is used to further assess the presence of changes in the monitored system by comparing the statistical properties of each newly acquired 8 hour subgroup of residuals to those established during the training phase. If the T² values remain below the upper control limit (UCL), the system is considered to be operating normally. If the UCL is consistently exceeded, it may indicate the presence of a structural anomaly. In this case, the control charts presented in Fig. 8 (with the UCL corresponding to the 99th percentile of the values occurring during the training period), indicate no permanent exceedances and support the conclusion that no significant anomalies have affected the Olona East (VI03) and Olona West (VI04) viaducts during the presented monitoring period. 6. Correlation between the dynamic characteristics of the investigated bridges Given the structural similarity of the bridges at study and the strong correspondence between most of identified mode shapes, the correlation between the dynamic characteristics of the Olona viaducts deserves a more detailed investigation, which is summarized in Errore. L'origine riferimento non è stata trovata. in terms of average natural frequencies, related discrepancies and coefficients of determination. As shown in Errore. L'origine riferimento non è stata trovata. , the natural frequencies of VI04 are systematically lower than those of VI03, with relative differences ranging from about − 1.5% to − 9%. This trend aligns with the structural geometry of the bridges: the larger central spans of VI04 result in increased flexibility and lower natural frequencies, with the largest frequency reductions being related to the lower bending (B 1 ) and torsion (T 1 ) modes as well as to bending mode B 5 . Errore. L'origine riferimento non è stata trovata. also highlights that most of the coefficients of determination between the modal frequencies of the two bridges are very high (≥ 0.8 5), indicating an almost linear relationship between the dynamic characteristics of the structures as well as the possibility of accurately predicting the frequency variations in one bridge from the knowledge acquired on the other bridge.