PSI - Issue 78

Elisa Tomassini et al. / Procedia Structural Integrity 78 (2026) 1831–1838

1837

Author name / Structural Integrity Procedia 00 (2025) 000–000

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structural configuration of the bridge can be considered particularly sti ff , a feature that strongly influences its dynamic response to external excitations, including seismic events. The bridge has been instrumented as part of a long-term structural health monitoring program. A total of 56 accelerometric channels are installed on the deck: each span hosts seven uniaxial MEMS accelerometers in the vertical direction, symmetrically placed at ¼, ½, and ¾ span along both edges, with an additional sensor at mid-span. A biaxial accelerometer is positioned to capture the deck’s longitudinal and transverse vibrations, and a triaxial sensor is located at the base of the western abutment. Furthermore, 14 accelerometers are mounted at the pier heads, and three are located on the abutment structure, bringing the total number of channels to 73. On April 23, 2025, at 23:31 CET, a seismic event of local magnitude M L = 3 . 6 occurred approximately 4 km north of Spoleto (see INGV - Istituto Nazionale di Geofisica e Vulcanologia). Given its proximity (approximately 2.5 km from the epicenter) the Spina Bridge was among the closest instrumented structures to the event. The acceleromet ric recordings associated with this earthquake were subsequently analyzed to assess the dynamic response of the bridge. Time histories from all available sensors are depicted in Fig. 5, with particular attention to the peak ground and structural accelerations. The highest recorded accelerations occurred at the pier head in the transverse (Y) di rection, reaching 0.0141 g, while longitudinal (X) components remained substantially lower at 0.0016 g. The vertical accelerations at deck level peaked at 1.0055 g, indicating a maximum deviation of 0.0055 g from the nominal gravita tional acceleration. These low-to-moderate levels of seismic excitation are consistent with expectations for a shallow, moderate-magnitude event and are well within the elastic capacity of the bridge. Finally, a verification was performed using control charts based on the residuals of the identified dynamic characteristics of the structure, with the ob jective of detecting potential outliers following the seismic event. The results confirmed that all indicators remained within the expected variance bounds, suggesting that no significant changes in the structural behavior occurred as a consequence of the earthquake. This outcome can be plausibly attributed to the inherently sti ff configuration of the structure, as the combination of low-height piers and short, wide spans tends to limit the bridge’s susceptibility to dynamic amplification. Moreover, the relatively low levels of recorded ground motion further support the assessment

Fig. 5: Seismic sequence on April 23rd near Spoleto, recorded by sensors on the Spina Bridge.