Modal dynamic identification of historic San Michele bridge (Italy, 1889)

The San Michele Bridge, also known as Paderno d’Adda Bridge, stands as a striking example of 19th-century iron architecture and a prominent symbol of Italian industrial archaeology heritage. Constructed between 1887 and 1889 by the Società Nazionale delle Officine di Savigliano (SNOS), this impressive infrastructure spans over the Adda River, linking the towns of Paderno d’Adda and Calusco d’Adda, near Milano. Since its inauguration in 1889, the bridge has continuously served as a vital connection between the provinces of Lecco and Bergamo in Lombardy, northern Italy. Following nearly 136 years of service, the bridge underwent extraordinary restoration works from 2018 to 2020, ensuring up to now its continued functionality for both railway and road traffic. After that, from May 7 to 10, 2024, a dedicated experimental campaign was conducted on the bridge to investigate its current structural response under various dynamic excitations, including ambient vibrations with no traffic, dynamic response with vehicular transit and controlled train passages. During such a campaign, diverse acceleration data were collected and then analyzed with the aim of extracting the salient modal properties of the bridge. A comparison of the processed data with available records prior to bridge restoration revealed a rather good correspondence with the identified modal properties, within a time span of about 14 years. This seems to suggest that the recent restoration works may have displayed a minimal impact on the global dynamic behavior of the structure, in terms of modal features. The findings from this study shall provide valuable insights for future research developments and contribute to the ongoing deployment of a Structural Health Monitoring (SHM) platform for the San Michele Bridge, contributing to the long-term goal of monitoring and preserving this historic viaduct.