Long-term dynamic monitoring of the historic “San Michele” iron bridge (1889)

The San Michele bridge is an iron arch bridge that crosses the Adda river about 50 km far from Milan. The bridge, built in 1889, is the most important monument of XIX century iron architecture in Italy and is still used as a combined road and railway bridge. In order to assess the structural condition of the bridge, ambient vibration tests were performed in 2009 and suggested the opportunity of installing a permanent dynamic monitoring system with Structural Health Monitoring purpose. Hence, a continuous monitoring system was designed and it is now active on the bridge since late November 2011. The paper, after a brief summary of the experimental studies developed since 2009, describes the monitoring system and the software developed in LabVIEW for processing the collected data. Subsequently, the tracking of natural frequencies automatically identified until the beginning of September 2012 is presented and the effects of environmental and operational conditions on the dynamic response of the bridge are investigated.