Structural Health Monitoring of a centenary iron arch bridge:1. Ambient vibration tests and condition assessment

Between June 2009 and March 2010, ambient vibration tests were performed at the San Michele bridge, which is a historic iron arch bridge, with a span of 266 m, completed in 1889 and still used as a combined road and rail bridge. These tests represented the first experimental survey carried out on the global characteristics of the bridge since the original static proof tests (1889-1892) and suggested the opportunity of installing a permanent dynamic monitoring system on the historic infrastructure with Structural Health Monitoring purposes. After a description of the bridge, this paper summarizes the main results obtained from the application of different output-only modal identification techniques to the data collected in the dynamic tests performed until March 2010, whereas a companion paper focuses on the dynamic monitoring system, the software developed to continuously process the recorded data and the results obtained in preliminary tests of continuous acquisition (November 2010 and June 2011).