Operational modal testing of historic structures at different levels of excitation

The paper focuses on the dynamic tests of two historic structures, performed in operational conditions at different levels of ambient excitation. The tests were aimed at checking the invariance of the dynamic characteristics (i.e. natural frequencies and mode shapes) with increasing excitation amplitude. A masonry bell-tower, dating back to the XVI century, is firstly considered and the modal parameters of key vibration modes were identified from the data collected in two ambient vibration tests: in the first test, the excitation was only provided by micro-tremors and wind, whereas in the second test a higher level of excitation was induced by swinging of bells. The second structure is a well-known iron arch bridge, erected in 1889 and still used as a combined road and rail bridge. Within the dynamic tests performed on the historic infrastructure since June 2009 to address a Structural Health Monitoring program, the bridge response at selected points was continuously recorded for several hours, in order to check the variation of the dynamic characteristics under different traffic conditions.