Ambient vibration testing and operational modal analysis of a historic tower

An accurate knowledge of the dynamical parameters of structures is definitely useful for seismic assessment and for the design of risk mitigation interventions. In this paper, the opportunities provided by dynamic identification techniques for the non-destructive evaluation of heritage structures are discussed with focus on the bell tower of Announziata (Corfù, Greece), a masonry tower, which shows a high damaged scenario and, consequently, a high vulnerability to dynamic and seismic forces. The paper presents the experimental investigations and operational modal analysis results, useful for defining the finite element model of the tower. The monitoring system consists of several elements properly connected: in total twenty-four accelerometers have been positioned, eight for each of the three floors and oriented according to the orthogonal directions x and y. This configuration has been also conditioned by many operative problems about the position of the instrumentation due to the limited accessibility of the structure, not only to the main access but also to reach the top. It is important to emphasize that the data obtained are not connected to external events detected during the acquisitions, so it is possible to identify with a certain confidence the first six frequencies of the tower and their corresponding mode shapes.