Automated modal identification of a historic bell-tower

Continuous monitoring of the structural response under ambient excitation is especially suitable to Cultural Heritage structures because of the fully non-destructive and sustainable way of testing, that is performed by just measuring the dynamic response under ambient excitation and does not involve additional loads rather than those associated to normal operational conditions. Within the context of vibration-based monitoring of historic masonry structures, the paper presents the development of an automated procedure of modal parameters estimation and tracking, as well as its application in the continuous dynamic monitoring of a masonry bell-tower. The proposed algorithm of modal parameters estimation is based on the interpretation of the stabilization diagram associated to parametric identification methods and consists of three key steps aimed at: (1) filtering a high number of spurious poles in the stabilization diagram; (2) clustering the stabilization diagram; (3) improving the accuracy of the estimates. The developed procedure is exemplified using the data collected on the bell-tower of the church of San Gottardo in Corte in Milan. Following the practice adopted by the authors for masonry towers, a simple dynamic monitoring system is installed in the tower: the monitoring system includes two bi-axial seismometers (electro-dynamic velocity transducers), one 24-bit digitizer (6 channels, A/D converter, 8 Gb Ram on board for data storage) and one UMTS modem for data transfer. After a concise presentation of the developed automated algorithm, the paper focuses on the results obtained in several months of continuous monitoring of the tower.