Continuous monitoring of a challenging heritage tower in Monza, Italy

A recent survey of the historic complex of “Santa Maria del Carrobiolo” in Monza (Italy) highlighted that two sides of the bell-tower are directly supported by the load-bearing walls of the apse and South aisle of the neighbouring church. After the discovery of the weak structural arrangement of the building, a network of 10 displacement transducers, integrated by five temperature sensors, was installed in the tower to check the opening variation of the main cracks. Subsequently, ambient vibration tests were performed and closely spaced modes with similar mode shapes were clearly identified: since the dynamic characteristics of the tower are quite different from those obtained in past experimental studies of similar structures and conceivably related to the construction sequence, a simple dynamic monitoring system was installed in the tower to complete the health monitoring aimed at the preservation of the historic structure. The paper—after a brief description of the tower and a summary of selected evidences provided by on-site survey, historic research and static monitoring—focuses on the dynamic characteristics identified in the preliminary ambient vibration tests and the main results of 1-year dynamic monitoring. In order to assess the effects of changing temperature on the natural frequencies of the investigated tower, especially in view of the removal of those effects needed for an effective performance assessment, simple correlation studies between modal frequencies and temperature are presented and discussed.