Continuous monitoring of the Milan Cathedral: dynamic characteristics and vibration-based SHM

The traditional collaboration between Politecnico di Milano and Veneranda Fabbrica del Duomo di Milano—the historic institution established by Gian Galeazzo Visconti in 1387 and having in charge all operational aspects related to the Milan Cathedral since more than 600 years—recently focused on the design and installation of a structural monitoring system, with the objective of assisting the condition-based structural maintenance of the historic church through the continuous interrogation of sensors installed in the structure and the extraction from measured data of features which are representative of the current state of structural health. The new monitoring system of the Milan Cathedral includes different types of measurements and sensors: quasi-static acquisition of strain in selected tie-rods and biaxial tilt of selected piers and the main spire, monitoring of inner and outer environmental parameters and dynamic measurement of the velocity response at the top of 14 piers and at 3 levels of the main spire. After a concise description of the historic church and of the monitoring system, the paper focuses on the dynamic characteristics of the Milan Cathedral, their evolution during the first months of monitoring (since October 16th, 2018) and the lessons learned in view of the structural health monitoring of the monument. The presented results from the vibration monitoring highlight that: (a) 8 global modes of vibration are automatically detected in the frequency range 1.0–5.0 Hz; (b) the resonant frequencies exhibit a distinctive trend of variation, which is mainly driven by temperature; (c) the mode shapes of the cathedral do not show appreciable fluctuations associated with the environmental effects.