Continuous monitoring the modal deflections of the Milan Cathedral

The Milan Cathedral is the most iconic symbol of Milan and is world-wide known for being one of the largest Heritage monuments ever built. Since October 2018, a large monitoring system is active in the monument with the main objective of providing the information needed for the condition-based structural maintenance; furthermore, the availability of large amount of experimental data will be conceivably useful to improve the knowledge of the historic building. The monitoring system of the Milan Cathedral includes different sensing technologies to allow appropriate tracking of different long-term structural behaviour. The dynamic monitoring of the horizontal response of selected piers and along the main spire is complemented by the measurement of the quasi-static tilt of the same sub-structures, whereas the static monitoring of strain in a certain number of tie-rods gives a direct evaluation of the structural condition of those elements (as the tensile force and stress in the tie-rods was previously evaluated). Since especially the temperature might affect the variation of structural parameters in historic masonry structures, the indoor and outdoor environmental parameters are extensively measured, as well. The paper is mainly aimed at presenting selected results from the dynamic monitoring of the Milan Cathedral. It should be noticed that the dynamic response of the structure is continuously collected in a reasonably well distributed measurement grid (27 channels of data), so that mode shapes-based methods could be conveniently used for anomaly detection and localization. Those methods conceivably exhibit advantages because mode shapes contain also local information on the structural behaviour and might be less sensitive than resonant frequencies to exogenous factors. After a concise historic background on the historic monument and a description of the dynamic monitoring system installed in the Milan Cathedral, the paper focuses on the dynamic characteristics of the monument, that were identified in the first hours of continuous monitoring, Subsequently, the results collected in the first two years of dynamic monitoring are presented and discussed, with special attention being given to the influence of environmental parameters on the variations observed in the resonant frequencies and mode shapes. It is worth noticing that the modal deflections and the mode complexity do not exhibit appreciable fluctuations associated to the environmental changes, so that the possibility of using the time invariance of those parameters to address the Structural Health Monitoring is discussed and investigated.