Continuous dynamic monitoring of the Milan Cathedral: influence of environmental changes on the dynamic characteristics

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 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. Different sensing technologies were adopted in order 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 selected tie-rods gives a direct evaluation of the structural condition of those elements (as the tensile force in the tension bars was previously evaluated). Since both the long-term static behaviour and the time evolution of dynamic signatures of historic masonry structures are expected to be sensitive to environmental changes, 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. After a concise description of the historic church and of the dynamic monitoring system, the paper focuses on the dynamic characteristics of the cathedral, their evolution during 3 years of monitoring and the lessons learned in view of the structural health monitoring of the monument.