Environmental effects on the dynamic characteristics of a historic cathedral

The maintenance and preservation of the Milan Cathedral is traditionally performed through well-established and time-scheduled programs of visual inspection and architectural restoration of surfaces, decorations, and statues in Candoglia marble. On the other hand, the structural condition assessment and preservation turns out to be a challenging task due to the dimensions and complexity of the building, the usual uncertainties on the material properties and also the difficulty in inspecting several structural elements. Therefore, a structural monitoring system was recently designed and installed in the Milan Cathedral to assist the condition-based structural maintenance of the monument. The monitoring system includes different sensing technologies to allow appropriate tracking of different long-term structural behavior. The dynamic monitoring of the horizontal response of selected piers is complemented by the static monitoring of the tilt of the same piers and of the strain in selected tie-rods. In addition, the indoor and outdoor environmental parameters are extensively measured as well. After a concise historic background on the historic monument and the 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 of the first year 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. In more details, the presented results highlight that: (a) 8 global vibration modes are automatically detected in the frequency range 1.0-5.0 Hz; (b) the variations observed in the resonant frequencies are mainly driven by temperature, with the effect of thermal changes being very peculiar; (c) the mode shapes and the related mode complexity do not exhibit appreciable fluctuations associated to the environmental changes, so that an appropriate strategy of SHM should be based also on the time invariance of those parameters.