Structural Health Monitoring (SHM) is a promising approach for assisting the condition-based structural maintenance of heritage buildings, via the continuous analysis of diagnostic data collected by a network of sensors. Within this context, vibration-based SHM is receiving increasing attention and a limited number of applications on heritage structures is documented in the scientific literature. In most cases, the experimental setup implemented in historic monuments consists of few sensors, so that natural frequencies are adopted as damage indicators and the detection of anomalies should include the minimization of the temperature effects on identified frequencies. On the other hand, when a reasonably well distributed grid of sensors is available, the monitoring of mode shapes seems a promising perspective to ease and improve the damage assessment procedure.

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.

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 and on the monitoring of environmental parameters. Subsequently, the results collected in various 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 modal behaviour of the building.