Continuous monitoring the cathedral of Milan: Documentary and preliminary investigations

Within the traditional collaboration between Veneranda Fabbrica del Duomo di Milano and Politecnico di Milano, significant efforts were recently devoted by the authors to the critical re-analysis of the past issues experienced by the Cathedral of Milan and to the evaluation of recent experimental evidences, with the objective of designing and implementing a new monitoring system, aimed at assisting the condition-based structural maintenance of the building. Appropriate strategies of Structural Health Monitoring have been developed for 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 conceptual design of the monitoring system is presented and discussed in the paper, as well as the documentary and preliminary investigations carried out to address the main choices. In summary, the new monitoring system of the Milan Cathedral includes different classes of measurements and sensors, such as: (a) quasi-static strain acquisition (through wireless vibrating wire extensometers) in the metallic tie-rods subjected to high tension loads; (b) quasi-static measurements of the biaxial tilt at the top of selected piers and along the height of the main spire (through innovative wireless tilt-meters, with high accuracy and compensation of the temperature effects); (c) quasi-static measurements of environmental parameters (through temperature and humidity sensors placed in the same points where the tilt-meters are installed); (d) dynamic measurement of the velocity at the top of selected piers and along the height of the main spire. The monitoring system is characterized by a distributed architecture, allowing easy modifications and/or adding of sensors; furthermore, all quasi-static sensors are wireless and powered through high capacity batteries.