Monitoring an iconic heritage structure with OMA: the Main Spire of the Milan Cathedral

One of the most remarkable structural elements characterizing the Milan Cathedral is its Main Spire, built in Candoglia marble and completed in 1769. The Main Spire, reaching the height of about 108 m and supporting the statue of the Virgin Mary, is about 40 m high and stands on the octagonal tiburio erected around the main dome. The structural arrangement of the spire includes a central column which is connected through a spiral staircase to 8 perimeter columns and each column is stiffened by inverse flying buttress. Metallic clamps and dowels connect the marble blocks and metallic rods connect the perimeter columns to the central core. A large monitoring system was recently installed in the Milan Cathedral, including seismometers and temperature sensors at 3 levels of the Main Spire as well as a weather station at the top of the spire. After a concise historic background on the Main Spire and the description of the sensing devices installed in this structure, the paper focuses on the dynamic characteristics of the spire and their evolution during a time span of about 16 months. The presented results highlight that: (a) a high density of vibration modes is automatically detected in the frequency range 1.0-7.0 Hz; (b) the lower identified modes correspond to global modes of the cathedral; (c) the normal evolution in time of the resonant frequencies is characterized by clear fluctuations induced by the environmental effects (temperature and wind); (d) especially the dependence of resonant frequencies on temperature is very distinctive and reveals the key role of the metallic elements in the overall dynamic behavior; (e) notwithstanding the remarkable effects exerted by the changing environment on the resonant frequencies, output-only removal of environmental effects and novelty analysis allow an effective monitoring of the structural condition.