PSI - Issue 64

Carmelo Gentile et al. / Procedia Structural Integrity 64 (2024) 677–684 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. View of the Milan Cathedral (courtesy of Veneranda Fabbrica del Duomo di Milano ).

The Cathedral (Fig. 1) has a Latin cross-shaped layout (Fig. 2) and covers an area of over 10400 m 2 with a volume of about 300000 m 3 . Its main dimensions along the E-W and N-S axes are equal to 158 m and 66 m, respectively. The transept, about 49 m long (E-W) and 86 m wide (N-S), intersects the longitudinal limb and has a main nave and two side aisles. The crossing between the main nave and transept is capped with a main dome, surrounded by a prismatic structure with an octagonal base (i.e. the tiburio ) and supporting the Main Spire. The perimeter load-bearing walls are made of dry-stone masonry of varying thickness, with coatings in pink-veined white marble from the Candoglia quarries (i.e., the same material used for the spires and more than 3400 statues adorning the Cathedral). Another characteristic of the Cathedral is the presence of 122 metallic ties, which were permanently installed between each couple of adjacent columns to absorb the lateral thrust of the vaults (Veneranda Fabbrica del Duomo, 1885). The institution named Veneranda Fabbrica del Duomo di Milano (VFD) (Ferrari da Passano, 1973) was established in 1387 to manage all the aspects related to construction, maintenance, and restoration of the Cathedral. After the erection of the church main structures, the VFD focus shifted to architectural details and maintenance, implying ongoing inspections and structural interventions especially on the Main Spire (Nava, 1845) and on the piers supporting the tiburio (Ferrari da Passano, 1988). The development of a Structural Health Monitoring (SHM) strategy has been progressing (Gentile et al., 2019) since October 2018 to complement the inspection efforts of the VFD and streamline the preservation of the historical monument. This paper focuses on highlighting selected findings attained in the long-term dynamic monitoring of the Milan Cathedral. The evolution of natural frequencies is presented and discussed across a span of five years, with special emphasis being given to the impact of environmental factors (Ramos et al., 2010; Gentile et al., 2016; Masciotta et al., 2016; Ubertini et al., 2017; Kita et al., 2019; Alaggio et al., 2021). Furthermore, the seismic response is presented referring to two events occurred on December 17th, 2020, and December 18th, 2021, respectively. 2. SHM strategy and dynamic characteristics of the cathedral Electro-dynamic velocity sensors were installed on the capitals of 14 columns (Fig. 2, Gentile et al., 2019): the sensors placed on piers (94, 92, 90), (65, 67, 69), (22, 85, 84), (9, 74, 75), and (47, 48) are wired to five 24-bit digitizers. The number of seismometers is 27 and the velocity time series, acquired at a sampling frequency of 100 Hz, are transferred in real time to dedicated workstations, located in the VFD offices and in Politecnico di Milano. The raw data are stored in separate files of 1 hour (3600 s × 100 samples/s × 27 channels = 9720000 samples) and each file received from the monitoring system is first processed to compensate for the low-frequency attenuation of the sensors.