Vibration-Based Assessment of the Tensile Force in the Tie-Rods of the Milan Cathedral

Continuous monitoring of the Milan Cathedral: dynamic characteristics and vibration-based SHM

“…The Milan Cathedral is one of the largest masonry monuments ever built. The construction of It is worth mentioning that the adoption of 122 iron tie-rods, still exerting an active role in resisting the lateral thrusts [9], is one of the distinctive characteristics of the Milan Cathedral:…”

“…During the assessment of the metallic tie-rods [9], quick preliminary investigation of the overall dynamic behaviour of the Cathedral was carried out using a very limited number of conventional (high-sensitivity) accelerometers. In those preliminary tests the sensors were (e) In mode C6 (f C6 =2.76 Hz, Fig.…”

“…During the recent assessment of the state of preservation and the tensile force of the metallic tie-rods of the Cathedral [9], the idea of implementing a Structural Health Monitoring (SHM) strategy has been taking shape and a monitoring system was designed [10] and implemented with the two-fold objective of providing the information needed for the condition-based structural maintenance and the creation of a large archive of experimental data useful to improve the knowledge of the monument.…”

“…The complexity and the large dimension of the church suggested the implementation of a monitoring system, that is fully computer based and easily expandable. The documentary research in the archives as well as the analysis of recent experimental information allowed to identify the sub-structures to be initially involved in the monitoring: (a) the structural elements which underwent important strengthening interventions, such as the piers [6] supporting the tiburio (i.e., the prismatic structure with octagonal base, which is built around the dome) and the main spire [1]; (b) the piers of the apse, as the absence of buttresses makes the apse region especially vulnerable in Gothic cathedrals; (c) selected piers of the transepts, because those regions are more exposed to the ambient vibration generated by the metro passages and the traffic in the neighbouring streets; (d) selected the tie-rods, which are subjected to high tensile stress (of the order of 100 MPa or higher) or affected by slight damage [9]. The designed monitoring system includes a relatively large number of sensors for measuring both static (15 bi-axial tilt-meters and 12 vibrating wire extensometers) and dynamic parameters (36 uni-axial seismometers).…”

“…5aare subjected to tensile stress exceeding 100 MPa, whereas the other tie-rods (orange colour inFig. 5a) are affected by slight damage[9];(b) 12 bi-axial tilt-meters (Figs. 5a and 5c), with a measurement range of ±0.5º and resolution of ±0.5 mm/m, located inside the Cathedral on the capital of piers 31 and 64 (façade), 69 and 90, 11 and 20 (transept), 74-75 and 84-85 (tiburio), and 47-48 (apse); (c) 3 bi-axial tilt-meters, installed at different levels of the main spire (+65.87 m, +74.99 m and +91.67 m, Fig.…”