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

One of the distinctive characteristics of the Milan Cathedral, built over a period of more than 400 years, is the presence of iron ties under the vaults of all 5 naves. Those tension bars date back to the age of construction and still have an important role in supporting the lateral thrust exerted by vaults and arches. After the recent failure (November 2011) of one tie-rod, an extensive research program was carried out to assess the structural condition of all iron ties. The investigation included the characterization of the metallic material, the direct inspection of the ties and the evaluation of the axial force in the structural elements through dynamic testing and system identification. After a concise description of the Milan Cathedral, this article presents the adopted methodology for the dynamic assessment of the tie-rods in the Milan Cathedral and the main results of the investigation, in terms of fundamental frequencies and axial forces in the metallic elements. Subsequently, the splitting of the first resonant frequency observed on two ties is presented: this anomalous phenomenon (which is occasionally reported in the literature as associated to the response of cracked reinforced concrete beams) is addressed and its correlation with the occurrence of local damage is discussed.