Estimating the Tensile Force in Ancient Metallic Tie-Rods from Vibration Tests

One of the distinctive characteristics of the Milan Cathedral 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.During the maintenance interventions preceding EXPO 2015, almost all the tie-rods of the cathedral underwent visual inspection, geometric characterization and hammer tests to evaluate their dynamic characteristics. After a brief summary of the experimental procedures, the paper focuses on presenting and exemplifying the structural identification approach adopted in the vibration-based assessment of the tensile force in the metallic ties. The presented procedure differs from the one proposed by Tullini and Laudiero (2008) since: (a) the Young's modulus of the metallic material is assumed to be unknown and (b) the resonant frequencies of higher modes are used to solve the inverse problem.The application of the proposed procedure is mainly exemplified with reference to the tension bars exhibiting the higher stress level (larger than 100 MPa) as those elements are now permanently instrumented with vibrating wire extensometers. Selected results from the long-term monitoring of the tie-rods are briefly discussed as well.