NON DESTRUCTIVE EVALUATION OF AXIAL LOAD IN TIE-RODS: THE CASE OF AN ANCIENT DOME

The knowledge of the tensile load in reinforcement tie-rods is of crucial importance for the structural integrity and safety in ancient buildings. This work deals with the problem of identifying the tensile load in the tie-rods of an ancient dome in Pistoia, Italy. The proposed procedure is a non-destructive and non-invasive method. It consists in matching the first six natural frequencies of the tie-rod, acquired by a classical accelerometer, with numerically obtained frequencies. This matching procedure is accomplished by an optimization algorithm in which the length of the rod, the presence of concentrated masses along it, and an elastic foundation at the edges are the optimization variables. In this way the main unknown, i.e. the axial load, is determined by a mathematical algorithm that automatically minimizes the difference between experimental and numerical results, on the basis of the choice of multiple parameters combinations. In particular, in this work we want to show a very interesting case study in which the method is successfully applied. An experimental campaign has been recently carried out on the Dome of Pistoia, Tuscany, see fig. 1, for the determination of the tension in the reinforcement rods. Several parameters have been considered, as the junctions, the constraints, and the presence of elastic supports. The estimation of the stress state in the tie-rods, which are from the 5th century, is carried out via a non-linear finite element analysis. Finally, considerations on the structural integrity of the rods and the safety of the buildings are drawn.