A hybrid method for the non-destructive evaluation of the axial load in structural tie-rods

The knowledge of tensile load in reinforcement tie-rods is of crucial importance to assure structural integrity and safety. This work addresses the problem of identifying this tensile load in structural tie-rods of ancient masonry arches and vaults. The proposed procedure is a nondestructive 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 optimisation algorithm in which the length of the rod, the presence of concentrated masses along it and an elastic foundation at the edges are the optimisation variables. In this way, the main unknown, i.e. the axial load, is determined by a mathematical algorithm that automatically minimises the difference between experimental and numerical results on the basis of the choice of multiple parameter combinations. A 'local zoom technique' is adopted around a parameter set, which gives a minimum, in order to determine the load with good approximation. A deep investigation about the stress state in the tie-rods is also carried out via a nonlinear finite element analysis