Experimental methods for estimating in situ tensile force in tie-rods

Several methods have been proposed for estimating the actual tensile forces in metallic tie-rods inserted into the masonry arches and vaults of historic buildings. Static and dynamic methods based on the experimental measurement of the vertical displacements caused by concentrated loads, accelerations, and the fundamental frequencies and periods of free vibrations have been applied to arrive at a more precise evaluation of the axial tensile forces acting on the tie-rod under examination. Each of these methods, with its corresponding experimental procedure, presents different advantages and disadvantages with regard to operating procedures, equipment requirements, and accuracy range in the determination of tie-rod stress. In this paper the writers propose a simple method and experimental procedure based on a single static test. The reference structural system consists of a bending moment- resistant tie beam with unknown restraint conditions at its ends. Therefore, the proposed method does not require any assumptions when modeling the rod's extremities, nor does it discount the increase in pull induced by the transverse load as irrelevant. The relevant experimental data are represented by three vertical displacements under a concentrated load and the strain variations measured in three sections of the rod. The reliability of the method has been verified through laboratory tests using tie-rods set within a sufficiently stiff metal framework in which the tension is known through continuous monitoring by means of a load cell. The correlation of the measurements with the actual tensile strength was checked; laboratory tests show good agreement between the analytical estimates and experimental measurements.