Identification of tie-rods tensile axial force in civil structures

This paper is concerned with the problem of identifying the tensile axial force of metallic tie-rods used in civil structures. Tie-rods are generally used to transfer a tensile force between different structural elements. Specifically in civil structures, they are often used to unburden the lateral load exercised by the vaults and arcs on walls or columns, in ancient monumentalmasonry buildings as well as in modern concrete/iron constructions. A direct measure of this force is impossible and the tension is an unknown variable, as are the boundary conditions expressed by the constraints. Thus, in such a scenario, the characteristic equation has no analytical solutions. The paper proposes an experimental technique developed to identify, in situ, the tensile force on the rod. The identification method uses the first modal frequencies of the structure, identified by measuring the frequency response functions (FRFs) with instrumented hammer excitation. The frequencies are used to characterize some correlation indexes that change according to the variation in the tensile axial force of the rod. An FEM code numerical model allows to identify where the change in correlation indexes along the tie-rod could be greatest, in order to arrange a suitable measurement set up. Then the numerical model provides an evaluation of the tensile force corresponding to the set of correlation indexes identified. An estimation of the error in the identification of the force is given by a reasonable interval of stiffness of the constraints. The technique has been tested on laboratory system. © The Society for Experimental Mechanics, Inc. 2014.