Chapter7 Identification of Tie-Rods Tensile Axial Force in Civil Structures S. Manzoni, M. Scaccabarozzi, and M. Vanali Abstract 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 monumental masonry 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. Keywords Tie-rod • Tensile axial load • Dynamic identification • COMAC • Structural health monitoring 7.1 Introduction In many civil structures, both historical and modern kind, the tie-rods, or tie-beams, represent a useful and common tool to balance the lateral force at the base of arches and vaults. The possibility to measure the axial load acting on them is definitely a great help in order to monitor the overall health of the structure in which they are placed. Due to structural settlements, the tensile force in the tie-rods could change in time. An overload applied on one of them can cause the tie-rod to exceed the elastic range of the material which compounds the beam. This state could yield displacement larger than those bearable by the static balance of the structure. Moreover, the beam might not support any increase in load or, in a worst-case scenario, it could suffer a mechanical failure. This is particularly dangerous in the event of any environmental forcing such as earthquakes or wind action. Conversely, due to any constraint yielding, the work of a tie-rod may be ineffective in ensuring structural equilibrium. This would yield subsidence in the static attitude of the whole structure. Often, in ancient or historical buildings, chemical corrosion can play a role in decreasing the strength of tie-rods. In any case, monitoring change in time of a tie-rod’s tensile force could provide timely warning of any change in the equilibrium of the structure. The only way to have an accurate measure of the axial load may be to fix a strain gauge or a load cell on the beam before tensing it. Obviously, this requirement is impossible to have on the ancient tie-rods of a historical building. Without this condition, there is no non-destructive technique for direct in situ measurements of the tensile force on the beam. In order to S. Manzoni ( ) • M. Scaccabarozzi Dipartimento di Meccanica, Politecnico di Milano, Via La Masa 1, Milano 20156, Italy e-mail: stefano.manzoni@polimi.it; matteo.scaccabarozzi@polimi.it M. Vanali Dipartimento di Ingegneria Industriale, Università di Parma, Parco Area delle Scienze, 181/A, 43124 Parma, Italy e-mail: marcello.vanali@unipr.it A. Wicks (ed.), Structural Health Monitoring, Volume 5: Proceedings of the 32nd IMAC, A Conference and Exposition on Structural Dynamics, 2014, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-04570-2__7, © The Society for Experimental Mechanics, Inc. 2014 59
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