Mechanical and metallographic characterization of iron tie-rods in masonry buildings: an experimental study

Historic metallic tie-rods are relevant structural elements in heritage masonry buildings, contributing to guarantee an efficient connection between walls and playing a decisive role in the control of horizontal thrusts (both permanent, as in the case of arches and vaults, and environmental, as in the case of seismic actions). However, few studies have conducted on the mechanical behaviour of metallic tie-rods in the last years and most of them are devoted to the identification of the axial stress. This results in rather unreliable models of these reinforcing elements, based on simplified constitutive laws (usually disregarding deformability issues, that play a fundamental role in modern seismic engineering) and on conventional material properties (usually extrapolated from those of modern ferrous alloys, or from some literature tests on XIX century steel bridges). At a bigger scale, this results in rather unreliable models of heritage buildings, leading to unreliable safety assessment procedures and unaware consolidation interventions (mostly replacements). The paper present the first results of an on-going study on the mechanical characterization of ancient iron tie-rods, based on experimental tests performed on a set of real tie-rods recovered from restoration works or building demolitions. Dealing with a structural ferrous material produced by bloomery smelting (an ironmaking process used in ancient items in Europe) and therefore potentially rich in inclusions and poor in alloying elements, tests of both metallographic and mechanical nature are introduced.