Use of induction heating in steel structures: material properties and novel brace design

Induction heat (IH) treatment technology is a very efficient way to increase several times the strength of a selected part of steel elements. This paper presents an experimental investigation on the material properties of IH-treated steel elements and a novel application to steel braces. The IH treatment technology and manufacturing process are first reviewed and then, the new material properties obtained by a series of coupon tests and Vickers hardness tests are reported. Compared with the conventional steel, the IH-treated steel offers two-to-three times higher yield stress and tensile strength, but three times lower fracture ductility. The proposed steel brace is a steel tube with a partial strength enhancement in its cross-section. One-half of the section is treated by IH, while the remaining maintains the properties of conventional steel. The conventional steel part yields earlier and dissipates energy, whereas the IH steel part remains elastic until large deformation. An intentional eccentricity is also introduced along the brace length to magnify further the contrast of material benefits. The effective combination of the partial strength enhancement and eccentricity provides the brace with a beneficial multiphase response. The brace exhibits a high tensile post-yielding stiffness nearly equal to 20% of the initial stiffness and stably dissipates energy during cyclic loading up to 2.0% story drift by delaying the onset of local buckling.