Induction-heat treated steel braces with intentional eccentricity

This paper presents a numerical and experimental study of a new steel brace design. The brace is a circular hollow structural section (HSS) of modified material properties by the induction heat (IH) treatment technology. The IH-treatment technology can increase the yielding stress of the steel to the desired value combining both the induction heating and quenching processes. In the proposed brace design, one-half of the section is treated by IH while the other one-half of the section maintains the original properties of conventional steel. The brace is designed with an intentional eccentricity along its length which inherently subjects the brace to bending moment in addition to axial forces. Based on the experimental data on steel braces with intentional eccentricity (BIE) and coupon tests of IH-treated steel, an accurate finite element model was created. A parametric study was conducted to evaluate the influence of major design parameters, such as the yielding stress ratio of IH-to-conventional steel, design configurations of brace-to-end plate connections and the bending direction with respect to the frame plane (i.e., out-of-plane and in-plane bending). Four IH-BIEs were tested quasi-statically under a standard cyclic loading protocol. Compared to the conventional design, IH-BIE provided up to a 60% lower elastic stiffness, reached a higher tension post-yielding stiffness equal to 20–30% of the elastic stiffness, while no strength deterioration was observed up to 2.0% story drift. The post-yielding stiffness depends on both eccentricity and IH-to-conventional yielding stress ratio, while fracture ductility can be further enhanced by special treatments at brace-to-end plate connections.