Development of Novel Buckling-Restrained Steel Braces Enabled by Partial Induction Hardening

A new type of buckling-restrained brace (BRB), called induction-hardened buckling-restrained brace (IHBRB), is proposed in this study. The IHBRB uses a round bar as the core steel and a hollow round section as the restrainer. The core steel undergoes partial induction hardening, which makes its strength lower in the middle but higher at the two ends. This strength difference along the length is expected to trigger energy dissipation in the middle but keep the ends undamaged. The restrainer has an inner diameter similar to that of the core steel, which minimizes the gap between them and avoids time-wasting grouting. This study introduces experimental and analytical research on four types of IHBRB. Tensile coupon tests revealed that the core steel of the IHBRB-600 series satisfied the target yield strength of 600 MPa in the middle and 1,200 MPa at both ends. The core steel of the IHBRB-800 series also satisfied the target yield strength of 800 MPa in the middle and 1,200 MPa at both ends. The ascending cyclic loading test showed that the IHBRB exerted a load capacity of over 1,000 kN as designed and exhibited tensile-compressive symmetric hysteresis behavior. IHBRB-600L, the specimen that failed by tensile fracture, showed that the high-strength brace end succeeded in remaining elastic as planned, while the low-strength brace middle showed a higher-order mode buckling. However, unexpected overall buckling at the joint occurred on other specimens. The low cycle fatigue specimens showed large cumulative inelastic deformation ratios of more than 200. A three-dimensional finite-element model of IHBRB-800L built by ABAQUS/Explicit (Dassault Systèmes Simulia Corp., Paris) captured the experimental results well. A parametric study revealed that the longer restrainer fixed at one side of the core steel end was one of the solutions to guarantee the great seismic performance of the IHBRB.