Development of curved braces partially strengthened by induction heating

摘要

This paper presents an experimental and numerical study on a novel brace which is curved and partially strengthened by induction heating technology (induction-heated curved brace, IHCB). Induction heating, comprised of heating and quenching processes, is an advanced technology to raise steel material strength to 2-3 times. Because of the partial strengthening, the untreated region yields first while the induction-heated region is still elastic before larger deformation, so the post-yield stiffness of the whole brace increases. Besides, the initial curved deformation along the brace length, which is due to the uneven heating, is also flexibly employed to delay the yielding behavior and stabilize the compressive behavior. Tensile coupon tests and Vickers hardness tests on the untreated normal-strength specimens and induction-heated high-strength specimens show that the induction heating technology effectively improves the material strength to 2.2-2.6 times. Cyclic loading tests on CBB (conventional buckling brace) and IHCB series show that under tension, IHCBs show approximately 57% lower initial stiffnesses compared to that of CBB but reach the yield loads at the cycle twice as large as that of CBB owing to the initial deformation. After yielding, although the stiffness of CBB dramatically drops to 0.9% to its initial stiffness, IHCBs succeed to maintain higher post-yield stiffnesses, of which the ratios to their initial stiffnesses are larger than 30%. Under compression, IHCBs show the smooth transition into flexural behaviors without apparent buckling behaviors, and the stable compressive loads (the load at 0.5% axial strain) become 1.49 and 1.67 times larger than that of CBB. The data obtained from the strain gauges and displacement transducers show that the strain and transverse deformation of IHCBs are uniformly distributed along the brace length rather than locally concentrated at the limited region as seen in CBB. The numerical analysis conducted by ABAQUS and the proposed design equations capture the experimental results well.