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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.

The purpose of this paper is to model mechanical properties of prestressing strand at elevated temperature for estimating the strength and deflection of prestressed concrete members during fire and/or after fire. Prestressing strands(SWPR7BL) of 12.7 mm in diameter were used as test coupons. Tensile tests on prestressing strands at elevated temperatures were conducted to investigate mechanical properties such as yield strength, Young's modulus and creep strain. Reduction in yield strength, Young's modulus and stress-strain curves at elevated temperature were formulated. Creep tests were conducted on prestressing strands. The initial stress applied was 70% of the yield strength. The maximum creep strain was as large as 545 microns at 250°C. Creep behavior of presstressing strands was approximated by a numerical formula proposed by Norton and Bailey. Using the formula together with time-hardening rule, an estimation method of relaxation of prestressing steel at high temperatures was proposed.

Seven half-scale post-tensioned precast concrete beams were tested under anti-symmetrical flexure. The experimental parameters were shear span ratio and shear reinforcement ratio. Multi-strands and high strength shear reinforcement with nominal yield strength of 785 MPa were used as post-tensioning steel and shear reinforcement, respectively. Yielding of shear reinforcement was not observed in any of the specimens. The flexural capacity evaluated by the ACI rectangular concrete stress block with plane section assumption agreed well to the experimental results. The shear capacity was estimated by the design equation 71.2a in the AIJ Standard for Structural Design and Construction of Prestressed Concrete Structures. The experimental results collected by literature survey to past research revealed that shear failure modes could be related to each of the experimental parameters (shear span ratio, shear reinforcement strength and shear reinforcement ratio).