Rotation capacity of HSS flexural members subjected to uniform and gradient moment

Although high strength steel (HSS) with tensile strength up to 800 MPa has been used for a long time, the use of HSS flexural members in building construction is often restricted because of concerns about their low rotation capacity. This study analytically evaluates the in-plane rotation capacity of H-shaped beams made from various steel grades (325–800 MPa) under uniform and moment-gradient loading conditions. Based on tensile coupon test results for SM490, SM570, and HSA800 steels, stress-strain material models were calibrated using the Haaijer model (for mild steel with yield plateau) and Ramberg-Osgood model (for HSS without yield plateau). Moment-curvature analyses were performed to calculate rotation capacities, and the effects of key parameters were investigated: tensile-to-yield strain ratio SR (= εu/εy), yield-to-tensile strength ratio YR (= Fy/Fu), and beam geometry. Results indicate that HSA800 (SR ≈ 15, YR ≈ 0.84) achieves rotation capacity Rm < 3 under moment gradient conditions, insufficient for current design code requirements. Parametric studies demonstrate that increasing SR from 15 to 25 provides 41–45% improvement in rotation capacity, significantly more effective than geometric modifications. Web thickening (20% improvement) outperforms flange thickening (3.4% improvement) for sections under moment gradient. The findings provide guidance for HSS material development and design specification modifications.