Flexural buckling of mild and high-strength steel hot-rolled sections - improvement proposal of the flexural buckling design rules

The determination of the bearing capacity of a member under compression, presenting an initial outof-straightness and residual stresses resulting from the production process, requires dealing with a stability problem governed by the geometry of the member and the mechanical properties of its constitutive material. Amongst these, the yield strength plays a key role. Indeed, it has a direct impact on the plastic resistance of the member but, also, the detrimental effect of initial imperfections decreases as the yield strength increases. The effect of these initial imperfections is considered by an imperfection factor α in the design recommendations provided in EN1993-1-1. However, the current and new upcoming versions of this norm give a step evolution of this imperfection as a function of yield strength by only distinguishing grades smaller or higher than S460. The present paper aims to briefly summarise the existing studies on this topic, to assess the recommendations of both versions of EN1993-1-1, and to propose a continuous form of the imperfection factor for hot-rolled sections for existing grades up to S500 and even higher grades as a prospective study. The validity of the proposed imperfection factor will be finally assessed for H-shaped cross-sections with height-towidth ratios h/b>1.2 and flange thicknesses tf ≤ 40mm.