Testing and cross-section design of hot-rolled steel I-section beams under moment gradients

Moment gradients are known to enhance the cross-sectional resistance of steel beams by restricting the development of local buckling. Recently, a design approach for cross-sections under combined bending and shear that incorporates this effect has been developed, but its applicability to stiffened and unstiffened I-section beams has yet to be experimentally validated. To address this gap, a comprehensive investigation into the effect of moment gradients on the local buckling behaviour and design of I-section beams is presented. Experiments on thirteen hot-rolled steel I-section beams under four-point and three-point (with and without stiffeners) bending, covering a range of cross-sectional slendernesses and moment gradients, are described. The development of strain at critical regions within the beams was monitored during testing using a Digital Image Correlation (DIC) system. The experimental results showed that the load-carrying capacities increased with higher moment gradients for the stiffened I-section beams, while for the unstiffened beams with relatively short span lengths, the capacities slightly decreased due to the negative effect of high shear forces outweighing the beneficial effect of high moment gradients. Nonetheless, the ultimate capacities of the unstiffened three-point bending specimens still exceeded those obtained in the four-point bending tests, highlighting that the effect of moment gradients is beneficial, until high levels of shear are reached. Current design approaches are evaluated against the test results, as well as additional numerical data from previous parametric studies. A new design approach, featuring simplified flange width factors rather than the local buckling half-wavelength, is proposed to determine crosssection resistances in the presence of local moment gradients, and shown to provide accurate results. Finally, a reliability analysis is carried out following EN 1990, which confirms the reliability of the proposals within the EC3 framework.