Simulation and cross-section design of steel beams under moment gradients

A systematic study into the effect of moment gradients on the cross-section resistance of hot-rolled steel square hollow section (SHS), rectangular hollow section (RHS) and I-section beams has been conducted and is presented in this paper. Finite element (FE) models were first developed and validated against test results from the literature; parametric studies covering different steel grades, cross-section geometries and moment distributions were then carried out. It was found that cross-section bending resistances increase with increasing moment gradient in the low to intermediate range, despite the necessary rise in the level of co-existent shear. Under high moment gradients, the negative impact of the high co-existent shear outweighs the positive influence of the moment gradient, and cross-section bending resistances fall. It was found that the benefits from moment gradients vary with the cross-section slenderness and the aspect ratio of the cross-section, and increase when intermediate web stiffeners are present. A new design method that captures the observed behaviour is devised, featuring a new parameter to describe the local moment gradient in beams under different loading conditions. The proposed design equations are shown to be more accurate than existing provisions in EC3 and to meet the reliability requirements set out in EN 1990. The new method is therefore deemed to be suitable for implementation within the EC3 framework.