Load height effects on lateral torsional buckling of steel beams validated by high-fidelity finite element analysis and code-based design

The lateral-torsional buckling (LTB) behaviour of steel beams is investigated through a combined methodology comprising analytical formulations, code-based beam-element checks, and high-fidelity shell-element linear eigenvalue buckling finite element analysis. Two end-restraint conditions, simple ( k z = k w = 1.0) and fixed ( k z = k w = 0.5), are examined under mid-span point loads applied at varying vertical load eccentricities relative to the shear centre (Z = + 100 mm, 0 mm, − 100 mm). The NCCI three-factor formulation for the elastic critical moment, used in conjunction with Eurocode 3 ( M cr , with C 1 , C 2 , C 3 ; \:χ\:LT = 0.34) yields up to 30% higher strength than the AISC M n –L b framework, which does not account for load-height and moment-gradient effects. Beam-element checks reproduce Eurocode predictions within a 3% margin when user-defined lateral–torsional buckling factors are specified. However, limitations in modelling warping stiffness independently of lateral stiffness result in a reduction of design moment resistance by approximately 4%. The finite element results show that critical moments align with Eurocode predictions within 1–6%, confirming the accuracy of shell-model-based buckling evaluation. The study also reveals that vertical load eccentricity significantly affects buckling capacity: moving the load from the top to the bottom flange increases the critical moment capacity, M cr by different code-based designs, due to stabilising counter-torsion induced by loading below the shear centre. These findings emphasise that accurate LTB assessment requires explicit consideration of both the magnitude and the application line of the load.