Ultra-high strength steel welded box section stub columns: Residual stresses, stub column tests, simulation and design

This paper details an integrated experimental and numerical analysis of the structural performance of axially compressed Q1100 ultra-high strength steel (UHSS) welded box-section stub columns. Material characterisation was performed through tensile tests on tensile coupons extracted from both the base metal and the heat-affected zone (HAZ) to obtain their constitutive relationships. Meticulous quantification of initial geometric imperfections and residual stress fields, comprising both membrane and bending components, was carried out, with measured peak tensile residual stresses near the weld seam attaining 37.2% of the nominal yield strength. All nine stub column specimens failed by local buckling, and a clear inverse trend was observed between the normalised axial strength and the plate width-to-thickness ratio. A high-fidelity finite element (FE) model was established, integrating the measured imperfections, residual stress patterns, and the strength reduction in the HAZ. The accuracy of FE model was confirmed through rigorous validation against test results. A subsequent parametric analysis expanded the data over a wide range of cross-section slenderness, enabling a thorough evaluation of structural local buckling behaviour. Comparisons with predictions from major design codes, such as EN 1993-1-1, AISC 360, and AS 4100, showed that these standards are consistently conservative for the studied UHSS sections, with the conservatism being most pronounced for stocky cross-sections. This research provides essential insights and suggests practical refinements to bridge existing gaps in design codes for UHSS structural applications.