Multi-Linear and Bi-Linear Stress–Strain Approximations for Finite Element Modelling of Extended End-Plate Moment Connections

This study investigates the finite element analysis (FEA) of beam-to-column bolted extended end-plate moment connections, with a focus on accurately reproducing plastic rotational stiffness. Existing FEA results for six experimentally tested connections from the literature show substantial discrepancies in the plastic range, despite acceptable elastic stiffness. These discrepancies are traced to conventional material modelling practices, where only yield and ultimate stresses are specified, engineering stress–strain data are used directly, and the minimum elongation is taken as the strain at ultimate stress. To address these limitations, the connections are re-modelled in ABAQUS using (i) a multi-linear approximation for the plastic stress–strain behaviour of mild steel plates, and (ii) a proposed bi-linear approximation that requires only measured yield and ultimate strengths but preserves the area under the reference curve. In both cases, true stress–strain values are supplied to the software for plastic analysis. These strategies reduce the average error in plastic rotational stiffness from 46–48% in the existing FEA to about 18% across all specimens, while maintaining good agreement in the elastic range. The results demonstrate that carefully constructed stress–strain approximations, combined with appropriate data formatting in ABAQUS, enable reliable validation of extended end-plate moment connection models and provide a practical basis for future parametric and design studies.