O.I.C.-based design for local/global/built-up interaction in laced built-up steel columns

This paper investigates the ultimate strength of laced built-up columns that buckle in the plane of the lacing system, where shear deformations significantly reduce the load-carrying capacity. These members are influenced by Local/Global/Built-up (L/G/B) buckling phenomena and their interactions. The built-up section consists of two C-shaped chords connected by double “X” flat lacing bars. Three main buckling modes are examined through detailed numerical analyses: (i) Local buckling of the cross-section, (ii) Global buckling of individual chords between connectors, (iii) Built-up buckling of the overall member and (iv) coupling/interactions between these modes. For members with slender elements, local buckling and its interaction with other modes are not adequately captured by current design codes, resulting in unconservative strength predictions. Similar issues are observed for global buckling of the chord. To address these limitations, this study extends the existing Overall Interaction Concept (O.I.C.), which was originally developed for Local/Global interactions, to include more complex Local/Global/Built-up interaction behavior. The extended O.I.C.-based approach demonstrates a strong ability to capture these complex interaction effects. Its accuracy and consistency are confirmed through comparisons with numerical results from validated shell finite element models. The proposed method provides more accurate, consistent, and safer strength predictions than the recommendations provided by Eurocode 3 and the American Standards for both tip-to-tip and back-to-back built-up sections. Its reliability is also supported by statistical analyses in accordance with EN 1990 guidelines.