Ferritic stainless steel built-up I-section columns: Testing, numerical modelling and design

• Local and local–flexural interactive buckling behaviour of ferritic built-up I-section columns was studied. • Tests were conducted on ten ferritic stainless steel built-up I-section column specimens. • FE models were validated against test results and then used for parametric studies. • Accuracy and reliability of the codified design rules were assessed based on test and FE data. Cold-formed steel built-up section members have attracted increasing interest due to their potential to provide higher load-carrying capacities than conventional single-section members. Ferritic stainless steels, characterised by their low nickel content, offer a cost-effective and sustainable alternative to austenitic grades, while maintaining adequate mechanical properties and corrosion resistance. This paper investigates the structural behaviour and capacity of ferritic stainless steel built-up I-section columns failing by local buckling and local–flexural interactive buckling. An experimental programme was first conducted, including tensile coupon tests, measurements of initial global and local geometric imperfections, and axial compression tests on ten built-up I-section column specimens. Each specimen comprised two identical press-braked ferritic stainless steel channel sections connected using self-drilling screws. Subsequently, finite element models were developed and validated against experimental results. Parametric studies were carried out to discuss the effects of key design parameters. The accuracy and reliability of the codified Effective Width Method and Direct Strength Method in predicting the capacities of ferritic stainless steel built-up I-section columns were evaluated, indicating that: (i) the Effective Width Method offers consistently conservative predictions, underestimating column failure loads by 10% on average; (ii) the Direct Strength method yields greater scatter but improved accuracy, underestimating load-carrying capacities by an average of 1%; (iii) the Effective Width Method provides accurate and consistent predictions for columns failing by local buckling, while its predictions for local–flexural interactive buckling are more scattered and conservative; (iv) both methods exhibit increasingly conservative and scattered predictions as the column slenderness increases. In summary, the existing codified design methods specified in AISI S100 can be extended to the design of ferritic stainless steel built-up I-section columns.