Flexural buckling behaviour and resistances of S700 high strength cold-formed steel (CFS) built-up I-section columns

This paper reports a testing and numerical modelling programme to investigate the flexural buckling behaviour and resistances of S700 high strength cold-formed steel (CFS) built-up I-section columns. Each built-up I-section is composed of two identical unlipped channel sections back-to-back assembled by G12–24TPI screws. Following the material testing of flat and corner coupons and initial geometric imperfection measurements of all the specimens, a total of 9 built-up I-section specimens were compressively tested under pin-ended boundary conditions. Upon testing, flexural buckling was identical for all the examined specimens. In parallel with the testing programme, a numerical modelling programme was conducted, with finite element models developed and validated against the experimental observations. Afterward, parametric studies incorporating 119 finite element models were adopted to produce further numerical data, with a wider range of cross-sections and member effective lengths involved. The relevant design rules for CFS structures, as specified in European code and American specification, were employed to evaluate their applicability to S700 high strength CFS built-up I-section columns failing by minor-axis flexural buckling. From the evaluation results, it was found that the resistance predictions obtained from the European code and American specification are scattered and respectively conservative by 41% and 36% on average, compared with the experimental and numerical data. A revised design approach, based on the American specification, was thus developed, with improved design consistency and accuracy observed from the quantitative and graphical evaluation results.