Structural behaviour of S700 high-strength steel built-up I-sections subjected to combined loading

This paper presents experimental and numerical investigations into cross-sectional buckling behaviour and resistance of S700 high strength cold-formed steel built-up I-sections under combined compression and major-axis bending. A testing programme, including tensile coupon tests, initial geometric imperfection measurements and a total of 8 major-axis eccentric compression tests, was firstly performed. Subsequently, a numerical modelling programme was conducted, in which the finite element models were established to replicate the experimental observations. The validated FE models were then employed in parametric studies to enlarge numerical data pool. The applicability of European (EC3) and North American (AISI) specifications for CFS structures to S700 high strength cold-formed steel built-up I-sections under combined compression and major-axis bending was assessed. The assessment results indicated that EC3 resulted in conservative ultimate load predictions due mainly to the conservative end points, while AISI exhibited higher level of design accuracy compared to EC3 counterpart, but still with conservative pure bending resistances. Finally, a new design approach, which employed a bi-linear interaction curve, as given in American specification for structural steel buildings (AISC), together with end points calculated by direct strength method, was proposed and offered improved ultimate load predictions.