Local buckling behavior and bearing capacity of cold-formed S700 high strength steel built-up box-section stub columns

This paper investigates the cross-sectional behavior and resistance of cold-formed S700 high strength steel (CFHSS) built-up box-section stub columns through an extensive experimental and numerical program. The experimental program contained tensile coupon tests, initial local geometric imperfection measurements and 12 stub column tests. The numerical program was subsequently performed, and finite element models were developed and validated against the experimental results and then employed in parametric studies to generate further numerical data. The obtained test and numerical data were used to evaluate the applicability of the existing design methods in European and American specifications to CFHSS built-up box-sections. It is revealed from the evaluation results that the codified slenderness limits in both codes can be accurately applied to classify internal web and outstand flange of CFHSS built-up box-sections in pure compression. The graphical and quantitative assessment results also show that the compression resistances predicted by the effective width method codified in two design codes are generally conservative but relatively consistent, and the current direct strength method specified in the American specification yields accurate ultimate loads for specimens of small local buckling slenderness (i.e. less than 1.35), but unsafe predictions for intermediate to large local buckling slenderness. A modified design approach based on DSM was proposed for CFHSS built-up box-section stub columns and demonstrated to improve design accuracy and consistency, with its reliability verified through statistical analysis, to address shortcomings of the current design methods.