Experimental investigation on cold-formed steel stiffened lipped channel columns undergoing local-distortional interaction

The objective of this work is to report a careful experimental investigation, planned at the University of Lisbon and carried out at The University of Hong Kong, concerning the behaviour and ultimate strength of cold-formed steel web-flange-stiffened (WFSLC) and web-stiffened (WSLC) lipped channel columns undergoing localdistortional (L-D) interaction. It involves 31 specimens (16 WFSLC+ 15 WSLC), brake-pressed from highstrength zinc-coated grades G450, G500 and G550 structural steel sheets, exhibiting critical distortional-tolocal buckling load ratios ranging between 0.75 and 1.88. The column geometries were carefully selected to enable testing fixed-ended columns undergoing true L-D interaction and secondary distortional-bifurcation L-D interaction (ensuring evidence of the latter required selecting rather slender columns) - all tested specimens exhibited the expected L-D interactive failures. The specimen material properties were obtained from tensile coupon tests and their initial geometrical imperfections were measured prior to testing. The experimental results presented and discussed consist of column (i) load-displacement equilibrium paths, (ii) photos evidencing the evolution of the column deformed configurations along those paths (including the failure mode) and (iii) failure loads. Finally, the experimental failure load data obtained are compared with their estimates provided by the currently codified DSM design approaches for columns failing in L and D modes, showing their inadequacy to handle L-D interactive failures - the fresh light shed by this comparison will contribute to the timely codification, in the near future, of a DSM design procedure for columns affected by L-D interaction.