Cold-formed members - comparison between tests and a unified design method for beam-columns

In [1] a unified method for the design of steel beam-columns is presented. The method has been checked for rolled steel beam-columns and extruded aluminium beam-columns. It is included in Eurocode 9 [19] for aluminium members and it is proposed to be included also in Eurocode 3 Part 1-3 [16] as well, but then it needs to be checked for typical cold-formed sections.
Cold-formed sections are usually un-symmetric and thin-walled, for instance channel sections or C-shaped sections (lipped channels). When used as compression members, local buckling causes a redistribution of the longitudinal stress which leads to a shift of the effective centroid. The shift causes overall bending and reduces the column strength when the member is compressed between pinned ends. In fixed-ended columns, however, the shift of the effective centroid is balanced by a shift of the applied force and bending is not introduced [6]. As a result, the strength of fixed-ended channel column exceeds that of a pin-ended column of the same effective length [7].
Using effective width for the flanges of channels e.g. according to EN 1993-1-5 [17] gives conservative result as the centroid of the effective section is too close to the web. The mixed effective width/effective thickness method for outstand elements given in Annex D of EN 1993-1-3 [16] is the basis in the following interpretations.