Keywords: cold-formed steel columns, local-distortional interaction, lipped channels, hat-sections, zed-sections, rack-sections, ultimate strength, direct strength method.

This paper reports the latest results of an ongoing investigation of the direct strength method (DSM) design of cold-formed steel columns affected by local-distortional mode interaction, which are aimed at extending findings unveiled earlier for lipped channels to other cross-section shapes, namely, hat, zed and rack-sections. Following a brief presentation of the most relevant aspects concerning the strength of columns with those cross-section shapes and identical local and distortional buckling stresses, the paper addresses the assembly of a significant column ultimate strength data bank, concerning fixed-ended hat, zed and rack-section columns undergoing strong local-distortional interaction and exhibiting several geometries (cross-section dimensions and lengths) and yield stresses. Then, this ultimate strength data is used to assess the performance of the existing DSM approaches to design columns against local-distortional interactive failures. Moreover, it is shown that a novel DSM design approach, recently developed and validated in the context of cold-formed steel fixed-ended lipped channel columns affected by local-distortional interaction and briefly reviewed in the paper, can also be successfully applied to hat, zed and rack-section columns under the same circumstances.

The above novel DSM design approach adopts the current DSM distortional strength curve in the lower distortional or local slenderness range (stocky column), where no relevant interaction occurs. In the high slenderness range (slender columns), characterised by considerable interaction, a modified local strength is proposed, which depends on the value of column critical half-wave length ratio. In practical terms, the novel approach combines (i) the current DSM design curve against distortional failure with (ii) an existing strength curve aimed at capturing local-distortional interactive failures, which is shown to yield excessively conservative column ultimate strength predictions. A linear transition between them is also proposed.

ABAQUS shell finite element analyses are employed to obtain ultimate strength data bank that involves (i) 210 pairs of hat and zed-section columns (with the same cross-section dimensions and lengths as the lipped channel columns analysed previously here each column trio shares the same local and distortional buckling behaviour) and (ii) 195 rack-section columns. On the basis of this ultimate strength data, it is possible to conclude that the application of the novel DSM-based design approach to hat, zed and rack-section columns affected by local-distortional interaction yields quite satisfactory (generally accurate and mostly safe) ultimate strength predictions. Nevertheless, the quality of such estimates is not homogeneous;indeed, it is higher for the hat and zed-section columns (virtually identical to that obtained previously for the lipped channel columns) than for the rack-section columns.

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