A numerical study on the critical bending moment for distortional buckling of cold-formed lipped C-sections

Applications of cold-formed profiles have become more common in the civil construction industry. In particular, the cold-formed lipped C-sections are frequently used. Mostly, these channels are submitted to bend about the major principal axis and, consequently, they get subjected to compression stresses that, due to their considerable local slenderness, can provoke instability. Particularly complex, the distortional buckling is one of those failure modes and may occur in cold-formed lipped C-sections causing translation of the edges. Thus, this article intends to evaluate theoretical design procedures, from the perspective of accuracy and implementation, when used to calculate the critical bending moment of distortional buckling. Four formulae were selected and, for comparison, two computational approaches were employed to determine, numerically, the critical moments in different lipped C-sections: the finite element method (FEM) and the finite strip method (FSM). The Hancock’s and Schafer’s formulations proved to be highly precise when confronted with the numerical data. An adapted procedure presented by AISI, based primarily on local buckling designs, showed itself imprecise, returning both unsafe and conservative results depending on the cross-section. The last one was the simplified method, also available on AISI standard, and as expected, it was conservative when predicting the distortional buckling moment. Conclusions regarding the critical moment and buckling modes with changes in geometrical parameters were also extracted. Thickness increase led the critical moment to grow, as expected, due to the reduction of local slenderness provided. Likewise, as the lip width gets larger, the critical moment rises, however the profile tends to fail by local buckling mode, evidencing the need for attention when using channels with too narrow or too wide lips.