Keywords: cold-formed steel columns, distortional post-buckling, local boundary conditions, rack-section columns, spline finite strip method, experimental analysis.

The structural behaviour of rack-section cold-formed steel columns (i.e., uniformly compressed members) is often strongly influenced by the occurrence of an instability phenomenon commonly designated as distortional buckling, a designation which stems from the fact that the corresponding buckling mode (distortional mode - DM) always involves the distortion of the cross-sections. Such distortion implies the motion of some internal longitudinal edges (fold lines) and is illustrated in figure 98.1.

This paper reports and discusses numerical (mostly) and experimental results dealing with the elastic distortional post-buckling behaviour of rack-section cold- formed steel columns. The numerical results have been obtained by means of geometrically non linear analyses, performed by means of the spline finite strip method and incorporating the influence of initial imperfections with the critical (distortional) buckling mode shape. The particular set of results displayed is selected with the aim of illustrating several distortional post-buckling aspects, namely (i) the effect of the local support conditions, (ii) the evolution of the column deformed configuration and normal stress distribution, as post-buckling progresses, and (iii) the influence of the initial cross-section distortion "sign" on the column post-buckling stiffness and strength. They include (i) post-buckling equilibrium paths, relating the applied stress with the flange-stiffener vertical displacement (see figure 98.1) and (ii) curves which describe the evolution, along such paths, of the most relevant displacement profiles and stress longitudinal and cross-section distributions. The experimental results presented concern stub-column tests performed at the laboratory of COPPE (Federal University of Rio de Janeiro) and involving specimens with (i) "natural" support conditions and (ii) "ensured" fixed wall element transverse edges and warping-free end cross-sections. Such results comprise (i) measured deformed configurations and initial geometrical imperfections, (ii) critical buckling stress estimates and (iii) post- buckling equilibrium paths. Moreover, several features of the experimental set-up, test procedure and data acquisition system are briefly described.

The columns analysed have four local support conditions, combining end sections with (i) free/prevented warping and (ii) pinned/fixed wall element transverse edges. Concerning the numerical results obtained, the following aspects deserve a special mention:

(i)

A non negligible asymmetry w.r.t. the cross-section distortion "sign", illustrated in figure 98.2, which involves displacements and stresses and is mostly noticeable in the flange and stiffener stress distributions.

(ii)

The qualitative similarity between the effects of either preventing the end section warping or fixing the wall element transverse edges: in both cases, the critical buckling mode exhibits two half-waves, instead of one, and the column experiences significant stiffness and strength increases.

Since the experimental tests on stub-columns with full end section warping and wall element flexural rotation restraint (GFRP plates moulded around the specimen end sections) have been closely monitored, it is possible to present a meaningful comparison between the experimental and numerical results (critical stress value, post-buckling equilibrium paths and deformed configurations) and to show that quite a good correlation was observed.

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