Cold-formed steel compression members with perforations

Abstract

<p>The research work of this thesis is concerned with the local buckling behaviour and the post-local buckling strength of perforated cold-formed steel (CFS) members subjected to axial compression loading.</p> <p>Cold-formed steel sections are widely used nowadays as primary and secondary framing members in low-rise steel buildings. Although CFS members are essentially thin-walled framing elements, with a major susceptibility to local buckling, these members maintain a considerable reserve of post-local buckling strength prior to yielding. The post-local buckling strength of CFS members is largely affected by the sectional non-uniform material properties after the forming operation, the initial imperfections, the large deformation behaviour after buckling, and the possibility having utility perforations in the plate components of the members.</p> <p>A finite element-based analytical model has been developed in this thesis to investigate the post-local buckling behaviour and the ultimate strength of non-performated and perforated CFS compression members. A large deformation degenerated shell finite element was used to model the surface of CFS sections. The kinematic formulation of the degenerated shell element was enhanced using the method of "assumed strain fields", to eliminate any locking program problem of the element. Special consideration was given in the finite element model to the geometric imperfections and the loading technique of CFS compression members.</p> <p>Tensile coupon tests and residual stress tests were performed on CFS channel sections, in order to determine the effects of the cold forming operation on the distribution of the material properties across CFS sections. based on the results of these tests, analytical models for the stress-strain relationship, the yield strength distribution, and the residual stress distribution across CFS sections were developed and incorporated in the finite element model of CFS compression members.</p> <p>A seris of CFS channel stub-column tests was performed to verify the deformation and ultimate strength predictions of the proposed finite element model. The tests were also used to investigate the effects of perforations on the behaviour and load capacity of CFS members in compression. The finite element model was then used to assess the axial stress distribution and the effective design width of perforated plates of CFS compression members. This assessment was performed through a parametric study on the perforation and the plate parameters.</p> <p>Two effective design width equations for stiffened compression plates with square and elongated perforations were developed, based on the analysis of the finite element results. The ultimate load predictions of the two equations were compared to the stub-column test results of this thesis, and several other test results from the literature. The proposed equations proved to give accurate and safe predictions for the effective design width and the ultimate strength of perforated CFS compression members.</p>