| This thesis entitled "Section capacity of cold-formed steel members by the Direct Strength Method" aims at advancing the Direct Strength Method under three objectives. The first objective is to provide and verify a general design method for prediction of reserve inelastic bending capacity in cold-formed steel members potentially subject to local, distortional, and/or lateral-torsional buckling modes. The strain capacity that can be sustained in inelastic local and inelastic distortional buckling is investigated through existing experiments coupled with nonlinear finite element analysis. The resulting relationships for inelastic local, distortional, and lateral-torsional buckling are provided in a Direct Strength Method format for potential adoption in the cold-formed steel AISI Specification.;The second objective is to explain why cold-formed steel angles may have significant post-buckling reserve in global buckling, as has been observed in testing, and to provide design guidance for locally slender cold-formed steel angles. In this work specific attention is paid to several unique aspects of cold-formed steel angles under compression: (1) the separation, or lack of, between local-plate and global torsional buckling, (2) the relative contributions between flexure and torsion in global flexural-torsional budding modes, (3) the specific impact of end boundary conditions, with particular emphasis on warping (longitudinal) deformations. Utilizing developed nonlinear collapse analysis with shell finite element models, and existing testing (including the work of Young), alternatives to current design methods are explored. Given the end boundary conditions are known new design procedures are recommended for strength prediction of cold-formed steel angles.;The third objective is to directly examine the stability and strength of cold-formed steel beam-columns under their actual combined actions: axial and bending, biaxial bending, etc. The current approach to cold-formed steel beam-column design is based on simplified linear interaction formulas that are based on separate axial and flexural stability and strength determinations. Since stability is stress dependent the appropriate combinations are strongly cross-section dependent and direct analysis of the stability under the expected combined action can yield fundamentally different (and more economical) solutions than assuming linear interactions always govern. This study provides the complete yielding and stability solution under direct combined actions for symmetric and non-symmetric cross-sections. These same sections are then modeled to failure with nonlinear finite element analysis, under all combined actions, to provide the predicted ultimate strength. These ultimate strength envelopes (a) do not agree with linear interaction equations used in design and (b) provide a basis for formulating a Direct Strength Method for beam-columns. Initial formulation of a Direct Strength Method for beam-columns is explored and these initial studies demonstrate that direct analysis under the combined actions can lead to more economical and more realistic strength predictions; particularly for un-symmetric sections. |
