Direct Strength Prediction of Cold-Formed Steel Beam-Columns
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The Direct Strength Method (DSM) of cold-formed steel member design employs local, distortional, and global cross-section elastic buckling analysis with empirically derived “direct” expressions to predict member strength. DSM is an accepted design method in national design specifications (e.g., AISI-S100-16) and enables a unified, robust, and flexible design approach. However, for beam-columns DSM in current design specifications employs simplified linear interaction expressions based on combining the isolated axial and bending elastic buckling and strength response. Today, local, distortional, and global elastic buckling under any combination of axial load and bending moments may be found using elastic buckling analysis tools such as the finite strip method (e.g., CUFSM). Thus, stability may be assessed under the combined actions, but new DSM expressions are needed to utilize this explicit stability information in determining beam-column strength. In this report, new strength expressions for each limit state are developed. In addition, the results of beam-column tests performed by the authors and those available in the literature are used to validate the performance of the new proposed DSM for beam-columns. The development of DSM for beam-columns has the potential to provide a more mechanically sound solution to the strength of cold-formed steel beam-columns, eliminate excessive conservativeness, and at the same time encourage the next generation of optimized, high strength, cold-formed steel shapes. This report covers: a new formulation for DSM that can account for stability and strength under multiple actions; targeted testing under P-M-M loadings to explore the beam-column stability space explicitly and find capacities; nonlinear FEA analysis to expand the studies and flesh out issues in the final design methods; and technology transfer to ease the use of the develop method and its related tools.