Impact of cross-section stability on cold-formed steel member stiffness and ductility

The objective of this paper is to provide work towards design methods to predict both the loss of stiffness and the available ductility for cold-formed steel members subject to cross-section deformations resulting from local and distortional buckling. Existing design specifications provide some guidance on how to predict the stiffness of cold-formed steel members, but no guidance on their available ductility. Further, the guidance on stiffness is limited, particularly for distortional buckling, and largely has not been validated against testing. The long-term goal of the research is to improve modern seismic design of cold-formed steel members. Existing four point bending tests on cold-formed steel lipped channel and lipped zee beams failing in local and distortional buckling, are used as the basis for the study conducted herein. The experimental results are converted from load-displacement to moment-rotation and simplified into multi-linear response curves. Stiffness of the conducted tests up to the peak load (moment) is compared with predictions based on the effective width method and the direct strength method. Relationships between local and distortional cross-section slenderness and the observed and predicted secant stiffness up to the peak load are examined. To explore models that include energy dissipation and post-peak behavior, as required for modern seismic design, simple hysteretic models are fit to the full response curve generated from the experimental data; including, an equivalent energy elastic plastic model, and an equivalent energy model that incorporates the pre-peak stiffness reductions predicted by current design methods. Relationships between local and distortional cross-section slenderness and the developed ductility parameters are explored. Formalization of a predictive method for the ductility parameters is underway, as is expansion of the database of studied sections through nonlinear finite element analysis. !"Visiting Student Scholar, Johns Hopkins University , and Ph.D. Candidate, Istanbul Technical University, "" 2 Professor and Chair, Department of Civil Engineering, Johns Hopkins University,