The Use of Average Bending Moments in New Explicit DSM for Bending-Shear Interaction in Cold- formed Steel Channels

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Cold-Formed Steel Research Consortium (CFSRC) Colloquium
The current Direct Strength Method (DSM) for the design of cold-formed steel sections under combined bending and shear in Specifications/Standards (AISI S100 and AS/NZS 4600) is based on the traditional bending-shear interaction curves for both elastic local buckling and post-buckling capacities. Specifically, the current design check is conducted indirectly using normalized ratios of the design actions of Maximum Bending and Maximum Shear to the corresponding buckling strengths of the members under pure bending and pure shear. This is an implicit design method and overestimates the local buckling strengths of sections governed by bending-shear interaction. By the use of an Average Bending Moment, a new explicit design approach in line with DSM curves is developed for ultimate strengths of cold-formed sections under different combinations of bending and shear. This concept comes from the interaction between shear and average bending moment rather than maximum bending moment, and gives accurate predictions for ultimate strengths, which can replace the use of traditional normalized design load-strength ratios. In addition, a combination of the new DSM curve for average bending moment and the current DSM curve for shear is also presented to provide strength prediction for the full range of bending- shear combinations. The full approach is validated using a series of reliable testing data from the literature. Recommendations for practical design in the use of the new explicit method are also included in this paper.