Experimental response of cold-formed steel Zee-section beam-columns

• The actual stress distribution is not explicitly considered in beam-columns.
• 43 Zee-section beam-columns were tested to evaluate AISI-S100-12.
• Applied stress distributions and failure modes were found to be correlated.
• The strength cannot be accurately reproduced by using only the anchor points.
• Tension on the sloping lips of the Zee-section results in conservative predictions.

The structural strength and stability of cold-formed steel Zee-section beam-columns including 22 structural Zee-sections at a length of 305 mm and 21 at a length of 1219 mm were tested under several load combinations of biaxial bending moments and axial force. This testing on Zee-section beam-column specimens complements an earlier test series on lipped channel specimens and provides an exploration of the impact of the principal axis configuration and the differing location of shear center for Zee-sections as relative to their location in lipped channels. The short length specimens are intended to mobilize local buckling and avoid distortional and global buckling, while the long length specimens are intended to mobilize other buckling modes such as distortional and local-global buckling. The combined axial force and biaxial bending moments were applied via a special test rig designed to apply axial load with eccentricities. The results were used to evaluate the current AISI-S100-12 specification for beam-column strength prediction via both effective width and direct strength methods. Both methods employ a simple linear interaction equation for strength prediction of the members under combined actions. The experimental results showed a considerable prospective for improvement in the current specification approach by providing more optimized and realistic results in design considering the actual stress resulted from combined actions on the cross-section. Accordingly, the potentials for further improvements of the direct strength design method for cold-formed steel beam-columns are discussed.