Tests to Examine Plastic Behavior of Knee Joints in Cold-Formed RHS

Abstract

This paper describes tests on various types of knee joints for steel portal frames constructed from cold-formed rectangular hollow sections (RHS) to examine the ability of the connections to form plastic hinges. Welded stiffened and unstiffened knee joints, bolted knee joints with end plates, and connections with a fabricated internal sleeve were included in the experimental investigation. Most connections tested under opening moment failed by fracture in the heat-affected zone of the RHS near the weld. The connections tested under closing moment failed by web local buckling, which occurred near the connection. While the stiffened and unstiffened welded connections satisfied the strength interaction requirements in the available design guides, the connections did not maintain the plastic moment for sufficiently large rotation to be considered suitable for a plastic hinge location. The unstiffened welded joints were not able to reach the plastic moment. The use of an internal sleeve moved the plastic hinge in the connection away from the connection center-line and reduced the stress on the weld between the legs of the connection. It was found that sleeve connections were capable of sustaining the plastic moment for large rotations considered suitable for plastic design.