Tests of K-joints using screwed connections in cold-formed steel floor joist systems

Cold-formed steel (CFS) framing systems have long been used in low-rise buildings for decades and are now increasingly employed in mid-rise construction due to their light weight and high strength-to-weight ratio. These CFS truss structures such as floor joists are typically built from lipped/unlipped C- and U-channel profiles, with screws connecting diagonal webs to the top and bottom chords through both flanges. In contrast to single-lap connections, the structural performance of screw-fastened K-joints is significantly influenced by additional factors, including member deformations and failures occurring in the gap between two diagonal members. Current design provisions, such as ANSI/SDI AISI S100:2024 and AS/NZS 4600:2018, as well as existing experimental studies, do not explicitly address these effects for K-joints using CFS channels, unlike the more extensively studied K-joints with hot-rolled sections. This paper presents an experimental program on K-joint connections cut from floor joists conducted at the University of Sydney, considering both single- and double-screw arrangements and various thickness combinations. Under the tensile force applied to one diagonal member, a triangular test rig was specially designed to rotate and apply compression force to the other diagonal member and tensile force to one end of the chord member, thereby reproducing their load transferring mechanisms in the full frames. Experimental observations with 3D DIC measurement revealed the curling and shear buckling of chord flanges in addition to bearing and screw tilting failure modes. Consequently, K-joint experimental results were considerably reduced compared to strength predictions using design equations of single-lap configurations in ANSI/SDI AISI S100:2024 and AS/NZS 4600:2018. The results strongly support the need for revised design equations that account for the effect of deformations and/or failures of connected members on the load capacity of CFS K-joint connections. A comprehensive analysis on the experimental observations and key findings is discussed in detail in this paper.