Experimental study of LSF truss floor-ceiling systems made of top hat sections

• Investigated the behaviour of load bearing LSF truss floor-ceiling systems made of top hat sections. • Identified the complex failure mechanisms of top hat trusses, including early local buckling failures of unlipped sections. • The use of nylon spacers and screws significantly delayed the early failures of diagonal web compression members. • Removing the lips and edge stiffeners at the ends of web members led to about 50% reduction in their capacity. • Highlighted the need to consider combined compression-bending actions and unlipped sections when designing web members. Light gauge steel framed (LSF) floor-ceiling systems, which employ cold-formed steel (CFS) joists, offer high strength, fast construction and cost-effectiveness. CFS trusses are examples of such joists with several advantages over conventional lipped channel joists. However, limited research exists on LSF truss floor-ceiling systems made of top hat sections, despite their popularity. This study addressed this gap by examining the flexural behaviour of LSF truss floor-ceiling systems made of top hat sections through a series of four-point bending tests. The experiments were conducted with variations in support conditions, truss configurations, top hat thickness, and the types of fasteners used in web-to-chord connections. Test results highlighted the complex failure mechanisms of top hat trusses, including early local buckling failures at the removed lips and edge stiffeners near the ends of diagonal web members. The use of nylon spacers and additional screws to the bolt connections proved effective in delaying early web member failures. Tested trusses did not fail abruptly, instead, the loads were redistributed to other members, resulting in higher ultimate failure loads. A comparison with the current truss design method which is based on elastic analysis to determine member forces and the direct strength method (DSM) to calculate member capacities showed that this approach overestimates the truss failure loads, particularly where web-to-chord connections had only one bolt. This research underscores the need to consider combined compression-bending actions when designing web members with eccentric connections and to strengthen these connections to prevent or delay early local buckling failures. It also highlights the importance of considering the unlipped portion of web members at connections when designing top hat trusses and provides design recommendations for top hat truss floor systems.