Development of Concrete/Cold Formed Steel Composite Flexural Members

Light-gauge steel (LGS) construction is widely used for commercial and industrial buildings. In such construction the wall framing is usually an assembly of cold-formed steel (CFS) studs held between CFS tracks while the floors are normally composite concrete/LGS decks on steel bar joists spanning between load-bearing CFS walls. Heavy hot rolled steel angles or HSS tubes are welded to the top of the CFS load-bearing walls to function as load distribution members (LDM) over the wall studs and as headers over wall openings. The use of hot rolled steel elements results in significant increase in construction cost and time. Such heavy steel elements would be unnecessary if the concrete thickening on top of the CFS wall can be made to act compositely with the CFS track. The resulting concrete/CFS composite beam would be a reinforced concrete beam where the CFS track serves as the tension reinforcement. The continuity at the interface would be provided by stand-off screws that are normally used in the construction of the composite deck. This research involved experimental and analytical studies to assess the structural performance and failure modes of concrete/CFS track composite beams and to develop optimum beam configurations for use in LGS construction. The parameters included the CFS track thickness, stand-off screw spacing, and stand-off screw configuration. Twenty-seven full-scale specimens were constructed, instrumented, and tested to failure. The flexural and shear strengths, flexural stiffness, and interface shear transfer were investigated. However, only the flexural strength and stiffness are reported in this paper. The results show that concrete/CFS track composite beams can be designed for ductile flexural failure and that the degree of composite action is dependent upon the stand-off screws intensity rather than configuration. Equations for the effective moment of inertia at service loads were derived.