Shear behavior of a novel cold-formed U-shaped steel and concrete composite beam

This paper discusses the shear behavior of a novel cold-formed U-shaped steel and concrete composite beam (RCUCB) composed of a rebar (reinforcing bar) truss for stiffening the open steel U-section, inverted U-shaped rebars for enhancing the continuity at the interface between concrete web and flange, and longitudinal bottom reinforcement for controlling the slip between steel U-section and encased concrete. A series of shear experiments on eight specimens were conducted, considering steel ratio (rho(s)), beam depth (H), and the number of bottom shear studs (n(d)). Two typical shear failure modes, depending on the shear span ratio (lambda), were observed: diagonal compression failure when lambda = 1.0; and shear compression failure when lambda = 1.5. The respective ductility factors are 2.5-4.0 and 9.0-9.7. This indicates that a transitional shear span ratio (lambda(0)) lies between 1.0 and 1.5. It is also obvious that shear lag effect exists in the concrete slab as the longitudinal strain of the concrete slab is 50-200% larger than the edge strain. Excellent integral action is ensured by the rebar truss and U-shaped rebars, thus enabling the concrete flange, web, and steel U-section to resist shear forces together. The steel ratio (rho(s)) has an obvious positive effect on the shear stiffness and capacity. The height to thickness ratio of the steel web is suggested to be no larger than 100 to avoid a local buckling before the peak load. A calculation method for the shear capacity of RCUCBs is proposed, which considers the contributions from the steel web, concrete, and dowel action.