Stiffness and Cracking Behavior of New U-Shaped Steel and Concrete Composite Beam under Negative Bending

This paper discussed the stiffness and cracking behavior of a new kind of reinforcement-stiffened cold-formed U-shaped steel and concrete composite beam (RCUCB) under negative bending. The static behavior of the RCUCB was discussed based on a negative bending experiment of 10 specimens. The reinforcement system consisting of the inverted U-shaped reinforcement, reinforcement truss, and bottom reinforcement was shown to maintain the integral action of the RCUCB, especially after the yield load. The RCUCB was found to behave well in deformability, ductility, and plastic rotation ability under negative bending: (1) the failure deflection and ductility factor reached L0/33 (where L0 is the effective span) and 17.7 respectively; (2) the ductility factor increased with decreasing steel ratio and decreasing reinforcement ratio; and (3) approximately 84.7%–94.0% of the web height yielded at the peak load, resulting in a plastification coefficient of 1.3–1.9. The damage process was defined with the cracking, yielding, and buckling loads approximately equal to 0.16Pu, 0.61Pu, and 0.90Pu, respectively. During the experiment, the degree of shear connection in the negative bending area showed little effect on the negative bending behavior before yield load. Calculation methods for initial stiffness, second stiffness, and cracking moment were proposed, and the calculation results agreed well with the test results.