Negative bending behavior of novel U-shaped steel and concrete composite beams

The negative bending behavior of the reinforcement stiffened cold-formed U-shaped steel and concrete composite beams (RCUCBs) is discussed in this paper. The negative bending experiment on 10 specimens were conducted and the damage process and corresponding static behavior in each stage are defined. Two distinct failure modes were found depending on whether the inverted U-shaped reinforcement (IUR) is installed or not: ductile plastic failure mode (when installed) and sudden elastic failure mode (when not installed). The IUR was observed to keep the integral action and improve the bending capacity and ductility of a RCUCB. The closed steel U-section stiffened by the reinforcement truss provides significant confinement to the encased concrete and even maintains its integrity after concrete cracking. The study results show that the plastic hinging of the framed RCUCB under vertical loads spreads to about L₀/6 and suggest that the effective slab width in the negative moment zone is B_e = L₀/3 + b, similar to the positive moment case. Lastly, a negative bending design method was proposed, giving good calculations compared to the test results. The slab reinforcement, steel U-section, encased concrete, and bottom reinforcement contribute 53%, 30%, 11%, and 6% respectively to the negative bending capacity on average.