Behavior and design of steel-UHPC composite beams subjected to negative moment

To improve the crack resistance of concrete deck slabs of steel–concrete composite beams in the negative moment regions, Normal Concrete (NC) and Ultra-high Performance Concrete (UHPC) were respectively used to cast the deck slab to investigate the effects of different slab materials on the failure modes, crack characteristics and flexural performance. The flexural performance tests under negative bending moment were conducted, and the impact of different slab materials on the mechanical performance of the composite beams were analyzed using the numerical simulation method verified by the tests. The method of calculating the ultimate flexural capacity of steel-UHPC composite beam, which considered the material micromechanics mechanism such as fiber distribution, orientation, burial depth and interfacial bond strength, was proposed based on the simplified plasticity theory and were verified with existing method by the test results of related literature. The results show that the force characteristics of the two composite beams were similar, and the failure modes were both flexural failures. Compared with the steel–concrete composite beam, the cracks of the steel-UHPC composite beam were more evenly distributed and narrower, with larger crack distribution spacing and fewer secondary cracks. The cracking resistance of composite beam can be effectively improved by using UHPC slab, and the flexural capacity improved slightly. The cracking load of the deck slab of the steel-UHPC composite beam was improved by 500%, the corresponding load when the crack width reached 0.2 mm was improved by 37.5%, and the ductility and ultimate flexural capacity were improved by 30% and 5 %. In addition, the ultimate flexural capacity of steel-UHPC composite beam should consider the tensile contribution of UHPC, and the proposed calculation method is suitable for steel-UHPC composite beam with different UHPC mix ratios.