Experimental and numerical research on ultimate bearing capacity of steel-concrete-steel composite deep beams with UHPC/ECC considering interfacial bonding

Using ultra-high performance concrete (UHPC) and engineered cementitious composites (ECC) in steel–concrete–steel (SC) structures can greatly improve ultimate strength, but the interfacial bonding between steel and concrete is also key to structural behaviors. Most studies focus on shear connectors and overlook this interfacial bonding. While UHPC and ECC help improve regular beams, deep beams can still fail in shear, so further research is needed to improve their safety and design. In this study, static tests were conducted on SC deep beams incorporating UHPC or ECC. Failure modes were identified, and the influence of concrete type, shear connectors, interfacial bonding, and steel plate thickness was examined. The results show that UHPC increased the peak load by 31.9%, while ECC improved the ductility coefficient by 575.4% compared with ordinary concrete beams. In addition, UHPC and ECC effectively enhanced resistance to interfacial slip and promoted better composite action. Finite element models were developed and validated against the experimental data. By integrating simulation and test results, the load transfer mechanism, typical load–deflection responses, and compression zone height were analyzed. Based on these findings, failure criteria for SC beams were established. Furthermore, a slip coefficient was introduced to quantify the degree of composite action, and predictive equations for the ultimate bearing capacity were proposed and validated. These equations provide a practical reference for the design and evaluation of SC beams in engineering applications.