Experimental study and theoretical analysis on ultimate bearing capacity of steel-concrete-steel deep beams with UHPC after freeze-thaw cycles

In cold climate regions, steel-concrete-steel (SCS) composite structures are often exposed to repeated freeze-thaw cycles, raising concerns about their long-term durability. Although the application of ultra-high-performance concrete (UHPC) with high strength and durability in SCS structures is becoming increasingly popular, there is limited research on its performance under freeze-thaw conditions. This study examines the mechanical behavior and failure modes of SCS deep beams filled with either UHPC or ordinary concrete following freeze-thaw exposure. Static loading tests reveal that UHPC enhances structural performance by improving shear resistance, preventing interfacial slip, and maintaining strength and integrity under freeze-thaw conditions. To gain deeper insight into the failure mechanisms, a concrete degradation model is developed using a multi-layer perceptron neural network combined with plasticity and damage mechanics. This model is integrated into a finite element analysis framework to simulate the internal stress distribution and failure progression within SCS deep beams. Based on the results from both experiments and simulations, a predictive formula for the ultimate bearing capacity of SCS deep beams is proposed. This formula considers the effects of composite action and fiber bridging, providing a practical and reliable tool for the structural design and durability evaluation of SCS deep beams subjected to freeze-thaw environments.