Interfacial Mechanisms and Shear-Key Improvement for an Assembled Integral Multi-Ribbed Composite Floor System

A novel assembled integral multi-ribbed composite floor system consisting of precast panels and a cast in situ topping has previously been validated through full-scale one-way and two-way slab experiments. Although the global load capacity of the system was initially verified, the experiments reveal that the concrete-to-concrete interfacial behavior between the precast panel and the cast in situ topping is a critical factor governing internal force redistribution and its post-cracking performance. To uncover the governing interfacial mechanism, this study develops a refined three-dimensional nonlinear finite element model using a coupled cohesive–frictional interface interaction, where a surface-based cohesive interaction captures the initial interfacial debonding and a penalty-based Coulomb friction model describes the subsequent shear-slip behavior. The model reproduces the cracking patterns, load-deflection relationship, and failure modes, with the relative error of load capacity, initial stiffness and crack load all less than 15%. Parametric analyses further indicate that the interfacial shear-transfer mechanism governs post-cracking stress redistribution across the multi-ribbed section, preventing premature delamination and ensuring efficient mobilization of the section’s flexural resistance. To effectively restrain this interfacial slip, an improved shear-key configuration is proposed to activate an enhanced mechanical interlocking mechanism. Numerical results confirm that the improved configuration effectively suppresses macro-sliding and redistributes local stress concentrations, thereby enhancing the structural integrity and flexural efficiency of precast composite floor systems.