Bearing capacity of spiral-reinforced concrete at the free edge: Experimental and numerical evidence for an additive bearing strength approach

The bearing capacity of concrete under partial area loading at the free edge is critical for designing joints in composite steel-concrete frame structures. Current design standards are often overly conservative, typically prohibiting strength enhancement for edge-loaded conditions. This study systematically investigates — both experimentally and numerically — the bearing strength of plain and reinforced concrete members subjected to partial edge loading. For the first time, it evaluates the efficiency of conventional ties compared to continuous spiral reinforcement in this specific loading configuration. Results demonstrate that spiral reinforcement consistently outperforms conventional ties, with bearing strength increasing proportionally to the reinforcement ratio and loaded area dimensions. Experiments on normal- and high-strength concrete specimens reveal that while plain concrete capacity scales proportionally with compressive strength, the additional gain from reinforcement is largely independent of the concrete grade. This indicates a transition in behavior: geometric effects in plain concrete are multiplicative and proportional to concrete strength, whereas the contribution of transverse reinforcement, which provides lateral confinement, is fundamentally additive. This additive nature is particularly evident in the case of highly efficient spirals. Consequently, traditional multiplicative approaches may lead to unconservative capacity predictions when applied to high-strength concrete members with significant confinement reinforcement.