A novel strengthening measure for steel-concrete composite beams with web openings based on partial steel fiber reinforcement: Mechanical performance and numerical modeling

Although steel–concrete composite beams with web openings can effectively optimize spatial configurations, the presence of openings severely degrades their cross-sectional stiffness and ultimate load-carrying capacity. To address this issue, this paper proposes a novel strengthening measure for steel–concrete composite beams with web openings based on partial steel fiber reinforcement, wherein the concrete slab within the opening region is precisely reinforced by localized placement of fibers. Through monotonic static loading tests and nonlinear finite element analyses conducted on six steel fiber-reinforced specimens and two reference beams, the effects of steel fiber (SF) volume fraction and configuration on the structural performance of the composite beams were thoroughly investigated. The results demonstrate that the partial steel fiber reinforcement can precisely strengthen the concrete slab in the opening zone, substantially enhancing the ultimate load-carrying capacity (by 18.64%–45.78%) and deformation capacity (by 9.58%–68.54%) of the beams. The crack-bridging and frictional effects of the steel fibers effectively suppress crack propagation and optimize the mechanical response within the opening region, causing the failure mode to transition from a catastrophic Vierendeel failure to a more ductile, incomplete Vierendeel failure. Comparative analysis reveals that the B-LOR (Bottom and Local Opening Region) group exhibits the optimum load-carrying capacity, while the D -PLB (Distributed Partial Length at Bottom) group saves 50% of the steel fiber dosage while simultaneously balancing structural performance and economic efficiency. Acoustic emission (AE) monitoring indicates that a high volume fraction (1.5%) stabilizes crack evolution, with the tensile-to-shear crack ratio of the B-LOR group approaching approximately 1:2 near the ultimate limit state, providing a pronounced early warning prior to structural failure. Furthermore, steel fibers with an aspect ratio of 75 achieve the superior anchorage performance, and yield the optimal strength-toughness balance when coupled with C50 concrete at a 1.5% volume fraction.