Post-earthquake repair of welded unreinforced flange-bolted web connections considering composite slab effects

This study investigates two proposed post-earthquake repair techniques aimed at restoring and enhancing the performance of previously fractured steel beam-to-box column connections with RC slabs. Initially, two full-scale composite subassembly specimens with welded unreinforced flange-bolted web (WUF-B) connection details were designed and cyclically loaded until fracture occurred. These damaged connections were then repaired using the proposed techniques and retested. The original specimens fractured at the beam bottom flange at 3% drift. A comparative analysis showed that after repair, the connections exhibited significant enhancements: maximum strength, cumulative plastic deformation, and energy dissipation increased by approximately 1.2, 1.5, and 1.8 times, respectively. The repairs involved installing wing plates on the bottom flange and supplemental welds to the web, along with welding the bottom weld access hole. However, adding inner flange stiffeners (IFSs) to the top flange did not markedly improve performance, mainly due to the RC slab's influence. Both repair schemes effectively countered the adverse effects of composite action, achieving notable connection plastic drifts over 4%. Additionally, the effects of the RC slab on the repairs were explored in comparison with test results from bare steel beam specimens utilizing the same repair techniques. The RC slab's presence altered the connection's neutral axis, which helped prevent top flange fracture in the wing plate repair scenario, while resulted in earlier fracture of wing plates in the scenario involving both wing plates and IFSs. The experimental results were confirmed by finite element analysis, validating the model's reliability in assessing the cyclic performance of repaired connections.