Seismic behavior of WUF-B connections repaired and strengthened using field-welded wing plates

This study examines the cyclic deformation performance of per-Kobe steel welded beam-to-box column connections repaired with wing plates after seismic fractures. Finite element (FE) models were developed to simulate the pre- and post-repair seismic behavior of tested bare steel and composite welded unreinforced flange-bolted web (WUF-B) connection specimens and investigate their mechanisms of stress transfer. The findings indicate that wing plate repairs lower the neutral axis to one-third of the beam height in bare steel connections, sharply increasing the rupture index in top flange. For composite beam connections, the presence of an RC slab intensifies stress concentration near the bottom flange, causing the fracture at the tip of wing plates. Subsequent parametric analyses indicate that, for bare steel connections, wing plate with an effective area 1.2 times that of the beam flange section ensure sufficient plastic rotational capacity. However, composite beam connections require wing plates with a larger effective section area (about 1.4 times the flange section area) to effectively mitigate strain concentrations at the wing plate tips and facilitate the transfer of the plastic hinge to the beam. The study also offers further repair modifications and design recommendations, revealing that adding transverse web stiffeners (TWSs) to wing plate repairs significantly reduces web buckling and enhances connection performance. The use of T-shaped stiffeners and modified wing plates provides flexibility under different construction conditions and demonstrates strong repair performance.