Study Of Experiment, Fracture Mechanism And Design Method On Diaphragm-through Irregular Joints Of Steel Box Column And H-section Steel Beam

In this paper, five diaphragm-through irregular joints of steel box column and H-section steelbeam with enlarged arc junctures and one conventional irregular joint were tested under low-reversed loading. The effects of enlarged arc junctures on failure mode, bearing capacity, plasticrotation, hysteresis loops, rigidity degradation and energy dissipation of diaphragm-throughirregular joints were investigated. The numerical simulation base on the steel ellipsoid fracturemodel considering welding defects and welding residual stress and coupled ellipsoid yield modelof irregular joints was performed. Seismic performance and plasticity developing process of theirregular joints with enlarged arc junctures and conventional irregular joints were studied. Failuremechanism of irregular joints was analyzed. Ductile structural study and design method ofirregular joints were put forward.The study results indicate that the conventional irregular joint brittle fractures at butt weldbetween the flange of stronger beam with large rotation rigidity and diaphragm-through rib, and theplastic rotation of the conventional irregular joint, couldnt match the minimum requirement0.03rad in FEMA. For the irregular joints with enlarged arc junctures, the plastic hinge is obviouslyformed at enlarged section of diaphragm and the plastic rotation reach0.033~0.044rad. Thebearing capacity and the energy dissipation capacity of the irregular joints with enlarged arcjunctures increase by41.7%~53%and173%~500%, respectively, compared with that of theconventional irregular joints. The sharp changes and stress concentration near butt weld in theconventional irregular joint are eliminated by using of enlarged arc junctures and moving butt weldto the location far away from the connection zone, and therefore the overlap and interaction ofwelding residue stress are mitigated. Buckling and ductile fracture of enlarged arc diaphragm,shear fracture of welding between diaphragm-through ribs and column walls and fracture ofwelding holes at beam web are typical failure modes of the diaphragm-through irregular joints withenlarged arc junctures. In this experiment, the strong beam fracture earlier than the weak beam, andtearing failure of welding between columns walls, which frequently occurred in the previousexperiments of joints with interior stiffened ribs, doesn’t take place. Box column was basically inelastic stage. The contribution of column end deformation for the plastic rotation is a little.