Welded flange-bolted web connection is one of the common joints in the steel structure,and the mechanical performance and failure mode of the joint is related to the collapse resistance of steel frame closely.To compare and analyze the failure mode,mechanical property and resistance mechanism under the condition of progressive collapse,the beam-column substructures of three different span ratios(1:0.6,1:1.0,1:1.4)with welded flange-bolted web connection in minor-axis direction were selected,which included three columns and two beams,to conduct monotonic static loading test by alternate load path method.The test results indicated that the fracture of specimens begins with the beam tension flange,and then part of the main internal force is transferred by the bolts on web,followed by buckling of the compression flange of the beam end.Finally,specimens are deactivated because of the shear failure on bolt holes or fracture of web and junction plate.The process of anti-collapse mechanism appears the flexure phase,flexure-catenary mixed phase and catenary phase.It could be found that joint with welded flange-bolted web connection possesses high redundancy,which has enough rotational capacity after the fracture of tension flange.The catenary could be developed and take the lead in the following great deformation in two factors,which are the magnitude of the axial tension generated in the beam-column members,and the sufficient rotation angle of the beam end.The deformation of the beam-column joint is beneficial for the beam-column substructure to bear the load by collaboration between the beam and the column.For the unequal span specimens,the initial fracture of substructure is always at the short-span beam with small span-depth ratio and the short beam has better catenary mechanism than the long one.
