I-section columns are still widely used in steel frames at present,while the conventional connection for weak axis of I-section column often brings about brittle fracture of welds which leads to poor seismic performance,ductility and stability.Experimental research and FEA(finite element analysis)have proved better seismic performance and ductility of box strengthen-joint region connection for weak axis of I-section column.Based on former study,the study subjects in this paper are the failure mechanism and design countermeasures of welded haunch reinforced of box strengthen-joint region connection for weak axis of I-section column.The pseudo-static experiment of cross-shaped frame joints was carried out to study the failure mechanism,hysteresis behavior,stiffness,bearing capacity and ductility of the bottom haunch joint.ABAQUS was used to stimulate the specimens in the experiment,then the accuracy and reliability of FEA were verified through comparative analysis.Finally,according to variable parameter methods of FEA,the influence of beam span-depth ratio,axial compression ratio,linear stiffness ratio of beam to column and sizes of haunch members on the mechanical property and failure mechanism of bottom haunch joint was studied.The experimental results show that the bottom haunch joint has similar failure characteristic to the standard joint with box strengthen-joint region.The two joint specimens both failed because of the weld cracking of column level stiffeners and interlayer tearing of covering plates.Compared to standard joint,the bottom haunch joint has better bearing capability,stiffness and ductility.Energy dissipation capacity of bottom haunch joint is better than that of standard joint.Due to the brittle weld cracking of level stiffeners,two specimens have ductility factors less than 3.0,which indicates that the ductility of this kind of connection needs to be improved.The connectional rotation mainly depends on the relative deformation of connection,while the shear deformation of box strengthen-joint region has little influence on it.Stess and strain data reveal that box strengthen-joint region was almost in elastic condition during loading process.That means box strengthen-joint region is a kind of ‘strong joint region '.In addition,buckling of beam top flange occurred in the haunch area on account of complex stress condition.The fracture of beam top flange was more serious than bottom flange,so the safety of top flange should be guaranteed in design.The FEA results show that the haunched beam connection applies to steel frames with beam span-depth ratio more than 15.0 as a means of joint strengthening and repairing.While guaranteeing enough mechanical property and seismic performance,the bottom haunch joint achieves moving beam plastic hinge outside the haunch area.The axial compression ratio of the bottom haunch joint should be limited to 0.6 or less,otherwise column buckling will occur.The bottom haunch and top cover plate connection is able to improve the butt weld stress condition of top flange and make plastic hinge outside the haunch area.The linear stiffness ratio of beam to column makes a great difference to the bottom haunch joint,it is suggested to be controlled in 0.3~0.7.The sizes of haunch members have little influence on the bottom haunch joint performance,and its main function is to improve stress condition of the beam end and force plastic hinge away from joint region.The suggested range of values for the angle of welded haunch is 30°~35°,that of the flange width for welded haunch is 0.75~1.0 times of the flange width of beam.The length of covering plate is also suggested to be taken as 0.5~0.6 times of beam depth. |