Seismic Performance Study On Linear Enlarged Diaphragm-through Joints Of Lightweight Aggregate Concrete-filled Square Steel Tubular Column With Variable Sections Of Steel Box Beam And Steel H-section Beam

In this paper,the effect of linear enlarged diaphragms on seismic performance of the joints were studied by applying cycle loading on linear enlarged diaphragm joints of lightweight aggregate concrete-filled square steel tubular column with variable sections of steel box beam and steel H-section beam and basic joint.And then obtained seismic performances of joints,such as failure mode,hysteresis loops,bearing capacity,plastic rotation,skeleton loops,displacement ductility coefficient,stiffness degradation and strain evolution of the joints.According to the dimensionless hysteresis loops and unloading stiffness curve,the restoring force model of linear enlarged diaphragm joints is established and its accuracy is verified.The mechanism of fracture and yield of diaphragm-through joints were analyzed based on the steel ellipsoid fracture model and ellipsoid yield model,the yielding mechanism and fracture evolution law are compared and studied between the basic joint with linear enlarged diaphragm joints,the design method of ductile structure for irregular joints is proposed,the formulas of flexural and shear bearing capacity of such joints are presented.The results of tests and finite element analysis indicates that the plastic rotation of steel box beam and steel H-section beam of the basic joint is respectively about 0.025 rad and 0.03 rad,so the plastic rotation of steel box beam is not up to the requirement 0.03 rad in FEMA.The bearing capacity of joints with linear enlarged diaphragms increased by 49.5%~89.1% compared with that of basic joint,and the displacement ductility coefficient,and the plastic rotation is more than 2.0 and 0.033~0.05 rad,respectively.Plastic hinges form at the section of linear enlarged diaphragms,ductile fracture of butt weld of beam flange,tear of beam web weld of the joint region are typical failure modes of linear enlarged diaphragm joints,however brittle fracture of the beam flanges butt weld is typical failure mode of the basic joint.Drastic geometric changes between beam flange and diaphragm are eliminated,the stress concentration of butt weld are relieved,the stress burden of butt weld are reduced,and the deformation capacity of nodes are greatly improved by using of linear enlarged diaphragms.At the time when the joints are loaded to failure,the panel zones of joints do not yield and no shear crack of welding between through diaphragms and steel tubular column walls occurs.Lightweight aggregate concrete filled in square steel tubular column is not crashed or separated from steel tubular column walls after the square steel tube column are carved.The seismic performance of the joints is mainly dominated by steel beams,through diaphragms and their welds(rather than lightweight aggregate concrete).