Experimental Study On Seismic Behavior Of Welded Joints Of Strengthened By Outer End Plates Of Concrete-filled Square Steel Tubular Columns And Steel Beams

Beam-column joints are the core areas in the frame system,which bears the function of transferring shear force and bending moment when the structure is under stress.However,the current types of joints are not abundant enough to meet the needs of practical engineering.At the same time,in order to solve the seismic load of traditional through-core bolt-stiffened end plate connection defects such as bolt relaxation and even brittle failure is likely to occur under the action.This paper proposes a fully welded joint form which is welded joints of strengthened by outer end plates of concrete-filled square steel tubular columns and steel beams,which not only has good seismic performance,but also the entire structure.No bolts are provided,and the structure is simple and beautiful.The combined method of experiment and numerical simulation is used to study the seismic performance of the joint.The main research contents of this article are summarized as follows:(1)In this paper,three specimens of two types of joints were designed,one is the traditional core piercing bolt-stiffening end plate beam column joint,which is used as the reference for the new type of joints,and the other two are welded joints of strengthened by outer end plates of concrete-filled square steel tubular columns and steel beams: one is the beam unreinforced welded joint,the other is the beam reinforced welded joint,so as to compare the seismic performance of the new type of joints at different beam-column line stiffness ratios.(2)Three specimens were subjected to low-cycle reciprocating load test,and the failure process and failure pattern of the joints were analyzed.The results show that the hysteretic curves of all specimens are full and show obvious shuttle shape,showing good ductility and energy dissipation capacity.For welded joints,when the beam-column line stiffness ratio of the joint is 0.28,its failure mode is the same as that of the core piercing bolt-stiffening end plate beam column joint,which is a typical plastic hinge failure mode and meet the requirements of seismic design,but the core piercing bolt-stiffening end plate beam column joint has severe deformation in the plastic hinge area and high-strength bolts have loosened,and the welded joint effectively solves this problem.;when the beam-column stiffness of the joint is 0.41,the bearing capacity of the joint increases significantly,but the ductility of the joint has been reduced and the weld in the core area of the joint has cracked,which does not meet the requirements of seismic design.Therefore,the beam-column line stiffness ratio of the new joint needs to be controlled,and the weld size,strength and the welding technology should meet the relevant requirements,so that the plastic hinge failure of the steel beam precedes the weld crack in the joint core area.(3)Based on experimental data,this paper establishes three-fold restoring force model with stiffness degradation for new joints by using experimental fitting method.The stiffness degradation laws of the hysteresis curve conform to the logarithmic function relationship.The analysis results show that the restoring force model established in this paper is reliable,which can lay a foundation for studying the seismic behavior of the new joints.(4)The ABAQUS is used to conduct numerical simulation analysis on the new type of joint and the core piercing bolt-stiffening end plate beam column joint.the results verify that the finite element simulation is more practical.Based on this,the main parameters of the joint are changed to study their influence.The main parameters such as concrete strength,axial compression ratio,steel beam web thickness,outer end plate thickness and stiffening rib.Numerical simulation results show that: the strength of concrete has almost no effect on the seismic performance of the joint;due to the effect of P-?,increasing the axial compression ratio greatly reduces the bearing capacity and ductility of the joint;increasing the thickness of the web of the steel beam significantly increases the bearing capacity of the joint;The change of the thickness of the outer end plate within a certain range has no obvious effect on the seismic performance of the joint;The existence of stiffening rib makes the plastic hinge area of the steel beam shift to the variable cross section,and the steel beam changes from brittle failure to ductile failure,thus greatly improving safety of structure.