Research On Energy Dissipation Mechanism Of The Overlapped K-joints Connecting Concrete-filled Double Skin Steel Tube And Steel Tube In Chord

The direct welded hollow section joints are often damaged due to the lack of radial stiffness of the chord.In addition,the hidden welds will also have adverse effects on the joints when they are not welded.In order to effectively avoid this phenomenon,give full play to the mechanical properties of the material,and seek efficient and reasonable strengthening measures,aiming at the hollow section overlapped K-joints of the steel tube truss structure,concrete was added to the interlayer between the chord and the inner steel tube to strengthen the joint.Based on the existing tests,the finite element model of K-joints was constructed by ABAQUS.The hysteretic performance of the joints was evaluated quantitatively from five aspects:failure form,bearing capacity,deformation characteristics,stiffness degradation law and energy dissipation capacity.The following aspects were studied:(1)ABAQUS was used to analyze the hysteretic performance of K-joints with hollow ratio ψ of 0,0.25,0.50,0.75 and 1.0 respectively.The unreinforced K-joints(ψ)were analyzed=1.0)the failure mode was as follows:the brace(toe)on the compression side has buckling phenomenon,the brace on the tension side has slight shrinkage phenomenon,the connection part of brace and chord should be concentrated,the surface of chord has plastic deformation,the chord wall on one side of the compression branch pipe has "depression",and the chord wall on the tension side has "bulge".The failure mode of the K-joints(0<ψ<1.0)with the chord interlayer poured into the concrete was shown as follows:the left compression brace has buckling deformation,the right tension brace has slight shrinkage,and the chord surface at the joint of the chord and braces has no obvious deformation,and the stress concentration of the chord wall has obvious improvement.When the chord was completely filled with concrete,the failure mode of K-joint(ψ=0)was basically the same as that of the reinforced K-joints,but the former has higher stress value of web member and lower stress value of concrete than the latter.(2)The strengthening method of grouting concrete into the interlayer between the chord and the inner steel tube can improve the bearing capacity of the joint.After strengthening,the stress concentration of the joint was obviously weakened,the plastic deformation of the chord wall was controlled,and the deformation capacity and energy dissipation capacity of the joint were also improved.When the hollow ratio of the chord increased,the bearing capacity of the joint decreased to a certain extent.However,the deformation capacity and the equivalent viscous damping coefficient of the joint increase to a certain extent.It can be concluded that when the hollow ratio ψ=0.75,the K-joint has the best hysteretic performance.(3)A large number of parameter analysis of the joint shows that when the strength level of the concrete in interlayer continues to improve,the bearing capacity and energy dissipation capacity of the joint were not obvious.The concrete stress in the chord was increasing,but the deformation capacity of the whole joint was getting worse.When the steel grade of the inner steel tube became larger,the bearing capacity and energy dissipation capacity of the joint had no relatively obvious change.The stress value of the inner steel tube increased continuously,but the deformation capacity of the joint also became poor.When the parameters β and τ increased gradually,the bearing capacity of the K-joints increased,but when the parameter Ov increased,the bearing capacity of the K-joints decreased.It can be seen comprehensively that the other parameters of the joint were constant,and the energy dissipation capacity and deformation capacity of the joint can be improved by appropriately reducing the diameter ratio β of the chord and brace;when the overlapped ratio was small,it was more beneficial to the deformation and energy dissipation of the joint;the joint with smaller wall thickness of the brace can be selected to weaken the braces,which was beneficial to the energy dissipation of the K-joints.(4)Based on the results of parameter analysis of K-joints,using the appropriate skeleton curve model,through the calculation of the loading and unloading stiffness changes in different stages of the K-joints,the restoring force model of the joint was constructed,which provides a certain reference for the hysteretic energy dissipation of this kind of strengthened K-joints under the action of reciprocating load.At the same time,the construction feasibility of the reinforced K-joints was briefly described,and the solutions to the construction difficulties were put forward.