Experimental Study On Seismic Behavior Of Prefabricated Joints With Composite CFST Column And Steel Beam

Composite CFST is a new type of structure based on ordinary concrete filled steel tube structure,which is formed by adding another steel tube inside the outer steel tube.Compared with ordinary CFST structure,composite CFST structure has better constraint effect on concrete in steel tube,higher carrying capacity,more reasonable mechanical performance,better fire resistance,and the construction is convenient.Composite CFST has a broader application prospect and research value.For the first time,composite CFST column and steel beams were connected by blind bolts.The connection of joints was simple and met the requirement of prefabricated structure.The load path of the joint was clear.And it overcome the disadvantages of large number of welding and complex construction of composite CFST joints in the past.In this paper,six prefabricated joints with composite CFST column and steel beam were designed.The seismic performance of this new type of joint was studied under the following test variables:flange plate thickness of T-stub,ribbing of T-stub,number of ribbed plate,beam-column linear stiffness ratio and bolt connection form.The failure characteristics,hysteretic behavior,skeleton curve,ductility and carrying capacity,energy dissipation capacity,degradation of strength and stiffness,comparison of test variables and strain variation at main locations were analyzed.The main conclusions are as follows:(1)The blind bolt connection of the prefabricated joints with composite CFST column and steel beam is safe and reliable,and the installation is convenient.There was no pull-out or break of blind bolts in the test,and only minor bending deformation occurred of the blind bolts.It could better play the role of the composite CFST column and greatly simplified the process of joints fabrication.It shows that it is feasible to apply blind bolts to composite CFST prefabricated joints.(2)The failure characteristics of the prefabricated joints with composite CFST column and steel beam were mainly divided into the four categories as follow:the plastic deformation of T-stub was large,and the beam end bent;connecting bolts of beam flange and T-stub were cut off;the plastic deformation of T-stub was small,and the beam end bent;T-stub flange and web were torn,and the beam end bent.From the failure characteristics,it can be seen that the connection of the prefabricated joint is safe and reliable.It meets the seismic design requirements of "strong column and weak beam" and "strong connection and weak member".(3)The hysteretic behavior of the joints had good performance.The hysteretic curve of the initial stage of loading was shuttle-shaped.The hysteretic curve was pinched as the loading continued due to the deformation of T-stub,but it was still full S-shaped and had good energy dissipation capacity.The ductility of joint was good.The ductility of joints could be divided into two ranges according to whether or not the T-stub was ribbed.The ductility of the joints without ribbing was obviously higher than that of the joints with ribbing.Increasing the beam-column linear stiffness ratio,adding double ribs to T-stub,adding single rib to T-stub and increasing the flange thickness of T-stub could increase the carrying capacity of joints,and the increasing effect decreases in turn.Increasing the beam-column linear stiffness ratio and the flange thickness of T-stub contribute significantly to the energy dissipation capacity of joints.The viscous damping coefficient of the joints increased greatly after the fracture of the weld seam of the ribbed plate.The strength of joint is stable and the stiffness degradation is uniform.(4)Because of the constraint effect of concrete,the strain of inner steel tube developed little,and it basically stopped developing after reaching yield strain under the tension of blind bolt.The strain of the outer steel tube developed little and was always in the elastic range.The strain of flange and web of T-stub was fully developed,and it was the main energy dissipation position of joint.The strain of ribbed specimens developed rapidly after the rib plate failure.The strain of steel beam showed good hysteretic behavior.Except for the specimen with large beam-column linear stiffness ratio,the plastic hinges at the end of beams were formed in all specimens.The steel beam was also the main energy dissipating member of the joint.