Seismic Performance Of Self-centering Concrete-filled Square Steel Tubular Column-to-steel Beam Connection Equipped With SMA Bolts

The self-centering structure has the advantages of small residual deformation after earthquake,good energy dissipation capacity and rapid repair,and has gradually become a recoverable functional structure with high seismic resilience.At present,the self-centering structure mainly uses post-tensioned prestressed steel strands and prestressed tendons as self-centering elements.However,the ultimate deformation capacity of post-tensioned steel strands and prestressed tendons is limited,and there is a risk of fracture.Under long-term load,prestress relaxation of steel strands and tendons will occur,which will affect the self-centering ability of the self-centering structure.In this paper,superelastic SMA(Shape Memory Alloy)bolts are introduced into the frame joints of composite structures by using the superior superelasticity and certain energy dissipation capacity of SMA materials.At the same time,brass friction energy dissipation elements with simple structure and stable performance are added to the beam-column joints to effectively reduce the residual deformation of the structural system and improve the stable energy dissipation capacity of the joints,thus providing a new idea for constructing a new self-centering structural system with controllable damage.In this paper,the basic mechanical properties of SMA bars and the seismic performance of self-centering concrete-filled square steel tubular column-to-steel beam connection equipped with SMA bolts are systematically studied by means of theoretical analysis,experimental research and numerical simulation.The main research contents and conclusions of this paper are as follows:(1)The mechanical properties of large-size NiTiSMA bars under different loading schemes were tested.According to the indexes of martensitic positive transformation stress,single-cycle energy dissipation,secant stiffness,equivalent damping ratio and residual strain,the effects of heat treatment strategy,loading cycles,strain amplitude,SMA bar diameter,loading rate and pre-strain on the mechanical properties of SMA bars were analyzed.The microstructure of the fracture surface of SMA bar was analyzed by SEM(Scanning Electron Microscope).An improved constitutive model of superelastic SMA strain hardening section is proposed and verified by finite element analysis.(2)A self-centering concrete-filled square steel tubular column-to-steel beam connection equipped with SMA bolts is proposed,and the structural form and working principle of the joint are introduced in detail.According to the ’flag-shaped’ constitutive model of superelastic SMA bar and the rigid-plastic model of web friction device,the mechanical behavior of the two is superimposed,and the bending moment-rotation model of self-centering concrete-filled square steel tubular column-to-steel beam connection equipped with SMA bolts is established.Through the force analysis of the self-centering connection,the formulas for calculating the bending,shear bearing capacity and stiffness of the SMA self-centring connection are established.(3)Low cyclic loading tests were carried out on 11 concrete-filled square steel tubular column-to-steel beam connection.According to the deformation characteristics,failure mode,stiffness,bearing capacity,ductility,self-centering ability,energy dissipation capacity and strain of key parts of the specimens,the effects of pre-strain of SMA bolt,initial pre-tightening force of energy dissipation device bolt,different axial compression ratios of columns and main aftershocks on the mechanical properties of SMA self-centring connections were analyzed.The test results show that the hysteresis curves of SMA self-centring connections are typical ’double flag’ type,with good self-centering performance,energy dissipation capacity and aftershock resistance.(4)The finite element model of self-centering concrete-filled square steel tubular column-to-steel beam connection equipped with SMA bolts is established by ABAQUS,and the nonlinear finite element analysis is carried out.By comparing with the hysteresis performance of the test connections,the feasibility of the superelastic SMA constitutive model to simulate the austenitic SMA bolt and its connections is verified.On this basis,the influence of SMA bolt diameter and friction coefficient on the seismic performance of SMA self-centring connections is analyzed.The numerical results show that increasing the diameter of SMA bolt can effectively improve the mechanical performance of SMA self-centring connections.Selecting a reasonable friction coefficient can obtain a self-centring connection with good performance and stability.(5)Two four-span five-story plane frame structure models were established by ABAQUS,and the seismic response time history analysis was carried out.The calculation results of ordinary semi-rigid connection frame(SRF)and SMA self-centering frame(SCF)were compared and analyzed.The results show that the inter-story drift angle of SCF frame structure is generally larger than that of SRF frame structure.The base shear force,residual inter-story drift angle and acceleration response of SCF frame structure are smaller than those of SRF frame structure,indicating that SCF frame structure has good self-centering ability and repairability after earthquake.According to the research results of SMA self-centering connections and frame structure,combined with the requirements of the current relevant specifications at home and abroad,the performance-based seismic design method of self-centering concrete-filled square steel tubular column-to-steel beam frame equipped with SMA bolts is proposed.