Seismic Performance Test And Analysis Of Self-Centering Prestressed Braces With Variable Friction Dampers

Self-centering Friction Energy Dissipative Brace（SCFEDB）has clear mechanical performance,stable energy consumption,good hysteresis performance and self-centering capability.However,the normal pressure of the friction surface is constant and the friction coefficient of the friction material is constant when the SCFEDB works,which leads to the constant output friction force of SCFEDB in an earthquake,and the brace can not change its friction force timely according to the deformation of the structure in actual earthquakes.To improve the performance of traditional SCFEDB,this paper proposes a new type of Self-Centering Prestressed Brace with Variable Friction Dampers（VFD-SCPB）which can change friction and stiffness with the loading displacement changed.In this paper,a series of hysteretic behaviors of the VFD-SCPB are studied,and the seismic performance of VFD-SCPB steel frame is analyzed.The main research contents and conclusions are as follows:（1）A new VFD-SCPB design is proposed by combining the variable friction energy dissipation system with the prestressed self-centering system,in which the variable friction energy dissipation system adopts the "disc spring slope" mechanism.The variable friction energy dissipation mechanism and self-centering mechanism of the brace are explained,and the stiffness and restoring force models of each stage are derived.The design method and assembly process of VFD-SCPB are proposed,and the hysteretic behavior and energy dissipation capacity by the load-displacement hysteretic model of VFD-SCPB are studied.（2）Two VFD-SCPBs were designed and manufactured for Pseudo-Static Test.The hysteretic curves of the braces in the plane sliding section present a traditional flag shape.After reaching the uphill section,the expected goal of variable stiffness is achieved,showing a new flag shape with obvious secondary starting point and three section stiffness.The hysteretic curve of VFD-SCPB is stable and full,and more seismic energy will be dissipated when the displacement is larger.Whether the brace is in the tension or compression stage,basalt fiber-reinforced polymer（BFRP）tendons show elongation,and the loss of prestress is small,which can provide stable restoring force.（3）Based on Open Sees,the secondary development of the VFD-SCPB material constitutive model is carried out.The hysteresis curve of the second development constitutive model is compared with the hysteresis curve of quasi-static test and theoretical hysteresis curve,which verifies the effectiveness of the development of the VFD-SCPB constitutive model.The effects of the key hysteretic parameters of VFD-SCPB on the hysteretic behavior of VFD-SCPB are studied.The results show that the energy dissipation capacity of VFD-SCPB can be improved by appropriately increasing the first and third stiffness of VFD-SCPB or the initial preload of the energy dissipator,or reducing the length of the plane sliding section of the variable friction dampers,but the residual displacement of the brace increases with the increase of the initial preload;Increasing the first stiffness and initial prestress of VFD-SCPB can reduce the residual displacement of the brace and improve the self-centering ability of the brace.（4）The analysis model of the VFD-SCPB hinged steel frame is established.The seismic response of the structure under design basis earthquake（DBE）and maximum considered earthquake（MCE）is studied by nonlinear time history analysis,and the influence of key parameters on the seismic performance of the structure is revealed.The results show that VFD-SCPB can reduce the story displacement angle response of steel frame,and effectively control the residual displacement angle of structure,meanwhile improve the post earthquake repairability of structure,and achieve the expected displacement control goal and self-centering function.Properly increasing the first stiffness and initial prestress and initial preload of VFD-SCPB can reduce the story displacement angle of the structure,and improve seismic performance of the structure;properly increasing the third stiffness of VFD-SCPB and reducing the length of plane sliding section can effectively reduce the displacement response of the structure under MCE.