| Buckling-restrained braces(BRBs)are one type of metal dampers that utilize the plastic deformation of steel core to dissipate the seismic energy,and they can provide lateral stiffness to structures as common braces.The BRB mainly consists of the steel core and the restraining member.The steel core is designed to dissipate seismic energy and to resist the axial load,and the restraining member is intended to prevent the BRB buckling.Although the physical configuration of the BRB is simple,the buckling mechanism of the BRB is difficult to reveal.The behavior of BRBs is related not only to the strength,stiffness of the steel core and the restraining member,but also to the gap and friction between them.Therefore,analyses on,and design methods of,BRBs have been receiving much attention both academically and industrially.And to ensure the BRB possessing the stable capacity of dissipate seismic energy,it is greatly meaningful for developing the study of stability of gusset plates connected to the BRBs.This dissertation focuses on the seismic behavior and stability design methods of BRBs considering the effect of friction and gusset plates,and it contains the following four parts:(1)Based on the existing research of our team,this dissertation investigates the effect of imperfection of the steel core with a simplified analytical model firstly.Then,two analytical models are built to investigate the buckling mechanical of the steel core for the BRBs with hinged and fixed ends considering the influence of the friction between the restraining member and the steel core.By using the moment equilibrium equations and boundary conditions of the steel core and the restraining member,the analytical expressions of contact force,the bending moment of the restraining member and the stiffening part,maximum BRB axial force capacity can be obtained.Furthermore,the results of finite element analysis validate these analytical ones.The global stability design criteria of BRBs with hinged or fixed ends are improved by utilizing the obtained formulae considering the effect of friction between the core and the restrainer,and the local stability formula of BRBs with the flat steel core plate is also improved with the acquired formula of maximum contact force.Finally,the global stability design criteria are preliminarily testified by the existing tests of our group.(2)Four full scale specimens are conducted to further validate the stability design methods of BRBs,and the specimens include three BRBs with mortar-filled steel tube and one with steel tube.The production and design process are introduced in detail,and the improved process technology is used to avoid the fracture occurring at the connection between yielding part and stiffening part.The maximum bending moments of stiffening part and the restraining member,buckling wave lengths of the steel core are measured from the specimens to check the analytical equations proposed in the previous chapter.Finally,the proposed stability design criterion of stiffening part of BRBs with fixed ends and local stability formula of the restraining member are confirmed by the experimental results.(3)Based on elastic-plasitic buckling theory of plate,a stability design method of gusset plates is proposed,which considers the effect of yielding of beams or columns around joints under severe earthquake.According to the actual behavior of gusset plates and the elastic-plasitic buckling theory of plate,the analytical model and the formula of stability bearing capacity of gusset plates are proposed firstly.Then,the structural finite element models are simplified to the simplified gusset models to parameter analysis based on the actual behavior of gusset plates.Finally,the buckling coefficients of gusset plates are obtained by nonlinear fitting according to the results of parameter analysis,and the design formula is checked by numerical examples.(4)The actual boundary condition of BRBs is not fully fixed rigid or ideal pinned,because BRBs is connected through gusset plates to main structure in reality.To validate the stability design methods of BRBs and gusset plates,a quasi-static test of buckling-restrained braced steel frame is conducted.The production process is introduced in detail,and the seismic behavior and failure mode of specimen are analyzed by the measured results from the test.Finally,the stability design criteria proposed in the previous chapter are confirmed by the test. |
