Buckling Mechanism Of Steel Core And Global Stability Design Method Of Buckling-restrained Braces

Buckling-restrained braces(BRBs) are a type of braces that share the virtues of both common braces and the metal dampers: they can provide lateral stiffness to structures and dissipate energy without buckling during an earthquake. The basic configuration of BRBs is simple: it consists of a steel core to dissipate energy and an outer member to restrain the core from buckling. However, buckling mechanism of BRBs is not so straightforward as their physical configuration appears, since the global and local buckling of BRBs are closely related not only to the stiffness, strength, and initial imperfection of core and restraining member, but also to elastoplastic deformation of the core, stiffening part of the core, gap and friction between the core and restraining member, connection between the brace and gusset plate, stiffness and strength of gusset plate, and so on. Therefore the analysis and design methods of BRBs have been drawing intensive attentions of academics and industries.This study focuses on the global stability design method of buckling-restrained braces considering buckling mode of steel core, and it contains the following parts:(1) Experiment conducted on five specimens is introduced, the specimens include a short specimen, two full scale specimens, a combined specimen test and an experiment done on an improved specimen for validation of the equation for buckling wave length calculation. The production process is introduced in detail and some key problems in producing process is pointed out. The construction of specimen is improved for pasting of strain gages which will be used for measuring the strains of the steel core during loading process.(2) Buckling mechanism of the steel core of buckling-restrained braces is done on a simplified model in Chapter 3. Assumptions are given to simplify the analysis process: there is no initial imperfection in the steel core as well as the restraining member, the restraining member is a rigid body. By using the moment equilibrium equation of the steel core and its boundary conditions, we get the deflection curve of the steel core and the expression of contact force between steel core and restraining member. The buckling mechanism of steel core is revealed through the analysis of the bending moment of steel core. The expressions of length between the contact points and from which the maximum contact force are given and the maximum bending moment of the restraining member is obtained. It is the first attempt that calculating the maximum bending moment of restraining member through the contact force and the buckling wave length. Afterwards, similar conclusion for BRBs with fixed ends is given and the conclusions for both fixed ends and hinged end are validated using finite element method. At last, the conclusions are extend to steel core gets into yield.(3) Global stability design method of BRBs considering buckling mode of steel core is proposed. The analytical model considers brace end and the deformation of the restraining member is used. Thus, both the moment equilibrium equations of steel core and restraining member need to be used to get the deflection curve of the steel core. The expressions of contact forces, maximum bending moment of the restraining member and stiffening part are given. The assumptions that the slope of the tangent lines and bending moment of the steel core at the contact points are zero are demonstrated can be used. The investigation of the stability are done for one and two points contact and then is extend to more general case. Unified equations are proposed to simplify the calculating process, and their precisions are checked.(4) A finite element analysis with ABAQUS is used to validate the global stability design method of BRBs considering buckling mode of steel core. First, the developing process of the buckling mode of steel core is investigated; second, the equations for calculation of buckling wave length and contact forces are validated; at last, the expression for bending moment of the restraining member is validated. The calculating equation for buckling wave length is also demonstrated by test results of the fives specimens introduced in Chapter 2. After been validated, the global design method proposed in this paper is compared with the existing method.