Hysteretic Properties Of Buckling-restrained Braces And Seismic Performance Of Buckling-restrained Braced Frames

Buckling-restrained brace is not only a type of metal damper,but also a lateral force resisting member without buckling,consisting of the core member,the restraining member,and the unbonding materials or air gap.Under small earthquakes,buckling-restrained braces will maintain elastic and provide the lateral stiffness for the structure.Under severe earthquakes,buckling-restrained braces will dissipate seismic energy after yielding,with the overall or severe local buckling of core member inhibited by the restraining member,which will reduce the seismic response of the structures and avoid heavy damage to the main structure.Buckling-restrained braces can be divided into two categories by the material of the restraining member,which are the concrete-filled steel tube buckling-restrained braces and all-steel buckling-restrained braces.Though all-steel buckling-restrained braces are easy to be fabricated,the configuration details have yet to be optimized and the cost-effective restraining system is to be developed.Moreover,the application in seismic rehabilitation is to be further investigated.To solve these problems,this paper studied all-steel buckling-restrained braces in terms of the common brace configuration details,new restraining systems and the application of buckling-restrained braces in seismic rehabilitation,through theoretical analysis,experimental research and numerical simulation methods.The main contents and conclusions are as follows:In Chapter 2,the effects of the configurations of the core member on the hysteretic properties of buckling-restrained braces were investigated,so as to provide design recommondations for the novel all-steel buckling-restrained braces.Through experiments,the influence of the configurations of the core member,including the transition mode in the transition segment and the length of the core projection and transition segment into the restraining members,on the cumulative plastic deformation and failure modes of all-steel buckling-restrained braces was evaluated.Moreover,strain concentration induced by the configurations of the core member was compared through finite element methods,which revealed the reason for higher low-cycle fatigue property of the improved specimens.The results show that the arc-shaped transition mode and the increase of the length of the core projections and transition segment into the restraining members can alleviate the strain concentration and stress concentration of the core member,which will significantly improve the low-cycle fatigue properties.In Chapter 3,the influence of the unbonding materials on the hysteretic properties of all-steel buckling-restrained braces was firstly studied,to provide theoretical foundation for the novel all-steel buckling-restrained braces.The hysteretic curves,failure modes and cumulative deformation capacity of all-steel buckling restraining braces with and without the unbonding materials were compared through experiments.Further,the multi-wave buckling behavior of the core member was predicted by theoretical derivation,and the formula of the compression strength adjustment factor was proposed.Moreover,the contact force between the core member and restraining member and the strain concentration through the longitudinal direction were investigated by finite element analysis.Finally,the design recommendations,including the out-of-plane gap width and the unrestrained width to thickness ratio,were proposed based on the test and finite element analysis.The results show that the hysteretic properties of the specimens with the unbonding materials are better than those without the undonding materials.In engineering practice,the unbonding materials are not necessary for all-steel buckling-restrained braces,but for high-performance buckling-restrained braces with long yielding segment and thin core member,the unbonding materials should be employed.In Chapter 4,a novel partly restraining mechanism was proposed,where only part of the core member was wrapped by the restraining member,improving the use efficiency of the restraining member and making the damage measurable after earthquake.A novel all-steel buckling-restrained brace was proposed based on the restraining mechanism and the design reccommations of the configurations in Chapters 2 and 3.Its hysteretic performance and the buckling behavior of the core member were compared with those of the conventional all-steel buckling-restrained braces.Further,the contact pressure,the wave length and the strain distribution of the core plate of the novel all-steel buckling-restrained brace were investigated through finite element analysis.The results show that,when the unrestrained width to thickness ratio bu/t is less than 5,the performance of the novel all-steel buckling-restrained braces is close to the conventional all-steel buckling-restrained braces.In Chapter 5,design recommendations of the novel all-steel buckling-restrained braces were proposed,which provide theoretical foundation for the novel H-shaped all-steel buckling-restrained braces and the application of the partly restrained mechanism.The unrestrained section of the core plate was equivalent to a one-direction uniform compressed simply-supported plate.Then,elastic buckling load,inelastic buckling load,and plastic critical thickness ratio(bu/t)cr was derived.Moreover,parametric studies was performed to study the influence of the key parameters,including the out-of-plane gap width d,the core thickness t,the width of the restrained section b-bu,the width of the unrestrained section bu and the unrestrained width to thickness ratio bu/t.Finally,the design recommendations of the novel buckling-restrained brace under two performance levels were proposed.In Chapter 6,the partly restrained system was applied to the H-shaped core member,and the deformation pattern of the partly restrained core member under different boundary conditions was studied,which provide design principle for the H-shaped buckling restrained brace and its application on the rehabilitation of the braced frames.End-strengthened and middle-weakened H-shaped buckling-restrained braces were proposed.The hysteretic properties and buckling behavior of the two types of H-shaped buckling-restrained braces were evaluated by tests.Further,the failure of the bolts and the contact force on the flange were analyzed through finite element analysis.Finally,the design recommendations on the unrestrained width to thickness ratio of the flange were proposed for the two types of braces by numerical analysis.In Chapter 7,two novel rehabilitation methods were proposed,which turn conventional H-shaped braces to buckling-restrained braces in braced frames,and the seismic performance of the buckling-restrained braced frames was investigated.3-storey and 9-storey existing concentrically braced frames in 8 degrees(0.20g)region were firstly designed,and then two member-level seismic rehabilitation schemes were proposed according to the ideas of end-strengthened and middle-weakened H-shaped buckling-restrained braces in Chapter 6.Besides,the design value of the restraining width of each side of the flange was recommended.Through pushover analysis and nonlinear time history analysis in OpenSees,the seismic performance of the rehabilitated buckling-restrained braced frames was assessed.Finally,the effect of the compression strength adjustment factor and the equivalent viscous damping on the seismic performance of the buckling-restrained braced frames was investigated.