Theoretical And Experimental Study On Design Methods For Truss-Confined Buckling-Restrained Braces

Buckling-restrained braces(BRBs)are widely adopted in high-rise frame structures,long-span spatial structures and bridge structures since they not only provide lateral force resistance,but also dissipate significant amount of energy under moderate to severe seismic actions,thus mitigating the adverse effect imposed on such structures.Based on innovation of the external restraining system,this paper proposes two novel types of all-steel BRBs,namely rigid truss-confined BRBs(TC-BRBs)and pre-tensioned cable stayed BRBs(PCS-BRBs).TC-BRBs are formed by introducing an additional structural system of rigid truss frames to the outside of a common double-tube BRB,whereas PCS-BRBs are formed by imposing an additional pre-tensioned cable stayed system to the outside of a common double-tube BRB.The external tube combines with the additional external restraining system(either rigid truss frames or cable stayed system)to form an integrated restraining system.As a result,the load resistance and hysteretic performance of the TC-BRBs and PCS-BRBs could be effectively enhanced by restraining the cores from overall buckling for their long-span and large-capacity designs,thus improving their load carrying efficiencies and saving materials.In addition,the architectural aesthetics of the TC-BRBs and PCS-BRBs can be enhanced when they are exposed externally in high-rise and spatial structures.Firstly,elastic buckling performance of various types of pin-ended TC-BRBs and PCS-BRBs are conducted.For TC-BRBs,expresisons for calculating their elastic buckling loads are derived thourgh adopting equilibrium method by considering shear deformation.Theoretical and numerical analysis reveals that their first order buckling modes appear to be single-wave symmetric buckling modes.For PCS-BRBs,expresisons for calculating their elastic buckling loads are derived thourgh adopting equilibrium method,energy method and curve-fitting method(based on a generalized stiffness coefficient of the cable stayed system).Theoretical and numerical analysis reveals that for various types of PCS-BRBs possessing different numbers of cross-arms,their first order buckling modes would transition from single-wave symmetric to double-wave antisymmetric or multiple-wave buckling modes.As a result,corresponding restraining ratios for TC-BRBs and PCS-BRBs are defined and expressed explicitly.Secondly,the respective interrelationship between restraining ratios and load resistance as well as hysteretic behaviour of various types of pin-ended TC-BRBs and PCS-BRBs is obtained through large-deformation elastic-plastic finite element analysis(FEA),and the ultimate failure modes for those two types of BRBs are revealed.Consequently,corresponding lower limits of restraining ratios are attained for preliminary design purposes.Furthermore,the effect of interactive buckling,which is triggered by asymmetric imperfection on the load resistance of the PCS-BRBs is investigated through FEA,and corresponding design methods considering the effect of different types of initial geometric imperfections for PCS-BRBs subjected to monotonic and cyclic axial loads are attained including the lower limits of restraining ratios and initial pre-tensioning force of the cables.Finally,experimental test has been conducted on TC-BRBs with triple rigid truss frames(TTC-BRBs)subjected to cyclic axial loads.Test results revealed excellent global stability perforamce of the TTC-BRBs as required,and verified rational constructional design of the end connections being adopted for the test specimens.The comparison between the experimental and FE simulation results validates the effectiveness and precision of the adopted FE models,thus providing reliable guidance for practical engineering applications of TC-BRBs.