Seismic Performance Of Reinforced Concrete Frame With Buckling Restrained Braces Subjected To Bi-directional Earthquake

A typical buckling restrained brace(BRB)consists of an energy-dissipating steel core and an outer restrainer that restrains the core from lateral buckling when subjected to compression,while allows free longitudinal deformation of the core.Compared to conventional steel braces,BRBs can dissipate a much larger amount of seismic input energy to structures by stable hysteretic capability.Thus,seismic responses of structures could be controlled efficiently when BRBs are applied.Currently,BRBs have been used increasingly in both newly constructed reinforced concrete frames(RCFs)and seismic retrofitting of existing RCFs.Although the performances of BRBs have been investigated extensively through experimental studies,researches on RCFs incorporating BRBs are limited,especially when bi-directional earthquake excitation is considered.To investigate the performance of RCF with BRBs(RCF-BRB)subjected to bidirectional earthquakes,a full-scale two-story spatial RCF-BRB was tested using substructured pseudo-dynamic(Ps D),quasi-static and pushover techniques.The test results showed that the RCF-BRB in this thesis designed based on the current Chinese building codes exhibited reliable seismic performance,as the selected performance objectives were achieved in the tests.The specimen displayed an effective and stable energy-dissipating capacity even when the inter-story drift was over 3%.The pushover test revealed excellent deformation capacity of the RCF-BRB,as the interstory drift reached 5.9% in both loading directions simultaneously.A conventional gusset plate is connected to both the column and the beam.This configuration could cause beam-and column-end plastic hinges to be shifted out of the gusset connection region,and the strain/damage level could be increased due to the shortening of effective lengths of the beam and column.Besides,it also results in complicated loading conditions for the gusset plate and the adjacent reinforcedconcrete(RC)members.To diminish these negative effects,researchers developed several joint configurations,among which the unconstrained one(i.e.,the gusset plate is connected only to column or beam)might be optimal for RCF as is easy of construction.When the unconstrained gusset plates are used,the beam-end section appears vulnerable to shear failure.This will be especially prominent if the zigzag configuration of BRBs is applied in the vertical direction.The strut-and-tie model was introduced to design the RC beam with unconstrained gusset plates so as to ensure a reliable working condition for BRBs.The two gusset configurations were used in two perpendicular directions of the full-scale two-story specimen.Compared to the conventional gusset plates,the unconstrained ones reduced the damage of the RC members significantly.Test result also shown that the strut-and-tie model was reliable for the design of discontinuity regions(D-regions)of the RC-beam ends with unconstrained gusset plates.With cheaper test equipment for conventional quasi-static testing,the hybridsimulation method can be as reliable and realistic as shaking table testing.To make a credible reproduction of the structural response using sub-structured Ps D technique,an overlap modeling technique together with tuning the mass of the lumped-mass model and stiffness of the numerical substructure was employed to reduce errors caused by the incomplete boundary condition between the experimental and numerical substructures.Unlike uni-directional tests,the relationships between length changes of actuators and displacement sensors and displacements of the mass center are nonlinear in bi-directional ones.The relationships between restoring forces of mass center and forces of actuators are also nonlinear in bi-directional tests.These nonlinear relationships render the loading control system more complicated,and new control issues appear.The working range of the arrangement scheme of displacement sensors was investigated using a geometric approach.The outer-loop control method based on Newton's iteration method was applied for the loading system to ensure the target displacements could be fulfilled satisfactorily.The redundancy issue,caused by the actuators outnumbering the degrees of freedom(Do Fs)of the floor to be controlled,was addressed with a force-displacement mixed control technique.The effectiveness of the above testing techniques was checked by the test as well as numerical-simulation results.In previous tests,global buckling of BRB with its gusset plates(BRB-GPs)occurred time and again.To investigate the effects of different design parameters on the global buckling of diagonal BRB-GPs considering out-of-plane structural deformation,analyses were carried out via finite element(FE)models.Theoretical analysis based on the FE analysis demonstrated different types of buckling processes of BRB-GPs.If the out-of-plane structural displacement is large,the BRB-GPs will remain stable when the moment between the connection and restrained regions reaches the moment transfer capacity.If the out-of-plane structural displacement is small,the system will buckle once the moment transfer capacity is reached.It was shown that the design methods recommended by the current design codes and references would overestimate the stability of BRB-GPs for certain cases.Based on the analysis results,a conservative design method was developed.