The Study On New Assembled Self-Centering Buckling Restrained Brace And Design,Optimization Metholody Of Braced Frames

After experiencing the rare or even unexpected earthquakes,the demolishment and reconstruction was inevitable with major economic losses based on the postearthquakes on-site survey,though the damaged frames survived without any collapse.To overcome the disadvantages,the self-centering concrete frame,where no steel bars crossing the joints and no tension existing in this area,is proposed to achieve quick reoccupation after earthquake.However,the unique joints make difficulties in on-site assembly and this new technique may be not feasible in retrofitting.The concentrically braced frames have unequal seismic performances in compression and tension.Moreover,the residual drifts in buckling restrained braced(BRB)frames also require a lot of repairing work.Inspired by the self-centering concrete frame,different kinds of self-centering braces are proposed with flag-shaped hysteretic characteristics.However,the application of these brace is limited by its inadequate deformation capacity and the difficulty in the replacement of damaged braces.A new assembly self-centering buckling restrained braces(ASCBRB)is proposed to improve the seismic performance of the braces.The numerical analysis and experiment was conducted to investigate the hysteretic characteristics and damage mode of the new braces.Then the new design methodology and optimization procedure of the frame with new braces are presented with examples.Work and conclusions include such aspects as follow:(1)4 identical disc spring groups and core plate with buckling restrained protection are installed in ASCBRB.The details,the assembly procedure and mechanics are discussed in this paper.And the negative effect due to assembly error is also taken into consideration.The design aims to create a braces with satisfying energy dissipating capacity and resilience to reduce the residual deformation.Since the prestress is attributed to 4 groups,the load carrying capacity of ASCBRB is improved.Moreover,the arrangement of 4 prestressed element can greatly eliminate the negative effect of assembly error;(2)9 specimens of the novel braces in certain orders were adopted in the qusaistatic experiment.The yield force of the core plate and the characteristics of the single disc spring group have been tested before the experiment of the specimens to estimate the maximum brace force when the core plate is ruptured.The damage mode shows that the core plate and the guide plates are the only elements requiring replacement.And the outer tube、disc spring and inner screws are reused since the specimens are tested in sequence.The assembly and disassembly procedure are repeated multiple times.The hysteretic model in mechanical study has satisfying agreement with the test result since the E-D and S-C systems are installed in parallel.In this paper,the self-centering coefficient is the ratio of prestress value in disc spring to the load bearing capacity of the core plate.The test result shows that the residual deformation is directly related to the maximum displacement ever when the self-centering coefficient is under 0.5;(3)Since multiple “S4R”elements are required in the numerical model which may cost more time.To improve the computational efficiency,dual tube system with closed-form formula is proposed in this paper to simulate the hysteretic behavior of the disc spring group.The asymmetric buckling deformation is observed in the numerical model of core plate as the result of inadequate confinement in loading end.And part of the guide plate is also damaged which is in accordance with the damage mode in experiment.To improve the computational efficiency further,the 1d numerical model of the brace is built in OpenSEES and has satisfying agreement with the test result.(4)The single-degree-freedom self-centering and Bouc-wen systems with different shear reduction factors and periods were built in Matlab/Simulink.Hypothetically,the damping force of the system is the product of the constant damping ration and the transient scant stiffness.The non-linear analyses under 64 seismic waves were conducted in this platform.The ductility spectrums are adopted in the design procedure of the multi-degree-of-freedom ASCBRB braced frames with different energy dissipating capacities.The load bearing capacity of braces at different floors are designed using Square Root of the Sum of the Squares methodology.And additional force is applied at roof considering the high mode effect.The braced frame is built in OpenSEES and analysis results under frequent/rare earthquakes verified that the ductility ratios of different braces are close to design level with uneven distribution pattern along the height of the buildings;(5)A new optimization procedure for ASCBRB braced frame is proposed in this study to minimize the ductility ratios of braces in different floors given by the limited material.The modification is conducted on the flag-shaped hysteretic model of ASCBRB and linearization process is complete given by the Gaussian distributed assumption.The single and multiple-degree-freedom self-centering system(with and without optimization)were built in in Matlab/Simulink.The result indicated that the linearization process with specific coefficients can reasonably estimate the nonlinear responses both for single and multiple-degree-freedom self-centering system.The result of the optimized system suggested that the ductility coefficients of braces are evenly distributed along the building.The highlights of this study as follows:(1)To achieve quick reoccupation post-earthquake,the ASCBRB is proposed in this study.Since the novel brace is blot assembled,the core plate and the guide plates can be replaced after earthquake which saves the repairing time and cost.The clamping force in high strength bolts can improve the buckling restrained effect of the guide plate;(2)To improve the computational efficiency,the dual-tube system is proposed to simulate the unique hysteretic behavior of the disc spring group.The mechanics of the dual-tube system is represented with explicit formula and the simplified system can be applied to the simulation of different disc spring groups as long as in appropriate working distance;(3)To account for the nonlinear behavior of ASCBRBs,the design methodology of multi-story braced frame is proposed based on the ductility spectrum of the nonlinear self-centering single-degree-freedom system.Since the uneven distributed pattern of ductility coefficients in braces is observed in the time history analysis result,the equivalent linear system is adopted in the optimization procedure to minimize the ductility demand of the braces.The optimal braced frames have evenly distributed ductility demand along the height.