Seismic Mitigation Mechanism And Performance Control Of SMA-Viscoelastic Hybrid-Control Brace System

As an effective implementation of resilience structure,self-centering brace has a wide range of application and favorable growth prospect.However,there are several defects in the traditional self-centering brace,such as limited deformation capacity,low stiffness,and insufficient energy dissipation,etc.Therefore,a kind of SMA-viscoelastic hybrid brace is proposed by this paper,and on the basis of that,the universal laws and methodologies of self-centering braced structure are studied by means of experiments,numerical simulation,probabilistic methods,and machine learning.Considering the lack of loading protocol for quasi-static testing of new structural components,such as self-centering braces,a method which can be used for developing the loading protocol is proposed.Then,based on this method and combined with the seismic response characteristics,a series of quasi-static testing loading protocols suitable for the testing of self-centering braced system and traditional energy dissipation system under different intensity levels are developed,with far-field and near-field earthquakes being separately considered.In the application,the applicable loading protocol can be selected according to the demand or developed with the method.According to the seismic response characteristics of different structures explored in the study of the loading protocols and the defects existing in the traditional selfcentering brace,a kind of SMA-viscoelastic hybrid brace(SVHB)which takes shape memory alloy(SMA)and viscoelastic damper(VED)as kernel elements is proposed.SVHB has a series of combination forms,such as SMA cables with VEDs,SMA ring springs with VEDs,etc.This paper focuses on the SVHB composed of SMA cables and VEDs,and the performance of SMA cables,VEDs and SVHB are studied through different experiments.The working mechanism and overall scheduling ability of SVHB are verified,and a hysteretic behavior prediction method is also put forward based on the superposition of element performance.Combined with a proposing four-stage selfcentering material model and the comparison results of different VED models,the best numerical simulation strategy of SVHB is constructed.Through the seismic analysis at system level,the performance and advantages of SVHB and other braces are studied compared.The SVHB designed reasonably has both self-centering and energy dissipation abilities,which can effectively suppress the seismic response of the structures,such as inter-story drift and floor acceleration.The essential working mechanism of SVHB are explained by detailed case study,and the suggestions for the design of the structures with SVHB are also given according to the sensitivity analysis.Then,a residual deformation predication model,which realizes the simultaneous consideration of the peak and residual response of the structure in the performance-based design,is proposed based on the probabilistic method.By means of theoretical analysis,experiments and numerical simulation,the element failure and corresponding mechanism in SVHB and traditional pretensiontendon(PT)self-centering braces are revealed.It is found that the conventional singlecore self-centering brace shows a progressive element failure mode,while the dual-core self-centering brace with enhanced deformation ability may suffer an instantaneous chain failure which causes more serious damage.Focusing on element failure,various structure models,such as SVHB and PT self-centering braced structure models,are established,and the impact of element failure on the structure and the mechanism behind the change of seismic response are explained through the seismic analysis and case study.Incremental dynamic analysis and probability statistics are used to evaluate the collapse,residual deformation vulnerability and life-cycle seismic risk of the target structures.It is verified that the SVHB structures have better seismic robustness in both dimensions of space and time,and it is also found that using the idealized model without considering element failure in design and analysis may overestimate the seismic performance of the target structure and underestimate the corresponding seismic risk,which can be up to more than 5 times.Post-earthquake economic loss is one of the important indexes to measure the impact on the structures,and the economic loss analysis method suitable for selfcentering braced structure could still be improved.Therefore,a kind of methodology which can be used for evaluating the economic loss of self-centering braced structure is proposed.Taking SVHB structure and other typical self-centering braced structures as representatives,the corresponding damage states are defined and the component fragility functions are derived through theoretical calculation and Monte Carlo simulation.Based on market research and database collection,the cost distributions of various self-centering braced structures are given for reference.The economic loss analysis and sensitivity analysis of the structures mentioned above are conducted,and the results show that the post-earthquake economic loss of SVHB structure is reduced by up to 50% compared with other structures,while the cost of that is slightly higher.Due to the increasing complexity of seismic components in recent years,there will be great difficulties and lag in the formulation of design methods.Therefore,machine learning(ML)and modified genetic algorithm(NSGA-II)are introduced to establish a multi-parameter hybrid brace optimization framework,which realizes the rapid acquisition of seismic response indexes through ML strategy and makes it possible to optimize the design of brace or other components with system-level indexes and largescale iterations.It also greatly improves the efficiency of the optimization process.Taking the optimization and design of SVHB as an example,the steps in optimization framework,including the optimization parameter selection,sample database establishment,ML strategy,automatic design and cost processing module and NSGAII algorithm implementation,are discussed in detail.The effectiveness of the optimization framework is also be verified through the comparison of the brace schemes before and after optimization.The optimization framework can be applied to optimizing other similar braces or components after modification.