Seismic-resistance Mechanism Of Novel Friction Spring Self-centering Braces And Design And Evaluation Methodology Of Structural Systems

In recent years,with the rapid development of urban modernization,the traditional seismic concept aiming at ensuring life safety can no longer meet the needs of modern society.Building a resilient city to reduce the economic loss and social impact caused by earthquake disasters has become a hot spot and an important content of research in the field of earthquake engineering at present.As the building structure is the main component of the city,proposing seismic resilient structures and realizing postearthquake recoverability of the buildings have gradually become a new requirement for structural seismic resistance.In this context,there is an urgent need to develop key technologies,design methods,and evaluation theories for recoverable functional structures based on seismic resilience improvement.Therefore,this paper develops a new type of self-centering brace based on paralleled high-strength steel friction spring groups(referred to as parallel friction spring brace)and its structural system,aiming to reduce the post-earthquake residual deformation of the structure through the new selfcentering brace,to improve the recoverability and seismic resilience of the structure.By integrating construction design,theoretical analysis,program compilation,experimental study,and numerical analysis,the seismic-resistant mechanism,performance-based seismic design theory and method,and life-cycle seismic cost evaluation theory and method of the brace and its structural systems are systematically investigated.The main research contents and contributions of this paper are as follows:A new type of parallel friction spring brace with simple construction and easy to use in practical engineering has been developed,which effectively increases the bearing capacity of the brace without increasing the size of the brace or effectively reduces the size of the brace without changing the bearing capacity of the brace.The brace can flexibly and individually adjust the deformation and load carrying capacity and has stable self-centering ability and energy dissipation capacity.A series of quasi-static tests were conducted on three proposed brace specimens to investigate the effects of different treatments of the friction spring surface,i.e.,different friction coefficients.The test results show that the braces have very stable “flag-shaped” hysteretic behavior with excellent self-centering ability and energy dissipation capacity.The large coefficient of friction on the friction spring surface can improve the load carrying capacity and energy dissipation capacity of the brace.Based on the test results,a macroscopic physical model specific to the brace was compiled in Open Sees software to take into account the differences in loading and unloading stiffnesses,as well as the differences in tensile and compressive initial stiffnesses,to provide a basis for the numerical analysis of the brace and its system.The dynamic response of structures often differs significantly from the quasi-static behavior.For this reason,a hybrid simulation experimental study was conducted on a beam-through frame system using the parallel friction spring brace to investigate the real response of the brace and its system under different types of ground motion.The test results reveal that the brace and its system can achieve the expected seismic performance under both far-field and near-fault pulse-like ground motions.Based on the calibrated numerical model,a parametric study was performed to investigate the effects of the ratio of the initial actual compression stiffness to the initial theoretical compression stiffness(K)and the ratio of the initial actual tension stiffness to the initial actual compression stiffness(ρ)of the brace on the seismic performance of the system The recommended values for the practical engineering application of the brace are given.The low cycle fatigue performance of a typical large-size parallel friction spring brace was investigated experimentally by a constant-amplitude quasi-static test.The test results show that the brace has stable hysteretic behavior under constant amplitude loading and exhibits excellent fatigue performance.On this basis,new cumulative seismic capacity and demand indices are proposed through theoretical derivation.The lower limit value of the cumulative seismic capacity of the brace was quantified based on the constant-amplitude quasi-static test.In addition,the cumulative seismic demand of the self-centering brace under mainshock-aftershock and long duration ground motions was quantified by structural system-level analyses.The analysis results reveal that the cumulative seismic capacity of the brace exceeds its demand under all the considered earthquake records,i.e.,the brace has a significant advantage that it does not need to be repaired and can be reused after earthquakes.These evaluation indices can provide a theoretical basis for the study of the low cycle fatigue performance of self-centering braces.Although the self-centering braced structural system can effectively reduce the residual deformation and the risk of demolition of the structure after earthquakes,the system tends to present larger peak deformation and floor acceleration,which increases the structural and nonstructural seismic losses.Based on this background,a braced structural system based on the hybrid control strategy is proposed,which enables a wellbalanced control of the peak deformation,residual deformation,and peak floor acceleration responses.Based on the parametric spectrum analyses of single-degree-offreedom systems,the influence of structural properties on the seismic performance of the braced structural system based on the hybrid control strategy,especially the partial self-centering structural system,is quantified.The comprehensive and extensive prediction models for inelastic displacement demand and residual displacement response are proposed,which provides a practical method for structural engineers to conduct preliminary performance-based design and evaluation.In order to alleviate the phenomenon of “high-mode effect” of self-centering braced structural systems under earthquakes,the influence of structural lateral force distribution on the seismic response of braced structural systems with different energy dissipation factors and post-yield stiffness ratios is investigated from the perspective of lateral force distribution pattern,and the corresponding recommended design values are given.Based on this,a performance-based seismic design process for braced steel frames considering modified lateral force distribution is proposed.The case design of the braced steel frame was carried out using the modified lateral force distribution pattern and the proposed performance-based design process,and nonlinear time history analyses were performed.The analysis results show that the designed braced steel frame can meet the expected performance objectives and the distribution of peak inter-story drifts are uniform,which proves the effectiveness of the design method.Finally,a life-cycle seismic economic evaluation framework that can consider the effects of post-earthquake residual deformation of the structure is proposed.This framework can consider not only the initial construction cost,operation and maintenance cost,and direct economic loss due to post-earthquake repair of the structure,but also the indirect economic loss due to the recovery time of the structure.Based on this evaluation framework,the cost-benefit of the performance-based designed self-centering braced steel frames compared to conventional bucklingconstrained braced steel frames is investigated from a life-cycle perspective.The analysis results imply that from the cost-benefit perspective,increasing the structural life cycle is beneficial to the self-centering braced steel frame compared with the conventional buckling-restrained braced steel frame;the high fatigue performance of the self-centering brace is beneficial to the cost-benefit of the self-centering braced steel frame;the cost-benefit of the self-centering braced steel frame is highly related to the initial cost,and reducing the initial cost of self-centering braces is critical to promote its widespread use in reality.The evaluation results of this framework can provide decision recommendations for the practical application of self-centering braces.