Research On Seismic Behavior And Damage Evaluation Of CFST Diagrid Structure System

In recent years,the diagrid tube structure system has been successfully used as a landmark building for its beautiful appearance and outstanding lateral resistance.The main bearing member of the structure is a two-way cross inclined column,which transfers the horizontal load and vertical load into the diagonal axial force.However,there are few studies on the failure mechanism and hysteretic performance of CFST columns subjected to the axial cyclic load,and no seismic damage evaluation method of the diagrid tube structure based on the inclined column damage model has been established yet.In this paper,experimental research,numerical simulation and theoretical analysis are used to study the failure mechanism and seismic performance of CFST columns under axial cyclic loads.On this basis,the axial restoring force model and seismic damage model of CFST columns are established.Based on the analysis of the distribution law and damage mechanism of hysteretic energy consumption of diagrid structure,an earthquake damage assessment method for diagrid structure system is proposed,particular contents are as follows:Axial cyclic loading tests of 8 concrete-filled steel tube(CFST)columns and 2hollow steel tube columns were carried out in combination with the mechanical characteristics of the inclined columns in diagrid structure system.The influences of loading path,aspect ratio,concrete strength and steel ratio on their seismic performance were studied.The mechanical behavior,failure mode and hysteretic behavior of CFST columns were investigated,and the interaction between steel tube and concrete was discussed as well.The results show that the failure of CFST columns under axial cyclic loading was caused by the fracture of steel tube,while the core concrete remains intact as a whole.CFST columns have better ductility and energy dissipation capacity in the axial tension direction,and higher bearing capacity and stiffness in the axial compression direction.Compared to the hollow steel tube column,the CFST columns have higher bearing capacity,deformation capacity and energy dissipation capacity due to the supporting effect of concrete on steel tube in tension and the restraining effect of steel tube on concrete in compression.The finite element model was eatablished by ABAQUS to simulate the behavior of CFST columns under axial cyclic loading.Based on the verified model,the mechanical mechanism and load distribution were further analyzed in different stress stages.Then the effects of concrete strength,steel strength,steel ratio,aspect ratio on the bearing capacity and deformation capacity of CFST columns were studied.The variation regularity of seismic behavior with various parameters of CFST columns under axial cyclic loading was obtained systemically.Based on the axial cyclic loading test results of CFST columns,the restoring force model was eatablished including the aspects of skeleton curve,hysteretic performance,stiffness and strength of CFST columns.The restoring force model of CFST column was fitted by the degenerated three-line model.The calculation formulas of the bearing capacity and the corresponding displacement of CFST columns are derived based on the limit equilibrium theory.Due to the difference performance of the CFST specimens under axial tension and axial compression,different hysteresis criteria and unloading stiffness functions were used in the positive and negative directions to fit the corresponding unloading stiffness function.Based on this,a complete set of restoring force model for CFST columns under axial cyclic loading was proposed.The rationality of the restoring force model is verified by comparison with the experimental hysteretic curve.Based on the damage evolution mechanism of CFST columns,the nonlinear combination of stiffness degradation and cumulative energy dissipation was used to establish the seismic damage model for CFST columns subjected to axial cyclic loads.Combined with the test results,the corresponding relationship between the damage degree and the quantitative index of the CFST columns under different performance levels was established.Based on the established damage model,the influence of different design parameters on the damage evolution of CFST columns was analyzed.An elastoplastic time-history analysis of the diagrid tube structure was performed using the established axial restoring force model of the CFST column.The energy distribution law of diagrid tube structure under different ground motions was studied.The distribution law of hysteretic energy consumption among members and stories was analyzed.The influence of structural dynamic characteristics and ground motion parameters on the distribution of hysteretic energy consumption was discussed.From the perspective of energy,the damage mode and energy dissipation mechanism of diagrid tube structure were investigated,which provides a reference for establishing the seismic damage assessment method of diagrid structure.Based on the damage model of CFST columns,combined with damage model of shear wall and coupling beam,the damage distribution mode and evolution characteristics of diagonal columns,shear walls and coupling beams in diagrid tube structure were studied.Considering the impact of damage location and damage degree on the overall performance degradation of the structure,a weighted combination method of component damage from layer to overall damage was established,and the damage mechanism and damage degree of different members in the diagrid tube structure were analyzed.On this basis,according to the different functions of various components in the structural system,the combination method from the overall damage of the component to the overall damage of the structure was determined,and then a complete set of seismic diarid structure damage assessment method was established to realize the full-scale and multi-layer seismic damage assessment.