Mechanical Performance Of Hexagonal CFST Column Base Under Biaxial Compression-Bending Load

Special cross-sectional shape of hexagonal concrete-filled steel tubular(CFST)columns,which have been employed in super-high-rise buildings,allows them to be connected with other structural components conveniently from different angles.However,previous researches on such members are still insufficient.The static behavior of hexagonal CFST columns under biaxial compression-bending load,and the seismic performance,design methods and detailing requirements of concrete-encased hexagonal CFST column bases under biaxial cyclic transverse load need more investigation.The main work of this paper is fourfold:1.Based on the analysis of experimental data from laboratory tests and in-situ engineering measurements(up to 2,687 days of continuous data acquisition),a model was proposed to predict the shrinkage strain of the core concrete in CFST members.A finite element analysis(FEA)model of hexagonal CFST columns under biaxial compression-bending load was established.The verified model was then used to analyze the full-range load-deformation relationship,the internal force distribution,and the composite action between the steel tube and core concrete.The effects of various parameters on the static behavior of hexagonal CFST columns were studied,including the loading angle,the loading eccentricity,the cross-sectional shape,the shear deformation,and the sustained load.Based on the parametric analysis,a simplified method was proposed for predicting the ultimate resistance of hexagonal CFST columns under biaxial compression-bending load.2.A total of 12 concrete-encased hexagonal CFST column base specimens under constant axial load and biaxial cyclic transverse load was tested,with their full-range load-displacement relationship,strain distribution,stiffness and strength degradation,and energy dissipation capacity measured and compared.The main experimental parameters included the height of the outer RC component,the loading angle of transverse load,the axial load ratio,the cross-sectional shape of the column,and the base type.Three typical failure modes of column bases were observed in the test,namely,failure at the CFST column section,failure at the bottom section of column base plate,and failure at the top section of column base plate.3.An FEA model was established to simulate the hysteretic behavior of the concrete-encased hexagonal CFST column bases under constant axial load and biaxial cyclic transverse load.The model considered the nonlinear material and contact behavior between different components.The feasibility of the model was verified against the experimental data and results showed that the numerical and experimental results reasonably matched.Based on the verified FEA model,the full-range load-deformation relationship was analyzed,with the failure mode,the load transverse mechanism,the internal force distribution,and deformation distribution investigated and revealed.4.The effects of main parameters on the seismic performance of the column bases were investigated,including the shear span-to-depth ratio of the CFST column,the steel yield strength of the tube,the steel ratio of the CFST column,the height of the outer RC component,the width of the outer RC component,the longitudinal reinforcement ratio of the outer RC component and the stirrup reinforcement ratio of the outer RC component,axial load ratio,and the shear connector.A simplified method to predict the flexural resistance of concrete-encased CFST column base was proposed.The detailing requirements for the outer RC component,the plate-anchor component as well as the shear connector were also proposed.