Seismic Behavior Of Concrete-filled Steel Tube Columns With Internal H-Section Steel Subjected To Combined Compression-Bending-Torsion Loads

The concrete filled steel tube columns with embedded H-section steel(H-CFST columns)not only has the characteristics of simple structure,convenient construction and strong fire resistance,but also has the advantages of large bearing capacity and rigidity,ductility and continuous collapse resistance.At present,there are many researches on the compressive-bending-torsion bearing capacity of the H-CFST columns.However,there are few studies on the seismic performance of the H-CFST columns under the combined actions of compression-bending-torsion.The seismic performance test of the H-CFST columns under compression-bending-torsion is carried out,and the combined strength formula of steel and concrete and the combined capacity formula of the H-CFST columns under the action of compression,bending and torsion are derived using mechanics theories.The following main research results have been achieved:(1)The pseudo-static reciprocating loading test of the H-CFST columns under different cross-sections,slenderness ratios and compression-bending-torsion was carried out,and the results of the column's failure modes,hysteresis characteristics,bearing capacity skeleton curve and key mechanical characteristics,stiffness degradation and energy dissipation were obtained under compression-bending-torsion;the bearing capacity,stiffness,energy dissipation capacity and Its influencing factors of the H-CFST columns under compression-bending-torsion are analyzed..This result can be used to analyze the failure mechanism and interrelationship of the H-CFST columns under compression-bending-torsion.(2)Based on the experimental failure mode,the stress distribution law of steel,concrete and H-shaped steel of H-CFST columns under compression-bending-torsion is analyzed,and the failure mechanism of the H-CFST columns under compressionbending-torsion is obtained.(3)The initial plane stress and principal stress of steel and concrete and their relationship under two-dimensional pure shear,compressive shear,tensile-shear stress state and two-dimensional bidirectional tensile stress or tensile-compression stress state are analyzed;based on the theory of the maximum shear stress(Tresca failure criterion)and maximum distortion energy failure theory(R.von Mises theory),the combined strength of steel and their related relationships under the two-dimensional pure shear,compressive shear,tensile-shear stress state and the under two-dimensional two-way tensile stress or tensile-compression stress are deduced.Based on the Mohr-Coulomb failure theory,the combined strength of concrete and their related relationships under the two-dimensional pure shear,compressive shear,tensile-shear stress state and two-dimensional two-way tensile stress or tensile-compression stress are derived.This result can provide the combined strength of steel and concrete for the combined bearing capacity formula of the H-CFST columns under the actions of compressionbending-torsion.(4)The normal and shear stress distributions of the cross-section of the H-CFST columns under compression-bending-torsion are analyzed,and the mathematical integration method are used to derive the calculation formulas of the combined bearing capacity of concrete filled circular steel tube columns with embedded H-section steel(H-CFCST columns)and concrete filled rectangle steel tube columns with embedded H-section steel(H-CFRST columns)and their correlation relationship of the bearing capacity under compression,bending and torsion.The results can be directly used to calculate the combined bearing capacity of the H-CFST columns under different axial compression ratios and torsion-bending ratios under the actions of compressionbending-torsion.(5)The stiffness and its affecting factors of the H-CFST columns under compression-bending-torsion are analyzed.The attenuation mechanism and calculation formula of the elastic-plastic stiffness under compression-bending-torsion of H-CFST columns are obtained.This result can be used for the design of the stiffness and deformation control of H-CFST columns under the actions compression-bendingtorsion.