Axial Compression Behavior And Design Method Of Elliptical Concrete-filled Steel Tubular Stub Columns With Initial Concrete Imperfections

Elliptical concrete-filled steel tubular(ECFT)structure combines the advantages of excellent hoop restraint of circular section and flexible primary and secondary axis stiffness distribution of rectangular section,its smooth streamline shape could also avoid stress concentration at sharp corners and reduce the flow resistance coefficient in wind and water flow,which has been widely used in large public projects such as airport terminals,large bridge piers and railway stations recently.However,statistics have found that most of the CFT structures in normal service have core concrete initial imperfection,which are mainly due to irregular construction techniques,unjustified material properties or too stringent service environment and other factors.Existing studies have shown that core concrete initial imperfection would weaken the combined effect between the steel tube and its core concrete,thus causing safety hazards to the structure.In view of the good application prospects of elliptical concrete-filled steel tubular structures and the objectivity of existence of concrete initial imperfections,it is necessary to carry out systematic studies on the performance of elliptical concrete-filled steel tubular structure with initial concrete imperfections.In this paper,the axial compression performance tests,numerical simulation analysis and calculation methods derivation are carried out for the elliptical concrete-filled steel tubular stub columns with two different types of initial concrete imperfections,as follows.(1)The axial compression performance tests of 12 elliptical concrete-filled steel tubular stub columns with spherical-cap gap(ECFTSG)were carried out.The impact of steel strength,concrete strength and spherical-cap gap ratio on its basic mechanical properties such as failure patterns,ultimate bearing capacity,initial stiffness and ductility were investigated respectively.The gap feature factor GFI was introduced to reveal the influence mechanism of spherical-cap gap on the bearing capacity.(2)The restrained stress-strain constitutive relationship for the core concrete of ECFTSG was proposed through introducing a reduction coefficient associated with the spherical-cap gap ratio to the existing constitutive relation of elliptical section concrete restrained by steel tube,then the FE models of axially-loaded ECFTSG stub column considering the influence of elliptical section characteristics and spherical-cap gap were established and verified by test data;The effects of five parameters including steel strength,concrete strength,diameter-thickness ratio,gap ratio and long-short axis ratio on the axial compression performance of ECFT with long-axis or short-axis direction spherical-cap gap imperfections were studied respectively;the impact of spherical-cap gap on the full-loading process stress distribution and the steel-concrete contact stress development law were further investigated.The results show that the existence of spherical-cap gap would weaken the bearing capacity and initial stiffness of CFT stub columns,and the stress distribution and steel-concrete contact pressure development near the gap area will also be significantly affected.(3)The testing of 12 elliptical concrete-filled steel tubular stub columns with circumferential gap(ECFTCG)under axial compression were carried out.The impact of steel strength,concrete strength and spherical-cap gap ratio on its basic mechanical properties such as failure patterns,ultimate bearing capacity and initial stiffness were investigated respectively.The GFI was also introduced to reveal the influence mechanism of circumferential gap on the bearing capacity.(4)Based on the research results of circular CFT with circumferential gaps,a stressstrain constitutive relationship model applicable to the core concrete of ECFTCG columns was proposed,and the FE model of ECFTCG stub column under axial compression considering the impact of elliptical section characteristics and circumferential gap was established and verified by experimental data.Similar to the ECFTSG stub column,the effects of key parameters including material parameters and geometric parameters on its axial compression performance of ECFTCG stub column were studied respectively,and the impact of circumferential gap on its full-loading process stress distribution and the steel-concrete contact stress development law were also investigated.The results show that the existence of circumferential gap would delay and weaken the steel-concrete contact action,resulting in a significant reduction in the axial compression strength,initial stiffness and ductility.(5)Based on the unified strength theory and superposition principle,the calculation formulas for predicting the axial compression strengths of elliptical CFT stub columns with two kinds of initial concrete gaps were proposed,and their accuracy were verified by some scholars’ research results,experimental and simulation analysis data in this paper.The results show that the calculation formulas proposed in this paper have good applicability for the axial compression strength of elliptical CFT stub columns with two kinds of initial concrete gaps.