Research On The Non-Uniform Confinement Effect Of Concrete-Filled Steel Tubular Column

The concrete filled steel tubular(CFST)column is characterized by its high bearing capacity,good ductility and convenient construction.Compared with the steel columns,the CFST member can fully utilize the material properties of steel.Thus,CFST is a good choice as the main load bearing members in high-rise and super-high-rise buildings.In contrast with the axially-loaded CFST members,the eccentric loaded members are more complicated,and the corresponding confinement effect to the core concrete is different from that subjected to axial compression.Under eccentric loadings,the bending moment causes the linear change for the longitudinal strain of the section along the height direction,and some of the sections are even under tension.In this context,the confining pressure is unevenly distributed in the hoop direction,leading to the lower confinement than the uniform confinement under axial compression.No experiment have revealed the mechanism of non-uniform confinement.To address this,the fiber model method and finite element(FE)model are used to conduct a systematic study on the non-uniform confinement of concretefilled steel tubular columns.The main findings of this research include:(1)The fiber model method and the finite element method are used to simulate the eccentrically-loaded CFST columns.In the finite element simulation,through systematic analysis,the reasonable constitutive relationships,element type,meshing,contact type and initial imperfections were determined.Furthermore,the reliability of the fiber model method program and the finite element model were verified by the current test data.(2)Carry out system parameter analysis to study the influence of slenderness ratio,core concrete strength,steel strength,steel ratio and eccentricity on the nonuniform confinement effect in the eccentrically compressed concrete filled steel tubes.Based on the results of the finite element analysis,the influence of the non-uniform confinement effect on the performance of the specimen under different parameter ranges is evaluated.Research shows that concrete strength,steel ratio,eccentricity,steel strength and slenderness ratio are all key parameters that affect the effect of non-uniform confinement effect.When the steel ratio and steel strength are high(steel content≥16% and steel strength grade≥Q355)and concrete is low(concrete strength grade≤C50),the difference in peak load and ductility caused by the nonuniformity of the confinement effect is more obvious.This difference peaks at an eccentricity of 0.6,which is 19% and 24%,respectively.The above results should be paid attention to in the design.When the slenderness of the component is relatively large(slenderness ratio≥48),the influence of the non-uniform confinement effect is small(less than 5%).At this time,the second-order effect of the component is significant,and the influence of the non-uniform confinement effect can be ignored in the design.(3)Based on the characteristics and limitation of the current setups for eccentrically-loaded structural members,this paper provided suggestions on the test setups for the CFST under eccentric loadings.