Performance Of Axially Loaded CFST Stub Columns With Inner Spiral Stirrup

Concrete-filled steel tube（CFST）is a kind of composite member formed by filling the steel tube with concrete,and CFSTs have good structural performance due mainly to the interaction between the steel tube and its core concrete.The effectiveness of saving steel,reducing cross-sectional area of member and lightening the self-weight of structure can be achieved while using high-strength steel or high-strength concrete in practical concrete-filled steel tube（CFST）structures.However,the local buckling of thin-walled high-strength steel becomes worse and the brittleness of high-strength concrete is more obvious with increase of materials’ strength,which results in a weak confinement of outer steel tube to core concrete within the common scope of steel ratio in the project with high-strength CFST.As a result,a new kind of CFST with inner spiral stirrup is proposed to improve the confinement of core concrete so that high-strength CFST members with inner spiral stirrup could have both high bearing capacity and good ductility.In this paper,by combining tests and finite element simulation,the structural behavior,mechanism and design method of axially loaded CFST stub columns with inner spiral stirrup are comprehensively studied.The following main works have been completed:（1）Tests on the structural behavior of 8 circular and 14 square specimens were carried out to mainly investigate the effect of yield strength of steel tube and volume-stirrup ratio on the failure patterns,load-displacement（strain）curves,bearing capacity,composite elastic modulus and ductility factor of axially loaded CFST stub columns with inner spiral stirrup.（2）On the basis of determining the constitutive model of steel tube,spiral reinforcement and concrete,a three-dimensional nonlinear finite element model of axially loaded CFST stub columns with inner spiral stirrup was established by using the ABAQUS.The finite element simulation results generally reproduced the measured results.（3）Based on the verified finite element model,the influences of volume-stirrup ratio,nominal steel ratio,concrete strength,yield strength of steel tube and spiral stirrup on the stress state and interaction of each component of axially loaded CFST stub columns with inner spiral stirrup were deeply investigated.（4）According to the parametric analysis results,the area coefficient of equivalent steel tube with spiral stirrup was determined,and the simplified formulae for calculating the bearing capacity of axially loaded CFST stub columns with inner spiral stirrup was further proposed.The simplified bearing capacity of axially loaded CFST stub columns with inner spiral stirrup were generally in good agrement with the measured results.