Axial Compression Behavior Of Circular CFST Columns Through Direct Measurement Of Load Components

To accurately understand the axial compressive mechanism of circular concrete-filled steel tube(CFST)short columns,an experimental method to directly measure the load components of CFST columns has been adopted.The axial compressive behavior during the whole loading process and composite action under the ultimate state of all specimens were investigated.Following main achievements are attained:(1)Twenty-two circular CFST short conlumn specimens and four corresponding hollow steel tube specimens were designed based on the experimental method which can directly measure the load components,and all specimens are subjected to axial compressive loading.The test specimens were varied in diameter-to-thickness of the steel tube D/t and cylinder compressive strength of the concrete infill f'_c,and divided into three types according to different strucutural configurations,including N1 and N2 type specimens which can directly measure the load components and corresponding C type specimens representing the ordinary circular CFST columns.It was found that N2 type specimens can better reflect the load behavior of the actual circular CFST short columns,and it could also be the data source to deriving the axial compressive mechanism for circular CFST short columns.It was demonstrated from the measured load components-axial average strain(N-?)relationships that the steel tube and concrete infill might not reach its peak load under ultimate state.The strain-hardening effect of the steel tube after yielding has significant infilences on the axial compressive behavior of all specimens.For the specimens with bigger D/t=29.1~53.5,there exists a descending portion in the N-? curve of the steel tube after reaching the first peak point.This result is attributed to the development of hoop stresses in the steel tube due to the lateral expansion of the concrete infill,which lead to a corresponding decrease for the axial stress;furthermore,the N-? curve of the steel tube entered ascending range due to the strain-hardening effect.For the specimens with smaller D/t=18.5,the N-? curve of the steel tube essentially did not decrease during loading because the compressive stress-strain relationship of the hollow steel tube exhibits continuous strain-hardening without a yield plateau,which means that a better stress-strain effect.(2)The theoretical study of axial compressive capacity under ultimate state for circular CFST short columns was investigated based on the test results.A reasonable analysis model of axial load capacity was established,which taking the confining stress coefficient k and hoop stress coefficient ?_h as the key variables.The suggested coefficient of confining stress for circular CFST short columns under ultimate state was presented based on the test results of N2 type specimens(k=4.1).Then,249 specimens from previous studies were summarized and classified according to different standards of yielding strength of the steel tube to derive the hoop stress models and the calculated formula for axial load capacity.The proposed formulas,which have wide range of applicable parameters,are in good agreement with the test results by comparing different existing empirical formulas.