Constitutive Model Of Thin-walled Circular Cfst Stub Columns Under Axial Compression Considering Local Buckling

In recent decades,the rare earthquakes in the worldwide have occureeed with a gradual increase.With the continuous advancement of modern cities,the seismic performance of(super-)high-rise buildings needed to be given full attention.To study the mechanical behavior of structural columns after local buckling under strong earthquakes,the full-process mechanical model for the compression behavior is one of the urgently established.In this paper,the main research object is the thin-walled concrete-filled circular steel tube stub columns(CFT-Cs)and the circular hollow steel tube stub columns(CHSs).Depend on series of axial compression tests of thin-walled CFT-Cs with three different strength concretes and CHSs with five diameter-thickness(D/t)ratios,the influences of local buckling of circular steel tube on mechanical behaviors of CFT-C specimens and CHS specimens were studied,and a complete mechanical model of CHS was proposed,as well as the combined constitutive model of CFT-C which is limited in the ultra-thin-walled CFT-C and thin-walled CFT-C with(ultra-)high-strength concrete.The main work and highlights are presented as follows:(1)The uniaxial complete stress-strain model of CHSs under compression is proposed.The model considers the influence of local buckling of steel tube.The mechanical model of CHS is a single parameter model expressed by a piecewise continuous function,and can be widely applied to the engineering steel structures.Based on 48 sets of available test data of CHS,through the parameter correlation analysis,the key mechanical indexes such as the peak stress,the corresspinding peak stress,the strengthening stiffness coefficiment in the ascending portion and the residual compressive strength in the descending portion are deternmined.The accuracy of the model is verified by comparison with the measured data.(2)The study of the mechanical behavior in the whole process of CFT-C shows that the local buckling of steel tube can cause a secondary stress redistribution,the non-uniformity along the columns will directly affect the failure mode of the concrete.With the comprehensive analysis of the avaiable related researches on confined concrete,it is pointed out that the critical condition of failure modes of the CFT-C is the ratio of the effective hoop stress to the compressive strength of concrete.(3)Based on the destructive tests of the shear-type CFT-Cs,the longtitudinal residual stress under large deformation of the steel tubes is obtained,which is used to modify the constitutive model of the CHS for the circular steel tube of the shear-type CFT-Cs.Based on the idea of superposition method and the modified model of circular steel tube,the nomial compressive strength-strain curves of the concrete core can be obtained separated from the measured curves.According to the study of the nomial concrete curves,three different curve types of the concrete core are summarized.The first curve type of concrete core filled in ultra-thin-walled circular steel tube belongs to the type of post-peak strain softening.The second curve type of(ultra-)high strength concrete core filled in thin-walled circular steel tube belongs to the type of post-peak strain softening and then hardening.The third curve type of the normal strength concrete core filled in common circular steel tube belongs to the type of strain harening and then softening.(4)A combined constitutive model for the ultra-thin-walled CFT-C and the thin-walled CFT-C with(ultra-)high strength concrete is completed.The concrete model of the combined constitutive model is based on the Sargin model and S&S model,in which some key mechanical parameters,such as peak stress,peak strain and effective confined stress are modified according to experimental data analysis and Mohr-coulomb strength Theory.By comparing with the measured curves of specimens,it is proved that the combined constitutive model has good accuracy and can reasonably predict the deterioration trend of stiffness of the descending portion,except the descending portion of CFT-C with ultra-high strength steel tube,which is conservative estimated.