Study On The Seismic Performance And Design Method Of Circular Concrete-filled Thin-walled Steel Tube Columns

Concrete-filled steel tube(CFST)columns are used in multi-story buildings,medium and small-span bridges with lower requirement on bearing capacity.But the existing CFST design specifications are strict on the diameter-to-thickness ratio of steel tubes,resulting in the unsatisfactory economic effect.Concrete-filled thin-walled steel tube columns have high bearing capacity,good plasticity and fire resistance,convenient construction and high economic effect,which are appropriate for above structures.In this paper,circular concrete-filled thin-walled steel tube column is taken as the structural model,and its seismic performance is studied by means of test,finite element analysis and theoretical analysis.The main content is as follows:(1)Eight circular concrete-filled thin-walled steel tube columns under constant axial load and cyclical horizontal load were tested to failure,which obtained their failure modes,strain development,and load-displacement hysteretic curves.The influence of axial compression ratio,diameter-to-thickness ratio and stiffener types on bearing capacity,ductility,and energy dissipation capacity were mainly analyzed.The tests showed that the specimens were damaged as steel tubes buckling and core concrete crushing at the bottom of specimens.All specimens had full hysteresis curves and high ductility coefficients greater than 3,indicating their good seismic performance.Longitudinal stiffeners and transverse tensile sheets increased the peak bearing capacity of specimens.However,the unnegligible damage caused by rough welding of stiffeners made the stiffened specimens have steel tube tearing,lower ductility and energy dissipation capacity than the unstiffened.(2)The finite element software ABAQUS is used to establish accurate solid models of test specimens,and mechanical analysis in the whole process of one-way pushover is carried out to obtain their stress development process,stress mechanism and failure mechanism.Simplified fiber models are established for test specimens by the finite element software Open Sees,and parameters of diameter-to-thickness ratio,length-to-diameter ratio,steel strength and concrete strength are analyzed.The results show that specimen with small axial compression ratio has stronger ductility,energy dissipation capacity and lower stiffness degradation.The specimen with an axial compression ratio between 0.4 and 0.5 has the highest bearing capacity.Specimen with large diameter-to-thickness ratio,length-to-diameter ratio or low steel strength has lower ductility,energy dissipation and capacity bearing capacity.Specimen with high concrete strength has lower ductility,energy dissipation capacity,but greater stiffness degradation and higher bearing capacity.The descending part of skeleton curves for specimens with different concrete strength tend to have the same final horizontal bearing capacity.The stiffeners effectively improve the stress distribution of specimens and enhance the bearing capacity of core concrete.(3)The design methods of load-carrying capacity for circular CFST columns under axial pressure and moment are summarized by existing specifications and then compared with test results in detail.The comparison shows that the test results are close to European specification EC4 and Japanese specification AIJ,while American specification AISC is conservative and Chinese specification GB50936-2014 is unsafe.Based on the parameter analysis,the correction coefficient K is proposed by linear regression.In accordance with specification AIJ,a restoring force model for circular concrete-filled thin-walled steel tube column is put forward,which is in good agreement with tests and finite element analysis.