Behavior of axially loaded concrete-filled steel tubular dumbbell shaped stub columns

Applying unified formula for stress-strain relations of concrete under unixial load,ABAQUS was used to establish the 3D finite element model and to analyze the behavior of concrete-filled circular steel tubular(CFST) dumbbell shaped stub columns under axial compression.The elasto-plastic method was also used to analyze the load-deformation curve of CFST dumbbell shaped stub columns when the CFST dumbbell shaped stub column was simplified as superposition with the two CFST stub columns,the two steel webs and the concrete inside the steel web space.The load-axial strain behavior of CFST dumbbell shaped stub columns was analyzed by both finite element method and elasto-plastic method.The analysis results are in good agreement with the experiment results from references.Analytical results by elasto-plastic method are slightly lower than that by finite element method.Then based on the two methods,the axial stress and perimeter stress of steel tube,the longitudinal and perimeter stress of steel web,the axial stress and radial stress of concrete core,and the longitudinal stress of concrete inside the steel web space in CFST dumbbell shaped stub columns were discussed.At yield state,the axial stress of section steel was slightly lower than its yield stress because of the confinement effect for concrete.The analytical stresses in CFST dumbbell shaped stub columns by the two methods were almost the same,and the finite element model could reflect that the longitudinal stress of the concrete inside the steel web space increases lightly for the concrete confined by steel webs.Based on the limit equilibrium method,simplified formula for the axially loaded CFST dumbbell shaped stub columns was proposed.The analysis results by limit equilibrium method are close to those by finite element method,and both are in safe range compared with the experiment results from references.