Nonlinear analysis of circular concrete filled steel tubular stub columns considering confinement effect

Abstract

Concrete filled steel tube(CFT) is a beam-column member with a round or rectangular steel pipe in-filled with concrete. This type of a structure has many benefits over steel or reinforced concrete columns such as improved performance of flexibility, strength and energy absorption capacity. Enhanced performances of CFT are mainly due to the confinement effect of concrete, and the increase of strength and ductility of confined concrete is proportional to the confinement pressure. Therefore the confinement effect must be considered to analytically investigate the behavior and the ultimate resisting capacity of CFT columns. To observe the influence of the confinement effect, circular CFT stub columns are selected in order to maximize the confinement effect. Because the confinement effect increases if the shape of the cross-section is close to circular and the length of the column is shorter. An exact estimation of the confinement pressure is crucial to evaluate the ultimate resisting capacity of CFT columns. However technically it is hard to measure the confinement pressure through the direct load-ing tests. Alternatively, the a new constitutive relationships for stress states of material components are configured and the corresponding numerical algorithm considering the dependencies of the modulus of elasticity and Poisson’s ratio on the stress level in the concrete is developed to definitely find out the con-finement pressure of concrete inside the CFT column. Yield of the steel tube is determined by numerically calculated stress state and an axial strength of CFT column is estimated using the confinement pressure when the steel tube yields. To validate the proposed numerical model, a total of 64 concentric axial tests are compared. And the relationships between the confinement pressure and the various design parameters are investigated using the verified numerical model. As a result, it was found that a confinement pressure of concrete and a maximum hoop stre...