STATIC BEHAVIOR OF AXIALLY COMPRESSED CIRCULAR CONCRETE FILLED CFRP-STEEL TUBULAR (C-CF-CFRP-ST) COLUMNS WITH MODERATE SLENDERNESS RATIO

Thirty-two specimens are experimentally tested in this paper to study the static performance of axially compressed circular concrete filled CFRP-steel tubular (C-CF-CFRP-ST) columns.The tested results indicate that, for columns with relative small slenderness ratio, failure is dominated by the strength loss of the materials.However, for columns with relative large slenderness ratio, failure is controlled by instability.The load versus deflection curves at the mid-height of the composite column can be divided into three stages, i.e., elastic, elasto-plastic and softening stage.Analyses of the tested results show that distribution of longitudinal strains in the steel tube over depth on the cross-section is approximately linear, and the steel tube and its outer CFRP material can cooperate both longitudinally and transversely.The longitudinal strain and the transverse strain at a point have opposite signs, and the steel tube under longitudinal tension has no transverse confinement effect on its concrete.The Deformed modes of the columns, the axial load verses deflection curves at the mid-height, the axial load verses axial shortening curves and the stresses in the steel tube are simulated by using finite element method.The calculated results agree well with the experimental results to prove the accuracy of the finite element model.Stresses in the concrete, the steel tube and the CFRPs are analyzed by using finite element method.The numerical results show that the interaction force between the steel tube and the concrete decreases gradually from compressive region to tensile region.The adhesive strength between the concrete and the steel tube has little effect on the critical buckling load and on the elastic stiffness of the columns.Equation for calculating the critical buckling load of the composite columns is presented, and the estimated results agree well with the experimental results.