Nonlinear analysis and behavior of concrete-filled steel tubular beam-columns

This paper presents a nonlinear fiber element analysis method for predicting the behavior of short concrete-filled steel tubular (CFST) beam-columns under axial load and biaxial bending. Nonlinear constitutive models for confined concrete and structural steel are considered in the fiber element analysis. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis within the composite section to satisfy equilibrium conditions. The accuracy of the fiber element method is established by comparisons with existing solutions. The fiber element analysis program developed is employed to study the effects of steel ratios and concrete strengths on the axial load-bending moment interaction curves for CFST beam-columns. The proposed fiber element analysis technique is shown to be efficient and can be used directly in the design of CFST beam-columns.