Nonlinear inelastic analysis of concrete-filled steel tubular slender beam-columns

High strength thin-walled concrete-filled steel tubular (CFST) slender beam-columns may undergo local and global buckling when subjected to biaxial loads, preloads or cyclic loading. The local buckling effects of steel tube walls under stress gradients have not been considered in existing numerical models for CFST slender beam-columns. This thesis presents a systematic development of new numerical models for the nonlinear inelastic analysis of thin-walled rectangular and circular CFST slender beam-columns incorporating the effects of local buckling, concrete confinement, geometric imperfections, preloads, high strength materials, second order and cyclic behavior. In the proposed numerical models, the inelastic behavior of column cross-sections is simulated using the accurate fiber element method. Accurate constitutive laws for confined concrete are implemented in the models. The effects of progressive local buckling are taken into account in the models by using effective width formulas. Axial load-moment-curvature relationships computed from the fiber analysis of sections are used in the column stability analysis to determine equilibrium states. Deflections caused by preloads on the steel tubes arising from the construction of upper floors are included in the analysis of CFST slender columns. Efficient computational algorithms based on the Muller’s method are developed to obtain nonlinear solutions. Analysis procedures are proposed for predicting load-deflection and axial load-moment interaction curves for CFST slender columns under axial load and uniaxial bending, biaxial loads, preloads or axial load and cyclic lateral loading. The numerical models developed are verified by comparisons of computer solutions with existing experimental results and then utilized to undertake extensive parametric studies on the fundamental behavior of CFST slender columns covering a wide range of parameters. The numerical models are shown to be efficient computer simulation tools for designing safe and economical thin-walled CFST slender beam-columns with any steel and concrete strength grades. The thesis presents benchmark numerical results on the behavior of high strength thin-walled CFST slender beam-columns accounting for progressive local buckling effects. These results provide a better understanding of the fundamental behavior of CFST columns and are valuable to structural designers and composite code writers.