Rectangular concrete-filled double steel tubular (CFDST) columns with inner circular steel tube possess higher structural performance than conventional concrete-filled steel tubular (CFST) columns. However, the local buckling of the outer steel tube of thin-walled rectangular CFDST columns has not been accounted for in the existing fiber element models and design codes that may overestimate the column ultimate axial strengths. This paper describes a computationally efficient fiber-based modeling technique developed for determining the behavior of concentrically-loaded rectangular CFDST short columns including the local buckling effects of the external steel tube and the confinement offered by the internal circular steel tube. The effective width concept is used to simulate the post-local buckling of the outer steel tube. Comparative studies are undertaken to verify the fiber-based model with the relevant test results. The computational model is then employed to investigate the axial load-strain responses of rectangular CFDST short columns with various key design variables. A design equation is developed for computing the ultimate axial loads of short rectangular CFDST columns and compared with design methods given in several international design codes. It is shown that the fiber-based modeling technique and the proposed design model predict well the structural performance of short CFDST columns.

Numerical analysis of axially loaded circular high strength concrete-filled double Numerical analysis of axially loaded circular high strength concrete-filled double steel tubular short columns steel tubular short columns

High-strength square concrete-filled double steel tubular (CFDST) slender beam-columns with a circular internal steel tube subjected to eccentric loads may undergo interaction local-global buckling. No computational studies on the interaction local-global buckling of slender square CFDST beam-columns have been reported and their behavior has not been fully understood.This paper describes a mathematical model for the simulation of the interaction local-global buckling behavior of square high-strength CFDST slender beam-columns under axial compression in combination with uniaxial bending. The mathematical model is formulated by the fiber approach, accounting for confinement provided by the internal circular steel tube, and geometric and material nonlinearities. An incremental-iterative numerical procedure is designed to quantify the local-global interaction buckling responses of slender CFDST columns. Efficient numerical solution algorithms implementing the inverse quadratic method are developed for solving the nonlinear equilibrium dynamic functions of CFDST columns. The formulation proposed is verified by existing experimental data on CFDST columns as well as

Square concrete-filled double steel tubular (CFDST) beam-columns consisting of an internal circular steel tube have increasingly been utilized in composite building structures because of their high structural performance. This paper describes experimental and numerical studies on the structural responses of square thin-walled CFDST columns loaded eccentrically. Tests on twenty short square CFDST columns were undertaken that included sixteen columns under eccentric loading and four columns under concentric loading. The parameters examined in the experiments included the cross-sectional dimensions, the width-to-thickness ratios of outer and internal tubes and loading eccentricity. The measured ultimate strengths, load-shortening responses, load-lateral displacement curves, stress-strain curves and observed failure modes are presented. A numerical model incorporating the fiber analysis is developed that predicts the moment-curvature responses and axial load-moment strength envelopes of CFDST columns.

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