Ahmed, Mizan (2020) Experimental and Numerical Studies of Concrete-Filled Double Steel Tubular Columns. PhD thesis, Victoria University.

Abstract

Concrete-filled double steel tubular (CFDST) columns are high-performance composite columns, which have increasingly been used in high-rise composite buildings and bridges as well as in strengthening conventional concrete-filled steel tubular (CFST) columns. The additional confinement provided by the inner circular tube in CFDST columns considerably improves their strength and ductility compared to CFST columns. However, research studies on the behavior of CFDST columns have been very limited and no design rules are given in current design codes. This thesis presents experimental and numerical studies on the fundamental behavior of circular and rectangular CFDST short and slender columns subjected to axial compression, combined axial load and bending, and preloads. Experiments on the behavior of square CFDST short columns with circular inner tube, circular CFDST short columns with circular inner tube and rectangular CFDST short columns composed of inner rectangular tube loaded concentrically and eccentrically are undertaken. Fiber-based mathematical models are developed for predicting the structural responses of CFDST short and slender columns under various loading conditions. The formulations of the mathematical models consider the influences of concrete confinement, geometric and material nonlinearities, and local buckling. New confining pressure models are proposed based on test results for ascertaining the compressive and residual strengths of confined concrete in CFDST columns, and incorporated in the mathematical models. The highly dynamic nonlinear equilibrium equations of CFDST columns under eccentric loading are solved by the efficient computer solution algorithms, which are developed based on the inverse quadratic method. The validations of the numerical models are made by comparisons with experimental results. The influences of various geometric and material parameters on the behavior of CFDST columns are examined. The results obtained from experimental and numerical studies are used to propose design equations. This research makes significant contributions to the knowledge by adding new test results on CFDST short columns to the database. The numerical models developed provide researchers and structural designers with accurate and efficient computer simulation and design tools, which lead to safer and more economical designs of composite structures. The design equations proposed can be utilized to design CFDST short and slender columns under various loading conditions.

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