Fourteen specimens of concrete filled double skin steel tubular (CFDST) stub columns were tested. Both outer and inner tubes were circular hollow sections (CHS). The main experimental parameters were diameter-to-thickness ratio and hollowness ratio. A mechanics model is developed in this paper for the CFDST stub columns. A unified theory is described where a confinement factor (ξ ) is introduced to describe the composite action between the steel tube and filled concrete. The predicted load versus axial strain relationship is in good agreement with stub column test results. Simplified models are derived to predict the load-carrying capacities of the composite sections.
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