Plastic mechanism analysis of concrete-filled double-skin (SHS inner and SHS outer) stub columns

A plastic mechanism to predict the collapse behaviour of concrete-filled double-skin stub columns is developed and analysed in this paper. Both outer and inner tubes are square hollow sections (SHS). In the analysis, the inner tube is treated the same way as that used in previous research on empty SHS stub columns. New mechanism models are developed for the outer tubes. The effect of local buckling in the outer tube is also studied. The concrete model adopted in this paper considers the effect of confinement of the concrete induced by the double skin tubes. It also considers the strength degradation of concrete for large deformation analysis. It has been found that the effect of local buckling on the collapse curve of the outer tube can be ignored. The concrete model including confinement and strength degradation should be used in order to simulate the collapse behaviour, especially for thin outer tubes. Good agreement was achieved between the plastic mechanism analysis and experimental results.