High performance concrete filled steel tube columns

Concrete-filled steel tube (CFST) column is one of the most effective forms of composite columns. CFST columns combine the favorable properties of steel and concrete. Steel tube usage provides high stress capacity and improves the ductility, whereas the concrete core increases the compressive strength and stiffness. External and internal strengthening techniques have been widely adopted for improving the structural performance of CFST columns. This thesis evaluates the structural behaviour of CFST columns stiffened by internal and external strengthening under axial compression. A total of thirty-four CFST specimens were tested in experimental programs. The first program investigates the structural performance of short CFST columns strengthened by three different methods. In this work confinement in the form of external reinforcing rings, stiffening in the form of internal reinforcing bars and a combination of both methods were employed. The second program evaluates the structural behavior of short CFST columns strengthened by external and internal continuous spirals. All CFST specimens were filled using self-compacting concrete (SCC) of normal compressive strength. From the test results, it was concluded that compressive strength, elastic axial stiffness, toughness, and ductility behavior were improved significantly as a result of stiffening the CFST specimens. In addition, a theoretical model to estimate the ultimate load of CFST columns strengthened by the continuous spirals was developed and close correlations were found between the model and the experimental results.