The behaviour of circular concrete-filled thin-walled steel tubes in flexure

![CDATA[Concrete filled steel tubes (CFTs) are efficient members in structural applications including bridges, buildings and piled foundations. Their design has been based on theory and tests on columns under load applied axially or at relatively small eccentricities. For loading at large eccentricities or loading in pure flexure in the beam situation, there has been relatively little research performed and design has been based on extrapolation from the column situation. Therefore, an extensive research program on the flexural behavior of CFTs has been undertaken at the University of Western Sydney with a particular emphasis on thin-walled steel tubes that can provide an economical form of construction. Numerical studies have been used to investigate the influence of imperfections and plasticity on the behavior of bare steel tubes and this work was extended to examine the effect of the concrete infill on the behavior, particularly local buckling of the steel tube. A series of flexural tests has been carried out on full-scale tube specimens, both filled and unfilled. Accurate measurements of the imperfections were taken using an innovative method allowing the complete imperfect shape of the tube to be determined. This was needed for the analytical studies. The results have allowed the influence of the concrete infill on the flexural stiffness and strength of the tubes to be established. The test results and the analytical studies are being used to develop a design procedure that accounts for imperfections, local buckling, concrete strength, steel strength and the interaction between the tube and the infill.]]