Composite action in concrete filled tubes

This paper studies composite action in concrete filled tubes (CFT) that have dimensions and proportions like those used in U.S. practice. CFT applications in buildings and the importance of bond stress and interface conditions to behavior are noted, past research is summarized, and the combined results are analyzed. An experimental study is described and evaluated. It is shown that shrinkage can be very detrimental to bond stress capacity, and the importance of shrinkage depends upon the characteristics of the concrete, the diameter of the tube, and the surface condition at the inside of the tube. The bond capacity is smaller with large diameter tubes and large d/t ratios. The bond capacity is interrelated with slip at the steel concrete interface. An exponential distribution of bond stress is expected prior to slip, and a more uniform distribution occurs after slip. An equation that estimates the bond stress capacity is developed, and this leads to design recommendations at different performance levels.