# Gupta, Sarda, and Kumar 2007

The paper presents an experimental and computational study on the behavior of circular concentrically loaded concrete filled steel tube columns until failure. The effect of diameter and D/t ratio of a steel tube on the load carrying capacity of the concrete filled tube columns was investigated. The effect of the grade of concrete and volume of fly ash in concrete was also investigated. A nonlinear finite element model was developed to study the load carrying mechanism of CFTs using the Finite Element code ANSYS.

## Experimental Study, Results, and Discussion

Eighty-one specimens of circular cross section with different concrete strength and wall thickness were tested under concentric axial quasi-static compressive loading. In these specimens, three parameters were tested: diameters of two, three, and four inches representing tubing with respective D/t ratios of 25.283, 32.598, and 38.948; fly ash compositions of zero, fifteen, twenty, and twenty-five percent mix by weight; and M30 (4.35 ksi compressive strength) and M40 (5.80 ksi compressive strength) grades of cement. Steel tubes were made of mild steel with a specific yield strength of 52.2 ksi and had a length of 340 mm. Nine of the specimens were hollow tubes.

During the experiment, all the two inch diameter specimens collapsed by Euler buckling while the three and four inch diameter specimens collapsed due to triggering of local buckling. It was observed that the strength of the concrete in the CFT reduced with increase in percentage of fly ash up until twenty percent. However, at twenty-five percent fly ash composition, the strength was observed to be higher than at fifteen and twenty percent. Smaller D/t ratios were found to provide good confinement effect to the concrete. Carrying capacity of the steel tube per unit volume decreases as the D/t ratio increases.

Two dimensional axis symmetric nonlinear finite element models were developed to study and to compare the experimental results of the axial load behavior of concrete filled tubes using the finite element code ANSYS. Multilinear elastic material and geometric nonlinear behavior were used for the computational model. In the computational study, failure modes identical to those found in testing were observed. The initial slope of the load-deformation curves obtained by the analytical model was found to be greater than that of the experimental one. Deformation of the CFT specimens at the yield point was about 20 to 30 percent less than their experimental counterparts.

## References

- Gupta P. K., Sarda S. M., Kumar M. S. (2007) "Experimental and computational study of concrete filled steel tubular columns under axial loads," Journal of Constructional Steel Research, 63 (2), pp. 182-193. doi:10.1016/j.jcsr.2006.04.004