# Ellobody, Young, and Lam 2006

This paper presents the behavior and design of axially loaded concrete-filled steel tube circular stub columns. The objective of this study is to develop an accurate finite element model to simulate the behavior of concrete-filled compact steel tube circular stub columns. The finite element program ABAQUS was used in the analysis.

## Analytical Study

In order to accurately simulate the actual behavior of concrete-filled steel tube columns, the main three components of these columns have to be modeled properly. These components are the confined concrete, the circular steel tube, and the interface between the concrete and the steel tube. A mesh size of 1 (length): 1 (width): 2 (depth) was used for the analysis. Due to symmetry only a quarter of the column was modeled. Experimental investigations conducted by Giakoumelis and Lam and Sakino et al. were used to verify the finite element model developed in this study. The measured stress-strain curves presented by Giakoumelis and Lam for circular steel tubes of 0.14 and 0.20 in. nominal plate thickness with 4.49 in. nominal external diameter were simulated as two multi-linear stress-strain curves. Tri-linear stress-strain curves were used to model the circular steel tubes of 0.18 in. plate thickness with 9.37 and 14.17 in. nominal external diaphragms tested by Sakino et al.. In order to define the full equivalent uniaxial stress-strain curve for confined concrete three parts of the curve have to be identified. The first part is the initially assumed elastic range to the proportional limit stress. The second part is the nonlinear portion starting from the proportional limit stress 0.5 (f_{cc}) and going to the confined concrete strength (f_{cc}). The third part is the descending part from the confined concrete strength (f_{cc}) to a value lower than or equal to rk_{3}f_{cc} with the corresponding strain of 11_{εcc}. The friction between the two materials is maintained as long as the surfaces remain in contact. For this study a coefficient of friction of 0.25 was used for the steel and the concrete.

## Comparison of Results

A comparison between the experimental results and the finite element results was carried out. The ultimate loads obtained from each test as well as the load-axial shortening curves and deformed shapes after failure were investigated. A good agreement between the two sets of results was achieved and a maximum difference of 7% was observed between the experimental and numerical results. The column strengths, load-axial shortening curves and deformed shapes of the columns have been predicted using the finite element program and compared well with the experimental results.

## Parametric Study

A parametric study of 40 concrete-filled compact steel tube circular columns with different external diameters of the steel tube-to-plate thickness (D/t) ratios ranging from 15 to 70 and concrete cube strengths ranging from 4.35 to 15.95 ksi was performed. It was seen that the ductility of the column is decreased as the concrete strength increases. The results were compared with design rules specified by the American Specifications and Australian Specifications and were found to be conservative while they were found to be unconservative for the European Code.

## References

- Ellobody, E., Young, B., and Lam, D. (2006). “Behaviour of Normal and High Strength Concrete-Filled Compact Steel Tube Circular Stub Columns,” Journal of Constructional Steel Research, Vol. 62, pp. 706-715. doi:10.1016/j.jcsr.2005.11.002