# Guo et al. 2007

The results of steel only loaded square CFT columns are presented along with a corresponding numerical model. The results from additional numerical studies are used to formulate design recommendations applicable to RCFT columns with a wide range of D/t ratios.

## Experimental Study, Results, and Discussion

Twelve square CFT and twelve square HT short columns tests were performed. The CFT columns were loaded on the steel only, 0.59 in gaps without concrete were left at each end of the columns, and the steel was greased before placing the concrete to inhibit bond. All of the tubes were fabricated from two L shaped mild steel plates welded at their tips to form a square cross-section. The yield strength of the steel was 40.6 ksi. The thickness of the steel plate was constant, 0.063 in., while the depth was varied from 3.14 in. to 7.88 in to achieve D/t ratios between 50 and 125. The length was varied to keep L/D ratio equal to 3.0. End plates with a thickness of 0.31 in were welded to the ends of the CFTs. Stiffeners were welded to both ends of each specimen in accordance with Chinese design. The concrete used had a compressive strength of 5.6 ksi.

The initial stiffness was nearly identical between the CFTs and HTs, indicating that the concrete was, indeed, not carrying any load. The presence of the concrete prevented the occurrence of inward local buckles and the wavelengths of the local buckles were considerably smaller than those occurring in the HTs.

## Analytical Study

Using ABAQUS, both the HT and CFT specimens were modeled. The steel was modeled using shell elements and a bi-linear elastic-plastic stress-strain relationship. The concrete was modeled using eight-node brick elements. The results from these analyses compared well to the experimental results.

An elastic buckling analysis was also performed. It was found that for small D/t ratios, the elastic results were unconservative, however, for large D/t ratios, the elastic results were conservative. This lead to the recommendation that, for CFT columns with D/t ratios greater than 120, it would be beneficial and safe to consider post-buckling effects in design.

The effects of initial imperfections and residual stresses were also investigated using the ABAQUS model. Both were found to have little effect for D/t ratios less than 50. Based on further studies and assuming certain values of initial imperfection and residual stress, an equation for the effective depth of a rectangular CFT column was derived. The ratio of effective depth to actual depth is equal to the ratio of critical buckling stress to yield stress. The strength predicted using the formula was compared the experimentally observed strength of previously published experiments, as well as current design codes. The advantage of the new formula being that it is applicable to CFT columns with large D/t ratios.

## References

Guo, L., Zhang, S., Kim, W.-J., and Ranzi, G. (2007). “Behavior of square hollow steel tubes and steel tubes filled with concrete.” Thin-Walled Structures, 45(12), 961-973.