## 5 Comparison with the Design Codes
### Results Discussion  
The possibility of adopting the mentioned design codes to predict the axial bearing-capacity (Nu) of the RC column strengthened with the steel jacket and fill concrete was examined. Comparisons between the predicted (N\({}_{u,c}\)) and the experimental (N\({}_{u,e}\)) compressive strengths are given in Table 5 in terms of the forces and the N\({}_{u,c}\)/N\({}_{u,e}\) ratios. Code AISC-360 gives conservative predictions, mainly because it does not consider the enhanced compressive strength of the concrete induced by the lateral confinement provided by the steel jacket. The mean value of N\({}_{u,\text{A}\text{ISC}}\)/N\({}_{u,e}\) ratio is 0.929, and its standard deviation is 10.3%. Code DBJ/T13-51-2010, also gives conservative predictions, where the mean value of N\({}_{u,\text{D}\text{B}\text{J}}\)/N\({}_{u,e}\) ratio is 0.943, and the standard deviation is 10.6%. Compared to the AISC and DBJ codes, EC4 yields the best predictions, with 1.033 as the mean value of N\({}_{u,\text{EC4}}\)/N\({}_{u,e}\) ratio and 6.8% as the standard deviation. That advantage probably arises since it takes into account the enhanced concrete compressive strength as a result of the confinement effect provided by the steel jacket. It has to be noted that EC4 overestimates some of the results, as was registered by [PERSON] et al. [43]. The comparison of sectional capacity between predicted and test results are presented in Figure 20. It can be noticed from the diagram that, for the columns filled with low-strength concrete (Fill 1), code AISC-360 and DBJ/T13-51-2010 both gave conservative predictions that were outside the \(\pm\)10% limits. For the columns filled with regular-strength concrete (Fill 2) and with high-strength concrete (Fill 3), all considered codes gave a good prediction, with results inside the \(\pm\)10% limits. When considering the high-strength concrete, all three codes overestimated the column plastic capacity. The dispersion of results was expected, mainly because all codes consider composite members made from one type of concrete, but in the presented research, the core column concrete and the fill concrete have different properties. The behavior and bearing capacity of concrete-filled tube members are highly dependent on the relative strength ratio, as demonstrated by [PERSON] et al. [19], [PERSON] et al. [36], and [PERSON] et al. [40]. The comparison of axial strengths predicted by the mentioned codes to the experimental results from this paper is presented in Figure 21. All three considered codes provide more conservative predicted results within the experimental range as the value of the confinement factor increases. The enhanced strength of the concrete core in a CFST column is less obvious as the compressive strength of fill concrete increases. That can be explained by the fact that the concrete with higher compressive strength shows much stiffer behavior than the concrete with lower compressive strength. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Label** & **N\({}_{u,e}\)** & **N\({}_{u,\text{EC4}}\)** & **N\({}_{u,\text{EC4}}\)/N\({}_{u,e}\)** & **N\({}_{u,\text{AISC}}\)** & **N\({}_{u,\text{AISC}}\)/N\({}_{u,e}\)** & **N\({}_{u,\text{D}\text{B}\text{J}}\)** & **N\({}_{u,\text{D}\text{B}\text{J}}\)/N\({}_{u,e}\)** \\ \hline Column–S0x & 1600.0 & 1531.6 & 0.957 & 1321.2 & 0.826 & 1330.9 & 0.832 \\ Fill 1–B2x & 1709.3 & 1798.9 & 1.052 & 1588.5 & 0.929 & 1633.8 & 0.956 \\ Fill 2–B4x & 1864.0 & 2029.5 & 1.089 & 1923.3 & 1.032 & 1943.4 & 1.043 \\ Fill 3–P0x & 1600.0 & 1531.6 & 0.957 & 1321.2 & 0.826 & 1330.9 & 0.832 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison between calculated member bearing capacities and test results. As a result of the presented analysis, the mentioned design codes could be applied directly, in order to calculate the compressive axial load-bearing capacity of RC columns strengthened with the steel jacket and the fill concrete.