# Research on Compression Behavior of Square Thin-Walled CFST Columns with Steel-Bar Stiffeners
## 3 Test Results
### Ultimate Load  
In order to study the effect of steel-bar stiffeners on the axial bearing capacity of the columns, nominal bearing capacity was used in this study. Nominal bearing capacity means that the axial bearing capacity of a square thin-walled CFST column was assumed to be composed of three parts without considering the confinement effect on the core concrete provided by the steel tube. The first part is the capacity of the concrete inside the tube, the second one is the capacity of the steel tube, and the last part is the capacity of the stiffeners. The formula used to calculate the nominal bearing capacity of the columns is shown in Equation (1):  
\[N_{0}=f_{c}\times A_{c}+f_{yt}\times A_{st}+f_{ys}\times A_{ss} \tag{1}\]  
where \(N_{0}\) is the nominal bearing capacity of the column, kN; \(f_{c}\) is the strength of concrete inside the steel tube, N/mm\({}^{2}\); \(A_{c}\) is the cross-section area of the concrete, mm\({}^{2}\);\(f_{yt}\) is the strength of the steel tube, N/mm\({}^{2}\); \(A_{st}\) is the cross-section area of the steel tube, mm\({}^{2}\);\(f_{ys}\) is the strength of the steel-bar stiffener, N/mm\({}^{2}\); and \(A_{ss}\) is the cross-section area of the steel-bar stiffener, mm\({}^{2}\). The axial bearing capacities of the eight column specimens obtained from the static-loading tests were compared with the nominal bearing capacities calculated according to Equation (1), which is shown in Table 4. In Table 4, \(N_{ue}\) is the actual bearing capacity of the columns obtained from the tests. Table 4 shows that the \(N_{ue}/N_{0}\) of C1 s only 0.377. Because of this, local buckling in the thin steel plate of the square tube led to an unstable failure of the column before the steel reached yield strength. As a result, the actual bearing capacity of the column was much lower than the nominal capacity. For C2, the supply to the tube from the concrete inside significantly increased the bearing capacity of the column. However, due to the high ratio of the cross-section width of the square tube to tube-wall thickness, local buckling occurred earlier leading to a premature failure of the column. Thus, the actual bearing capacity of the column was still lower than the nominal capacity. For the column specimens with stiffeners inside or outside the tube, bearing capacities were increased significantly as compared with C2, and especially the bearing capacity of C6, with a 16 mm diameter steel bar outside the tube, which increased by 68.7% compared with C2. The ratios of \(N_{ue}/N_{0}\) of these columns were all around 1.0, and the ratio of C6 even reached 1.127. Such observations verified that the steel-bar stiffeners were able to provide effective support to delay the local buckling that occurred in the steel plate of the tube, which improved the confinement effect from the steel tube on the core concrete and made better use of the steel's material properties. The actual bearing capacity of C3 with 12 mm diameter steel bars inside was higher than that of C5 with the same-diameter bars outside the tube. This phenomenon can be explained as follows. When the 12 mm diameter steel bars were used as stiffeners, the cross-section area ratio of steel to \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Column Number** & \(A_{ss}\) **(mm\({}^{2}\))** & \(N_{ue}\) **(kN)** & \(N_{0}\) **(kN)** & \(N_{ue}/N_{0}\) \\ \hline C1 & - & 145.0 & 384.9 & 0.377 \\ C2 & - & 802.0 & 1014.9 & 0.790 \\ C3 & 452.4 & 1154.5 & 1119.6 & 1.031 \\ C4 & 804.2 & 1200.0 & 1200.9 & 0.999 \\ C5 & 452.4 & 1100.0 & 1119.6 & 0.983 \\ C6 & 804.2 & 1353.0 & 1200.9 & 1.127 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of axial bearing capacity of columns. column increased from 3.96% to 5.07% (Table 1). This increase can lead to the increase of the stiffness of the steel tube, and can provide support to resist local buckling. But when the increase of the steel cross-section area ratio was 1.11%, the support provided by the steel-bar stiffeners was still not sufficient.