# Behavior of an Internally Confined Hollow Reinforced Concrete Column with a Polygonal Cross-Section
## 3 Verification
### Behavior of Polygonal ICH RC According to Parameters
#### 3.2.2 Square Cross-Section with Square Inner Tube  
Similar to the case of rectangular ICH RC columns, the behavior of the square ICH RC columns with the square inner tube (S-S ICH RC) is analyzed according to the variation in concrete strength and inner tube thickness by selecting the representative cross-sections of S-S ICH columns. Table 3 shows the material properties of the concrete and reinforcements used in the representative section and column members, as well as the dimensions of the columns.  
Analysis of the axial load-bending moment  
The analysis results of the S-S ICH RC column with different parameters are shown in Figures 16 and 17. Similar to the case of the rectangular ICH RC column, the unconfined concrete case, which uses 20 MPa of concrete strength, is considered for confirming the confining effect of concrete. The cases with varying concrete strength use 3 mm of the inner tube thickness, and those with varying inner tube thickness use 25 MPa of concrete. The initial axial load is assumed to be 0, similar to the case of the rectangular ICH RC column.  
Unlike the case of the rectangular ICH RC column, the analysis results are the same between the \(x\) and \(y\) directions in the case of the S-S ICH RC column, which has the same length in both vertical and horizontal directions. It is found that the resistance to axial load and bending moment increases when the confining effect of concrete occurs. With the same condition as 20 MPa concrete strength, the column, considering the concrete confining effect, performed 126% and 122% of the column without the confining effect on the maximum lateral load and the maximum bending moment, respectively. In addition, it  
Figure 15: Lateral load-displacement of each member (rectangular cross-section).  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{1}{c}{**Properties**} \\ \hline  & \(d_{h}\)(mm) & 13 \\ \hline \(d_{v}\)(mm) & 19 \\ \hline \(s\)(mm) & 50 \\ \hline \(f_{c}^{\prime}\)(MPa) & 20, 25, 30 \\ \hline \(t\)(mm) & 1, 2, 4, 6 \\ \hline \(f_{yh}\)(MPa) & 237.6 \\ \hline \(f_{yv}\)(MPa) & 237.6 \\ \hline \(H\)(mm) & 2500 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Cross-section and specification of the S-S ICH RC column.  
The results of the lateral load-displacement analysis according to the change in the concrete strength also show that the maximum lateral load increases when the confining effect of concrete occurs. In addition, the lateral load increases with concrete strength. In the case of varying inner tube thickness, the maximum lateral load increases with an increase in thickness. The minimum inner tube thickness required to prevent the yielding and bending failure is 1.68 and 0.42 mm, respectively. As shown in Figure 19, when the inner tube thickness is 1 mm, the lateral load of the column decreases drastically because the inner tube failed so that the confining effect of concrete does not occur. Figure 20 shows the lateral load-displacement for each member of the S-S ICH RC column.  
The lateral load-displacement analysis for each member is set with a concrete strength of 25 MPa and inner tube thickness of 3 mm. The lateral load of the inner tube is smaller than that of the longitudinal reinforcements, unlike the result of the rectangular ICH RC column. This is because the area of the inner tube in the S-S ICH RC column is narrower than that of the rectangular cross-section case. For the cover concrete, the lateral load decreases sharply after specific lateral displacement because the confining effect of concrete does not exist. Therefore, it is found that the effect of the concrete cover on the total column behavior is the largest.