# Axial Compression Behavior of Steel Angle-Corrugated Steel Plate-Confined Concrete Columns
## 3 Experimental Results and Discussion
### Strength Index of Core Concrete  
In order to evaluate the improvement degree of the compressive strength of the core concrete in the different column types, the strength index (\(I_{\text{s}}\)) of core concrete was adopted, which is defined as the strength of confined core concrete (\(f_{\text{cc}}\)) normalized by the  
Figure 7: Load–strain curves.  
prism compressive strength of concrete (\(f_{\rm c}\)) [27]. The strength index can be calculated as Equation (4):  
\[I_{\rm s}=A_{\rm c}f_{\rm cc}/A_{\rm c}f_{\rm c}=f_{\rm cc}/f_{\rm c} \tag{4}\]  
where \(A_{\rm c}\) is the area of core concrete.  
As we were unable to measure the stress of the core concrete directly, the compressive strength of the core concrete was determined by separation analysis, which is a numerical analysis method. Calculating the stress of the steel angle and steel tube, the stress change of the core concrete was determined according to the balance condition. Combined with the load-strain curves of the specimens (Figure 7) and Equations (1)-(3), the stress state of the steel angle and steel tube can be determined under the ultimate bearing capacity (\(N_{\rm u}\)). The corrugated steel plates did not bear the axial load for the SA-CS columns, which was proven in previous studies [18; 19]. Based on the principle of superposition, \(N_{\rm u}\) can be written as Equation (5):  
\[N_{\rm u}=A_{\rm s2}\sigma_{\rm vu}+A_{\rm c}f_{\rm cc} \tag{5}\]  
where \(A_{\rm s2}\) is the area of the steel angle (for SA-CS column) or steel tube (for CFST column); \(\sigma_{\rm vu}\) is the vertical stress of the steel angle (for SA-CS column) or steel tube (for CFST column) corresponding to the peak load. \(\sigma_{\rm vu}\) is calculated from the vertical strain under peak load (\(\epsilon_{\rm vu}\)) and the stress-strain relationship (Equation (1)). Judge the relationship between \(\epsilon_{\rm vu}\) and the vertical component of equivalent elastic strain (\(\epsilon_{\rm ve}\)). If \(\epsilon_{\rm vu}\geq\epsilon_{\rm ve}\), the value of \(\epsilon_{\rm ve}\) is taken as the value of \(\epsilon_{\rm vu}\). If \(\epsilon_{\rm vu}<\epsilon_{\rm ve}\), the value of \(\epsilon_{\rm vu}\) is taken as the actual value. The calculated results have some deviations but are within the acceptable range.  
Thus, the strength of confined core concrete can be calculated by: \(f_{\rm cc}=(N_{\rm u}-A_{\rm s2}\sigma_{\rm vu})/A_{\rm c}\).  
Figure 8 illustrates the strength index \(I_{\rm s}\) of the core concrete in the different specimens. For all the samples, the \(I_{\rm s}\) was larger than 1.0, indicating that a steel skeleton (made of steel angles and corrugated steel plates) and steel tubes can provide confinement to core concrete. However, the \(I_{\rm s}\) of the SA-CS-230 column was 17.84% greater than that of the CFST-1 column and 12.29% greater than that of the CFST-2 column at a similar steel ratio (\(\alpha\)). The results presented that the steel skeleton (made of steel angles and corrugated steel plates) confined the core concrete more effectively than the steel tube. For the SA-CS columns, the \(I_{\rm s}\) decreased with the decline of the steel ratio, but the degree was very slight. It is speculated that the reason for this phenomenon is that the transverse stress of the corrugated steel plates of all the specimens did not reach the yield strength under the peak load, which provided sufficient confinement for the core concrete. On the other hand, with the increase in the section size, the ratio of the size of the steel angle to the section size gradually decreases, and the shape of the core concrete tends to be more regular, which is conducive to reducing the arch effect. In conclusion, the sensitivity of \(I_{\rm S}\) to the steel ratio is low.  
Figure 8: Strength index of core concrete.