## 3 Analysis of Test Results
### Skeleton Curve  
The skeleton curve of the column specimen is formed by connecting the peak points of each stage of the hysteretic curve obtained from the low-cycle reversed loading test. Usually, the shape of the skeleton curve is similar to that of the monotonic loading curve but slightly lower than that of the monotonic loading curve. The skeleton curve reflects the mechanical characteristics of the column specimen in different loading stages, such as stiffness, ductility, and energy dissipation. It is an essential reference for determining the characteristic points of the column specimen, such as cracking load, peak load, and failure load. The skeleton curve of each column specimen and the comparison curve are shown in Figure 11.  
The variation of the skeleton curve shown in Figure 11a indicates that the PRC tube column specimen is in the elastic stage before the column concrete cracking, and the change of the specimen stiffness is not significant. Thus, the slope of the skeleton curve remains unchanged. After the column concrete cracks, the slope of the skeleton curve for the PRC tube column specimen decreases significantly, and the skeleton curve tends to be flat. This indicates that the specimen stiffness degrades rapidly. Then, the skeleton curve of the PRC tube column specimen has a longer descending section. When the ultimate load is loaded, the bearing capacity of the PRC tube column specimen falls below 85% of its ultimate bearing capacity, and the column specimen is damaged. As illustrated in Figure 11b, the variation of the skeleton curve for the PHC tube column specimen is similar to that of the PRC tube column specimen, with the difference that the descending section of the skeleton curve is shorter than that of the PRC tube column specimen. The slope of the skeleton curve shown in Figure 11c does not change much overall. This indicates that the BFRPC composite column specimen has significant elastic characteristics during loading and unloading, and the stiffness degradation is slight. The failure of the BFRPC composite column specimen is caused by the sudden drop of the specimen bearing capacity due to the fracture of the FRP tube inside the specimen under the action of the ultimate load. The physical phenomena of the three column specimens displayed in Figure 11 are consistent with the test phenomena stated in Sections 2.3 and 2.4. The feature points of the skeleton curves for the three types of column specimens shown in Figure 11a-c are summarized in Table 3. The cracking point in the table refers to the points where the concrete first cracks, the peak point refers to the points where the load values reach the maximum, and the extreme point refers to the points where the displacement values reach the maximum. Figure 11: The skeleton curve of each column specimen and the comparison curve: (**a**) PRC tube column; (**b**) PHC tube column; (**c**) BFRPC composite column; (**d**) comparison curve. From the comparative skeleton curves shown in Figure 11d, the skeleton curves of PHC and PRC tube column specimens basically overlap in the elastic phase, indicating that whether the non-prestressed steel bar is configured or not has little effect on the stiffness of the tube column in the early stage of loading. Differently, the slope of the skeleton curve for the BFRPC composite column specimen is smaller than that of the PHC and PRC tube column specimens in the early loading stage but inverse in the later loading stage. This reveals that the stiffness of the BFRPC composite column specimen is smaller than that of the PHC and PRC tube column specimens in the early loading stage, while the stiffness in the later loading stage is more significant than that of the PHC and PRC tube column specimens. Moreover, the ultimate bearing capacity of the PRC tube column specimen (175 kN) is more significant than that of the PHC tube column specimen (120 kN), indicating that the configuration of non-prestressed steel bars can enhance the bearing capacity of the column specimen to a certain extent.