## 3 Analysis of Test Results
### Hysteric Curves  
In the structural seismic performance test, under low-cycle reversed load, a certain residual deformation still exists in the structure or member after complete unloading due to the elastic-plastic characteristics of reinforced concrete structures. A hysteretic loop is formed between the load and the displacement of the column specimen under the action of the forward loading-unloading and the reverse loading-unloading. A series of hysteretic loops are formed under the action of low-cycle reversed loading and thus constitute the hysteretic curve of the specimen. The hysteretic curve is a comprehensive reflection of crack development, steel yielding, concrete breaking, and other phenomena for the column specimen during loading. It is an important tool to analyze the deformation behavior, stiffness degradation, energy dissipation performance, and other mechanical properties of the column specimen. In our test, the load-displacement curves (hysteretic curves) in the mid-span of BFRPC composite column, PHC tube column, and PRC tube column specimen were automatically recorded using the MTS hydraulic servo loading system and corrected by the displacement meter data. The resulting hysteretic curves are demonstrated in Figure 10.  
Figure 10: Load–displacement hysteretic curve of each column specimen: (**a**) PRC tube column; (**b**) PHC tube column; (**c**) BFRPC composite column. The mechanical properties of the BFRPC composite column, PRC tube column, and PHC tube column specimens are compared below in terms of shape, slope variation, and symmetry of the hysteretic curves shown in Figure 10.  
From the shape of hysteretic curves displayed in Figure 10a,c, the load-displacement curves of the PRC tube column and the BFRPC composite column specimens are relatively steep at the beginning of unloading, and the displacement recovery is slow. However, with further unloading, the curves tend to be gentle, and the displacement recovery is expedited. In the whole process of low-cycle reversed loading, the accumulations of residual deformation for the BFRPC composite column and the PRC tube column specimens are relatively large, and their hysteretic curves generally present an inverse S-shape. Differently, as illustrated in Figure 10b, the hysteretic curve of the PHC tube column specimen is generally S-shaped, and the accumulation of residual deformation is small. At the beginning of unloading, the load-displacement curve is relatively flat, and the displacement recovery is fast. However, as the load decreases, the load-displacement curve becomes steep, and the displacement recovery slows down. Furthermore, the load-displacement curve of the BFRPC composite column specimen is approximately a straight line during the overall loading process, and the hysteretic curve is not full enough. It reflects the obvious elastic characteristics of the BFRPC composite column specimen during the overall loading process. The hysteretic curves of PRC and PHC tube column specimens are similar to those of the BFRPC composite column specimen at the initial loading stage. The load-displacement curves are approximately a straight line, showing the elastic behavior of PRC and PHC tube column specimens. However, with the increase of the load, the residual deformations of PRC and PHC tube column specimens continue to accumulate, showing apparent plastic behavior for PRC and PHC tube column specimens. Therefore, the hysteretic curves of PRC and PHC tube column specimens are relatively full. From the slope change of hysteretic curves displayed in Figure 10a,b, the curve slope of PRC and PHC tube column specimens decreases significantly with the increase of load after cracks appear in the column specimens, and the reduction amplitude of the curve slope increases with the increase of load. It reflects the plastic behavior of PRC and PHC tube column specimens. However, the slope of the hysteretic curve for the BFRPC composite column specimen decreases gradually with the increase of load, but the decrease is slight.