## 3 Results and Analysis  
### Failure Process  
#### 3.1.1 Group One (FCB1-FCB4)  
(1) FCB1: As the loading starts, no visible crack is noticed at the surface and the loading-displacement curve develops in a linear manner, during which the specimen reaches the elastic stage. Under a loading of 150 kN, the top part of the specimen starts to crack slightly and vertically. The increase in loading encourages the cracks to further develop along the mortar joint. At this point, the axial and longitudinal deformations take place at an accelerated pace, meaning that the structure has entered the elastic-plastic stage. As the loading tops 200 kN, cracks cross through the specimen along the direction of the mortar joint in the middle and expand in width, accompanied by a minor cracking sound and evident exfoliation. Under a loading of 251 kN, the bearing performance of the specimen declines drastically, resulting in its utter failure. Please refer to Figure 5a for the state of the specimen upon failure.  
(2) FCB2: Like FCB1, FCB2 starts with the elastic stage, then enters the elastic-plastic stage and ends with the failure stage. The difference lies in the split limit, regarding which FCB2 stands out because its split limit tops 150 kN. As the loading imposed hits 300 kN, the exterior clay starts to exfoliate slightly. Under approximately 330 kN, the specimen crashes, while the loading-displacement curve declines. At this point, the crashed square wood stem indicates that it works with the clay bricks in tandem to bear the force imposed and promises fairly good results. Please see Figure 5b for the specimen state at the failure stage and Figure 5c for the bent stem.  
(3) FCB3: As the elastic stage, FCB3 bears resemblance to its counterpart FCB1. Under a loading of 430 kN, the first crack appears vertically and slightly some distance from the central line of the wall's elevation. As the loading increases, cracks--both vertical and longitudinal--are noticed at other sides, which, though minor, are slowly extending. When the loading further increases to 550 kN, the vertical and longitudinal deformations take place at an accelerated pace, while the cracks expand at both ends and cross through the bricks. As the loading tops 670 kN, through cracks are noticed along the mortar joint. Therefore, the loading pace is slowed down; however, the clay bricks start to exfoliate. Upon a loading of 830 kN (the compressive strength of the square-shaped pipe measured stands at 767 kN), the clay bricks crash, while the loading-displacement curve starts to decline. At this point, the specimen is destroyed, with minor buckling observed at both ends of the concrete-filled steel stem. Please refer to Figure 5d for the specimen state at the failure stage and Figure 5e for the state of the square-shaped concrete-filled steel stem upon specimen failure.  
(4) FCB4: The damage process of FCB4 generally follows the same pattern of FCB3, with the split limit as an exception, which exceeds that of FCB3. As the loading reaches 530 kN, the first crack--minor  
Figure 4: Loading imposed: (**a**) loading device; and (**b**) test scene.  
and vertical--is noticed at the top part of the specimen near the central line. The increase in loading causes a growing number of slight cracks, which develop at a slow pace. Under a loading of 660 kN, the vertical and longitudinal deformations increase, accompanied by the extension of cracks. As the loading further increases to 755 kN, through cracks are noticed along the mortar joint. At this stage, the loading starts to be imposed slowly, upon which cracks expand in width and exfoliation takes place at the clay bricks. The specimen crashes at a loading of 945 kN. At this point, the loading-displacement curve declines drastically, and the circular concrete-filled steel pipe starts to exfoliate and buckle at both ends. Please refer to Figure 5f for the failure state of the specimen and Figure 5g for that of the circular concrete-filled steel stem.