# Behavior of Circular CFST Columns Subjected to Different Lateral Impact Energy
## 3 Test Results
### Load-Strain Curves  
Tensile strain is defined as positive strain and compressive strain is defined as negative strain to analyze the effects of lateral impact on the residual ultimate axial capacity of the circular CFST column. In other words, the strain obtained from compressive test is positive, which indicates that there exists tensile strain. Strain obtained from the compressive test is negative, which indicates that there exists compressive strain. The vertical strain measurement points are labeled No. 1, No. 3 and No. 5 and the transverse strain measurement points of the corresponding specimens are labeled as No. 2, No. 4 and No. 6. The load-strain curves are shown in Figure 12. It is obtained from the Figure 12 that the strain value increases with the increase of axial compressive load. When the axial compressive strength increases within the elastic stage, the strain is linearly increased with the increase of axial compressive strength. When the axial compressive strength increases beyond the elastic stage, the increased rate of strain is greater than that within the elastic stage.  
Figure 11: Typical load–displacement curve.  
Figure 10: Load-displacement curves: (**a1**) C20-L0.50; (**a2**) C20-L0.33; (**a3**) C20-L0.25; (**b1**) C30-L0.50; (**b2**) C30-L0.33; (**b3**) C30-L0.25; (**c1**) C40-L0.50; (**c2**) C40-L0.33; (**c3**) C40-L0.25.  
Figure 12: _Cont._  
By comparing the load-strain curves of different specimens without the damage of lateral impact, the increased rates of strain are mostly identical when axial compressive load increases within the elastic stage, elastic-plastic stage and strengthened stage. For the most specimens with damage of lateral impact, the increased rate of strain is greater at impact location than those at other strain measurement points within the elastic stage. Increased rates of strain at the No. 3 and No. 4 measurement points are greater than those at the No. 5 and No. 6 measurement points. When axial compressive load increases within the elastic-plastic stage and strengthened stage, the increased rate of strain at the impact location is greater than that the within elastic stage. The increased rate of strain at the impact location is greater than the increased rate of strain at the other measurement points. Results indicate that the steel at the impact location becomes plastic faster than other locations and load-strain curves of measurement points 1 and 2 are symmetrical. It also indicates that increased rate of strain at vertical strain measurement point and transverse strain measurement point are mostly the same. Because of the complicated deformation, load-strain curves show a turn back tendency for specimens C30-L0.50-E10000, C30-L0.50-E15000 and C30-L0.33-E10000 when the axial compressive strength increases within the later load stage. The curves of vertical strain and transverse strain of these specimens do not have a clear trend. Results indicate that compared with other locations, impact location damage becomes serious when the columns are subjected to the lateral impact. So the reinforcement of possible impact location should be taken into account when the CFST columns are designed in the civil engineering project.