# Evaluation of Eccentric Compressive Behavior Between Active and Passive Preloading in CFST
## 4 Finite Element Analysis and Discussions
### Stress on Components
#### 4.2.2 Step 2: Service Loading Stage  
Figure 16 shows the longitudinal stress-strain curve for each concrete unit in the middle cross-section of the specimen, with the number of each unit in the concrete cross-section indicated. In the curve, the symbol \"\(\bullet\)\" represents the peak load on the specimen. The horizontal axis represents strain and the vertical axis represents stress. Although the active preload was not substantial for CFST-SP and CFST-CP, the concrete strength was enhanced, as seen in the graph. Furthermore, for CFST-CC, even though the pre-compression was passively applied, the concrete was subjected to an initial three-way compression, resulting in a slight increase in the concrete's strength in the bending compression zone compared to the ordinary CFST. For CFST-SC, however, it can be seen from the diagram that the concrete initial stiffness in each CFST-SC unit was lower due to the absence of steel tubes working together, while the peak stresses in the concrete were also the lowest compared to the other specimens, even lower than those in the ordinary CFST. Finally, it is worth noting that the concrete in the CFST-SP compression bending zone reached its maximum at the same time that the specimen reached its peak load. The biaxial stresses in the steel tube in Step 2 were also influenced by the preload in Step 1. The evolution of the longitudinal and hoop stress-strain curves, as well as the Von Mises curve for the steel tube, is shown in Figs. 17 and 18. In the figures, the characters \"C\" and \"T\" denote the compressive and tensile bending zones, respectively, and the symbol \"\(\bullet\)\" denotes the peak load in the curve. First, as shown in Figure 17, the steel tube started as longitudinal compressive stress in the tensile bending zone due to the small eccentric ratio and became longitudinal tensile stress as the load increased. This characteristic was  
Figure 16: Longitudinal stress versus strain curves of the concrete. consistent with the ordinary CFST. Second, at the beginning of Step 2, the hoop stresses in CFST-SP, CFST-CP, and CFST-CC were non-zero, with CFST-CP being the largest, due to the highest active radial compression in CFST-CP. In contrast, the initial hoop stress in CFST-SC was zero, which was attributed to the lag in the steel tube confinement effect caused by the compressive pre-stress in the steel tube. The von Mises evolution shown in Figure 18 could better elucidate the development of the bi-directional stresses in the steel tube. As can be seen from the graph, the Von Mises stress in the compressive bending zone of the steel tube only yielded when CFST-SP and CFST-CP reached their peak loads, while CFST-CC and CFST-SC reached their peak loads only after the Von Mises stress in the compressive bending zone yielded. Compared to the ordinary CFST, the steel tube of CFST-SC exhibited significant ductility in the compressive bending zone. Figure 17: Stress versus strain of steel tube: (**a**) CFST-SP; (**b**) CFST-CP; (**c**) CFST-CC; (**d**) CFST-SC. Figure 18: Von Mises yield curves: (**a**) CFST-SP; (**b**) CFST-CP; (**c**) CFST-CC; (**d**) CFST-SC. Figure 19 shows the development of contact stresses at the concrete-steel interface in the tensile and compressive bending zones in Step 2. First, it can be seen from the graphs that the stresses in both zones changed in a similar way. Second, the preloaded specimens, except for CFST-SC, started at Step 2 with non-zero interfacial stresses. As the load increased, the contact stresses in the specimens all increased. However, the stress curves of CFST-SP and CFST-CP suddenly drop and then rise. The reason for this phenomenon is that the Poisson's ratio of steel is larger than that of concrete, so the deformation of the steel pipe is also larger than that of concrete in the early stage of service loading.