# Evaluation of Eccentric Compressive Behavior Between Active and Passive Preloading in CFST
## 6 Conclusions  
In this study, a comprehensive comparison of the compressive behavior of four representative CFST columns with active and passive preloads was carried out using the finite element method. The model we established can accurately predict the mechanical behavior of steel tube concrete columns under eccentric compression. The results show that the model is accurate and reliable and can be used for further parameter analysis. The conclusions of the study are summarized below. (1) When the four representative preloaded CFST columns were subjected to compression loads with an eccentricity of 0.2, the peak loads of CFST-CP, CFST-SP, and CFST-CC are 3244 kN, 3020 kN and 2977 kN, which are 1.15 times, 1.04 times and 1.02 times of the  
Figure 22: Effect of various parameters on difference ratio: (**a**) Difference ratio about \(a_{1}\); (**b**) Difference ratio about \(a_{2}\). peak loads of the ordinary CFST column. The CFST-SC only had 0.96 times the peak load of the ordinary CFST. In terms of ductility, CFST-SC had the highest ductility index of 9.8, and CFST-CP had the lowest ductility index of only 5.4. For the stress-strain relationship between the steel tube and the concrete, the concrete strength increased for CFST-SP and CFST-CP and increased slightly for CFST-CC compared to the concrete in the compression bending zone for the ordinary CFST, but for CFST-SC, the concrete strength was lower than that of the ordinary CFST. Additionally, for the stress-strain relationship of the steel tube, the service loading stage started with CFST- CP having the highest hoop stress and CFST-SP having zero hoop stress, but the steel tube of CFST-SC showed significant ductility in the compression bending zone. Finally, for the contact stresses between the steel tubes and the concrete, the contact stresses increased with the increasing load for all columns from the service loading stage, with only the contact stress curves for CFST-SP and CFST-CP dropping abruptly and then rising again. Furthermore, despite the different initial preloads, the later stages of all the contact stress curves (after the peak load) overlapped with and developed gently with the normal CFST curve. (2) Different influencing parameters such as the concrete strength, steel tube strength, and thickness had a significant effect on the compressive load capacity of the preloaded CFST columns. For CFST-SP and CFST-CP, the compressive load capacity was always higher than that of the ordinary CFST, the peak load ratio is always within the range of 1.0-1.2. Moreover, no matter how the influencing parameters change, the compressive load capacity of CFST-CP was slightly higher than that of CFST-SP. For CFST-CC, when the eccentricity increased, the peak load ratio increased significantly, even beyond 2.0. The rate of change of the peak load ratio was approximately the same when the concrete strength increased, but for the steel tube strength and thickness, the rate of change decreased with the increasing grade. For CFST-SC, again, the peak load ratio increased with the increasing eccentricity, regardless of the grade of the influencing parameters, and the peak load ratio was greater than 1.0 when the eccentricity was greater than 0.5. The trend in the peak load ratios was approximately the same as the influence parameter grade increased. Finally, it was also found that although the compressive load capacity of both CFST-CC and CFST-SC is higher than that of the ordinary CFST, the actual maximal service load was always lower than that of the ordinary CFST due to the presence of the preload. (3) The effects of the concrete strength, steel tube strength, and thickness on the _M-P_ envelope curves of the specimens were also prominent. Regardless of the influencing parameters, the _M-P_ curves for CFST-SP and CFST-CP were always significantly more convex than those of the ordinary CFST, and the _M-P_ curve of CFST-SC was less convex than that of the ordinary CFST, but the difference between the curves for CFST-CC and the ordinary CFST was not significant.