# Evaluation of Eccentric Compressive Behavior Between Active and Passive Preloading in CFST
## 1 Introduction
### Passive Preload  
During the construction of high-rise or super high-rise buildings, it is necessary to install several layers of the structure before pumping concrete into steel tubes. Hence, steel tubes are subjected to initial gravity, which is called the preload (Figure 0(a)). In the construction of a CFST bridge, it is also occasionally the case that the concrete core at the end of a CFST pier is slightly above the edge of the steel tube due to field operation defects; so, the gravity of the upper beam acts on the concrete core before service loading, resulting in initial stress in the concrete (Figure 0(b)). Similarly, there is also a situation that is dueto the shrinkage of the concrete after casting to form a tiny gap at the end of the CFST columns so that the preload is only applied to the steel tube (Figure 1c). Generally, before the service loading of members, CFSTs may be subjected to these preloads as self-weight, wet concrete weight, member weight, and temporary construction weight (Figure 1d), and this will generate pre-stress in the CFSTs. Several researchers have focused on the influences of these undesired preloads during construction on the behavior of CFST structures. [PERSON] et al. [12] conducted comparative tests and finite element calculations on CFST columns under axial compression with and without the preloading of steel tubes. It was reported that the existence of a steel preload reduced the bearing capacity of the compressed column. As the preload increased, the stability reduction coefficient (\(\varphi\)) increased. Ultimately, the column lost its stability. [PERSON] et al. [13] found that the additional deformation produced by a preload greatly affected the axial compressive capacity of intermediate and slender CFST columns because of the local buckling of steel tubes. The reduction of the axial compressive capacity was greater than 20% when the preload level (\(\beta\)a) was high (\(\beta\)a > 0.6). Therefore, [PERSON] et al. [14] proposed the reduction of the preloading ratio during construction to minimize the effects of preloads on CFSTs. [PERSON] and [PERSON] [15] also conducted a series of experiments to compare the performances of CFST columns subjected to different axial preloads (Figure 2). It was found that the initial preload had the greatest effect on the SFS column under axial compression. [PERSON] et al. [16] examined the influences of different preload levels on the mechanical properties of CFST columns during eccentric compression utilizing the finite element method. The effects of the eccentricity and the slenderness ratio were taken into consideration. It was reported that the compressive ultimate capacity decreased and the deformation increased significantly due to the high preload level. Moreover, the larger  
Figure 1: CFST columns during construction: (**a**) Concrete-filled steel tubes during building construction [10]; (**b**) Preloading on the concrete core; (**c**) Preloading on the steel tube; (**d**) Preloading on the side of the steel tube [11]. eccentricity and slenderness ratio further intensified the preloading disadvantageous effect on the CFST columns. [PERSON] et al. [17] asserted that a high preload decreased the stability and weakened the stiffness of an eccentrically compressed CFST column. The steel tube entered the plastic state in advance during preloading, and the descending section of the load-deflection curve appeared earlier. Concrete-filled double steel tube (CFDST) columns are very similar to ordinary CFST columns. [PERSON] et al. [18] investigated the mechanical behavior of axially loaded CFDST columns with preload applied either to the outer steel tube or to both steel tubes. It is found that the strength of CFDST column may be decreased moderately when the preload was applied. Therefore, for the abovementioned studies on the compressive behavior of CFSTs during preloading, the following inferences can be summarized: 1. the preload during building construction is neither deliberately nor actively applied. 2.