## 1 Introduction  
With the rapid development of high-rise buildings, concrete-filled steel tube (CFST) columns have received extensive attention and investigation due to their high strength and performance. Concrete-filled steel tube (CFST) columns have higher bearing capacity and ductility than normal steel reinforced concrete (RC) columns [1, 2]. However, their bearing capacity degrades rapidly after fire exposure, so a series of research studies have been performed on the fire performance of CFST column structures [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. [PERSON] et al. [5] analyzed the post-fire bond between the steel tube and concrete in 37 CFST columns. [PERSON] et al. [6] carried out 16 fire tests of high strength CFST columns. [PERSON] et al. [7] reported a numerical study on the temperature distribution of high-strength concrete-filled steel tube subjected to a fire. Generally, it was found that the outer steel tube is easily damaged, especially when subjected to high temperature without protection [4, 8]. [PERSON] et al. [9, 10, 11] investigated the post-fire performance of concrete-filled circular hollow section columns and concrete-filled elliptical hollow section columns. It was found that the post-fire performance of circular columns was better than that of square columns, while the eccentricity of columns had a significant effect on their fire performance. [PERSON] et al. [12] investigated the fire performance of high strength self-consolidating concrete-filled steel tubular stub columns. [PERSON] et al. [13] and [PERSON] et al. [15] revealed the behavior of CFST joints subjected to standard fire loading. [PERSON] and [PERSON] [17] and [PERSON] et al. [18] observed the fire resistance of CFST frames. A number of research studies have been performed on the fire resistance of columns under non-uniform fire [19]. To sum up, previous studies revealed that the residual strength of CFST columns after fire exposure is less than that of columns at ambient temperature because the outer concrete is brittle and easily damaged. To improve the mechanical properties of CFST columns, a new type of composite of CFST columns wrapped in reinforced concrete (RC) was proposed, which are also called concrete-encased CFST composite columns or steel tube-reinforced concrete composite columns [14; 16], as shown in Figure 1. The outer RC wraps the inner CFST column, so its fire resistance is better than that of conventional CFST columns. The outer RC component improves the durability of concrete-encased CFST columns. Additionally, the outward buckling of the inner steel tube can be limited due to the existence of outer RC. Generally, it was found that concrete-encased CFST composite columns have a higher bearing capacity and ductility than conventional CFST columns at ambient temperature [20; 21; 22; 23; 24; 25; 26; 27]. Due to their excellent mechanical properties, concrete-encased CFST composite columns have been widely used in high-rise and super high-rise buildings. However, research studies on the fire and post-fire mechanical performance of concrete-encased CFST composite column structures are relatively few. [PERSON] et al. [28] carried out experimental research on the fire resistance of concrete-encased CFST composite columns subjected to ISO-834 standard fire, which mainly evaluated the temperature distribution, fire resistance, displacement versus time relationship, and failure mechanism of the composite columns. [PERSON] et al. and [PERSON] et al. [29; 30] conducted numerical analysis investigations on concrete-encased CFST composite columns subjected to 3-side fire and ISO-834 standard fire, respectively. The influences of important parameters, such as fire duration time, sectional dimension, slenderness ratio, and sectional core area ratio, on the temperature field of the composite columns were investigated.