# Behavior of Circular Hollow Steel-Reinforced Concrete Columns under Axial Compression
## 1 Introduction  
The circular hollow steel-reinforced concrete (HSRC) column is a new type of composite concrete-steel member, consisting of outer circular hollow reinforced concrete (RC) and an inner circular hollow steel tube, as illustrated in Figure 0(a). These composite members have been extensively used in high-rise buildings and bridge structures [1]. Circular HSRC columns exhibit superior stiffness and ductility compared to concrete-filled steel tubular (CFST) columns thanks to the outer RC. Additionally, they possess lower weight than concrete-encased CFST columns, attributed to the inner hollow steel tube [2]. Moreover, the constraints offered by the outer RC effectively mitigates the occurrence of external buckling in the steel tube (Figure 0(b)). As depicted in Figure 2, the construction process of circular HSRC columns bears resemblance to that of the concrete-encased CFST columns. Previous literature primarily concentrates on the restraint and contact between the steel tube and concrete in hollow steel-reinforced concrete (HSRC) members. [PERSON] et al. [1] examined the mechanical behavior of square, octagonal, and box-HSRC columns under axial and eccentric compression. However, they did not consider the restraining effect of the steel tube on the concrete due to the shape of the internal steel tube's cross-section. Likewise, [PERSON] et al. [3] stated that the embedded hollow steel tube not only restricted the peripheral hollow RC section of HSRC members during axial compression, but also enhanced their ductility and prevented brittle failure. In a separate study, [PERSON] et al. [4]investigated the restraining effect of the steel tube on the concrete in circular HSRC members using FE analysis. Additionally, [PERSON] et al. [5] performed triaxial constraint tests on 14 fan-shaped circular HSRC specimens, providing experimental and theoretical evidence that the embedded hollow steel tube section exerted an internal triaxial constraint on the hollow RC section. Furthermore, [PERSON] et al. [6] investigated the behavior of circular HSRC columns with a glass-fiber-reinforced polymer (GFRP) reinforcement section under axial compression. They developed a restraint model that accounts for the interaction between longitudinal and transverse reinforcements during transverse restraint. [PERSON] et al. [7] performed eccentric compression tests on eight square HSRC columns and determined that the primary parameter influencing the stress at the interface between the inner steel tube and outer RC was \(e/B\). Various calculation methods are available for determining the bearing capacity of composite columns under axial compression, including the Australian standard AS 5100.6-2004 [8], the American Institute of Steel Construction code ANSI/AASC [9], the Chinese code DBJ/T13-51-2010 [10], the Architectural Institute of Japan (AIJ) design method [11], and Eurocode 4 [12]. The findings indicate that the above methods tend to be conservative when calculating the bearing capacity of short and slender columns under axial compression. Consequently, some studies have proposed new calculation methods, primarily categorized as the stress-equilibrium method ([2,13,14]) and the nominal slenderness ratio method ([2,11,13,14]). The stress-equilibrium method is employed to calculate the axial compressive  
Figure 1: Circular hollow steel-reinforced concrete columns. Figure 2: The construction process of circular HSRC column. bearing capacity of hollow composite members, steel-reinforced concrete members, and box-HSRC members. The AIJ design method [11] incorporates a formula for calculating the axial compressive strength using the nominal slenderness ratio method. There are few reports available on the behavior of circular HSRC columns under axial compression. This study conducted a test investigation of circular HSRC columns subjected to axial compression and established a validated finite element (FE) model based on the software platform ABAQUS (version 6.14).