# Behavior of an Internally Confined Hollow Reinforced Concrete Column with a Polygonal Cross-Section
## 1 Introduction  
Recently, the trend of enlargement of structures, such as skyscrapers, pier for long-span bridges, and wind turbine towers, has been growing. According to [PERSON] [1], the number of skyscrapers over 200 meters high has been increasing (Figure 1). The increase in building height is accompanied by an increase in building weight and moment due to lateral force. The performance of a building's structural system is determined by resistance to lateral force, which increases with the building height. Various studies have been conducted on the column structure for improving the building performance. [PERSON] et al. [2] proposed a concrete-filled steel tube (CFST) and evaluated its seismic performance by comparing the static and dynamic behavior with a steel column. The growth of the lateral force with an increase in building height is closely related to the outline of the building. [PERSON] et al. [3] conducted a study related to wind load at the building edge according to the shape of the buildings. [PERSON] et al. [4] conducted a study about thin-walled CFST columns to enhance seismic collapse performance of high-rise steel frames.  
In the case of bridges, both the span and height have been increased, and several studies have been conducted on piers and girder-supported bridges [5, 6]. In the case of a wind tower, the installation height of the turbine has been increased to obtain high-quality wind, which increases the power generation efficiency; hence, the slenderness ratio increases due to an increase in the tower height to support the wind turbine [7]. This increase in slenderness can cause problems due to the reduced safety of the wind tower. According to [PERSON] et al., 48 tower accidents collapsed by various causes during the 16 years since 2000 [8]. A wind turbine tower also broke by the buckling failure in 2019 [9, 10].  
[PERSON] et al. [11] proposed a new column-type internally confined hollow reinforced concrete (ICH RC) structure to improve the safety and performance of these large structures. In the case of RC columns that can reduce the column weight and save material usage, it is possible to use the cross-section more effectively than the solid reinforced concrete column. However, it might exhibit low ductility behavior due to the brittle fracture inside the column. The ICH RC structure is a structure in which a steel pipe is inserted into the hollow surface to prevent the brittle fracture of the inner wall of the hollow RC member to make the concrete into a three-axis confinement state (Figure 2). The ICH RC structure was found to be more effective than conventional RC columns in terms of weight and strength [11]. In addition, the ICH RC columns showed better resistance to strength and bending moments when compared to steel columns of the same diameter [12]. The ICH RC structure can be applied to the supporting structures of skyscrapers, super long-span bridges, and new types of wind towers, and it can contribute to safety improvement of the structure. To date, the nonlinear behavior analysis of a circular cross-section ICH RC structure has been extensively researched and applied to a wind tower, but ICH RC columns having polygonal cross-sections have not been researched [13].  
In this study, an ICH RC member with polygonal cross-section is introduced, and a nonlinear concrete model, as well as a column model considering the confining effect of concrete, is developed for accurate analysis of ICH RC columns having a polygonal cross-section. The developed model can derive an accurate analytical value of the ICH RC structure when it applies to supporting structures such as super long-span bridges, high-rise buildings, and wave dissipating blocks.  
Figure 1: Average height of the tallest buildings.  
Figure 2: ICH RC structure: (a) circular cross-section ICH RC; (b) polygonal cross-section ICH RC.