## 1 Introduction  
Reinforced concrete (RC) frames without considering seismic designs are vulnerable due to their strength, stiffness, and ductility. Therefore, these factors are generally considered when retrofitting non-seismic designed structures. Jacketing is the most typical retrofitting method for RC structural members, and it involves either concrete jacketing, steel jacketing, attaching a fiber-reinforced polymer (FRP) or a method using concrete-filled steel tubes (CFS). In concrete jacketing, an additional cross-section is produced by placing the rebars and pouring concrete on the outer section of the existing columns [1; 2; 3]. The disadvantages of concrete jacketing are the integration between the existing members and the added jacket, the need for additional rebars to ensure total integration, and the need for drilling on beams. It also reduces the available space by increasing the size of the members, and increases the overall cost of construction due to the limited use of the structure during the construction period. Recently, a few studies have been conducted by [PERSON] et al. [4] and [PERSON] et al. [5] to increase the strength and ductility of new members and reduce the size of the jacket by using high-performance materials such as fiber. Steel jacketing is a method of welding steel plates or steel tubes to existing members and grouping the gap between the steel jacket and the RC members with mortar. Compared to concrete jacketing, steel jacketing can reduce the construction period by prefabrication, although it is difficult to work because of its heavy weight. In addition, the attached steel member can buckle. [PERSON] et al. [6] and [PERSON] et al. [7] proposed optimal steel jacketing shapes and retrofit lengths, whereas [PERSON] et al. [8] proposed a lightweight prestressed steel jacket (PSJ) with good retrofit effects. [PERSON] et al. [9] proposed a method to enhance the seismic performance and prevent buckling using post-compressed steel plates. The jacketing method using FRP has been one of the most popular seismic retrofit methods of RC members over the last 20 years, and relevant research is still ongoing [10]. In this method, FRP sheets or strips are attached to the concrete surface with an epoxy adhesive. The strength of FRP jacketing is high compared to its weight, and there is no need to increase the size of the concrete members. However, FRP is relatively costly and air-voids between the FRP and the concrete surfaces can occur. A number of studies have focused on the partial wrapping method to save construction costs and time and prevent air-voids [11; 12; 13; 14]. CFSTs are members filled with concrete inside steel channels to address the above-mentioned problems of existing jacketing. CFST members are known to have excellent seismic resistance capacity as well as structural benefits such as stiffness, resistance, and deformation performance, because the steel channel constrains the concrete inside [15; 16; 17]. Therefore, studies on the seismic retrofit method using CFST have been widely conducted, and techniques about enhancing the seismic performance have been studied, such as using carbon-fiber-reinforced polymer composites [18]. However, there are problems related to investigating the level of damage in concrete caused by accidental loads such as earthquakes. In addition, there are scant data available for the effectiveness of repair and retrofitting methods for damaged steel tubes which are jacketed in concrete [19]. A seismic retrofit method that can deal with many problems related to CFSTs was proposed. The proposed method is to assemble the CFST member on the outside of an existing RC frame. It does not require lots of rebar, or additional formwork. Additionally, it is assembled outside of the structure, the structure can be available for use during construction, and damage of existing RC frames can be investigated after earthquakes; this method can also be used to easily repair damaged CFST members.