## 1 Introduction  
In the construction of concrete buildings without seismic details, the seismic load is not taken into account. In particular, concrete moment frames without seismic details pose serious risks for structural damage and collapse due to a lack of strength, stiffness, and ductility. To prevent damage caused by earthquakes, the techniques for the seismic rehabilitation of existing buildings by the federal emergency management agency (FEMA 547) [1] presented guidelines to improve the seismic performance. Ensuring sufficient strength and ductility in concrete moment frames is emphasized in FEMA 547. FEMA 547 [1] also proposes methods such as adding new structural elements (e.g., a concrete or masonry shear wall and steel braced frame) and increasing the cross-sectional size of the columns or beams in order to improve the seismic performance of concrete moment frames. However, the method of adding new elements can lead to the space inefficiency or retrofitting costs becoming uneconomical [2]. Furthermore, if the newly added elements and existing members do not behave in an identical manner, the structure may not resist an increasing seismic load, and brittle fractures may occur [3]. The method of increasing the cross-sectional size has disadvantages, such as reducing the space of a building and requiring foundation reinforcement as a result of an increase in the building weight. Therefore, a number of studies have been conducted on seismic retrofitting methodsthat can mitigate the disadvantages of the existing seismic retrofitting approaches. The research has focused on seismic retrofitting methods using fiber-reinforced polymer (FRP) [2; 4; 5; 6; 7; 8], steel beams [3; 9], and dampers [10; 11; 12; 13; 14]. Although new seismic retrofitting methods have been studied to improve the existing retrofitting methods, there are some drawbacks; the drawbacks of the seismic retrofitting method using FRP are the relatively high cost of epoxies, inability to attach on wet surfaces, difficulty conducting post-earthquake assessment of the damage suffered by the reinforced concrete that is behind the undamaged FRP jackets, and so on [4]. There are some disadvantages with the seismic reinforcement method of absorbing the seismic energy using dampers, namely, aesthetic problems, maintenance difficulties, and highly nonlinear behavior [10]. In particular, in the case of retrofitting with FRP, steel beams, and dampers, because of the characteristics of these methods, construction is only possible when the members requiring retrofitting are exposed to the outside. Therefore, if a member requiring retrofitting is not exposed to the outside, an additional demolition process is required, which may increase the construction period and cost.  
This paper proposes a seismic retrofitting method using welded concrete-filled steel tubes (WCFSTs) that improve both the strength and energy dissipation. The WCFST system functions by attaching the CFST frame to only the part of the structural member exposed to the outside environment. A base plate is installed to the existing reinforced concrete frame, and integration is ensured by welding the CFST frame to the base plate. This technique has an advantage in that it is possible to utilize the space during construction and it is easy to check the status of the existing members after an earthquake. To evaluate the performance of the WCFST seismic-resistant system, both a non-seismically designed reinforced concrete frame specimen and a specimen retrofitted with the WCFST system were subjected to cyclic lateral loading tests.