## 1 Introduction
So far, a range of methods for retrofitting the seismic performance of existing RC structures have been proposed: methods for seismic strengthening and improving deformation performance [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19], methods for improving energy absorption capacity using vibration dampers [20; 21; 22; 23], and the base isolation system [24]. According to [PERSON] and [PERSON] [25], most of the mid- and low-rise RC structures of six stories or fewer with non-seismic details are highly prone to shear failure because the spacing of transverse reinforcing bars in their columns is only about 30 cm. Unfortunately, this was actually demonstrated in the 2017 Pohang Earthquake, as shown in Figure 1. It was also reported that seismic retrofitting methods that focus solely on improving ductility capacity are less efficient, because the ultimate horizontal strength of such structures is much lower than desired. The researchers thus concluded that seismic strengthening is a more efficient retrofitting method to improve the seismic performance of those domestic mid- and low-rise RC structures with non-seismic details [26; 27]. Existing seismic retrofitting methods that focus on increasing strength mainly include methods for adding infill shear walls inside the frame; methods for installing K-shape or X-shape steel braces in the frame; methods for inserting steel plate panel walls into the frame; and internal connection methods, including cross-sectional extension methods. These existing seismic retrofitting methods based on internal connection can effectively improve the strength of structures against the horizontal force being applied [28; 29; 30]. However, existing seismic retrofitting methods involving strength increase add to the weight of structures. Given the weak foundations of RC structures with non-seismic details, the application of such methods may require foundation reinforcement work to support the weight increase. Furthermore, these methods do not allow for enough workspace during the retrofitting process. In particular, methods for installing walls with seismic design and using steel braces, among the most widely used retrofitting methods, require additional work to be done in the structure to facilitate the transport of reinforcement materials and ensure more workspace. This means that there will be less space available for the retrofitting work itself. This limited workspace then leads to a reduction in efficiency and potentially lengthens the construction period because more effort needs to be paid to ensuring integration between the existing frame and the internal reinforcement joints, as well as construction precision [30]. Therefore, it is necessary to develop a new, internal connection-based seismic retrofitting method by strength increase, that can overcome the limitations described above, best suit the seismic structural characteristics of mid- and low-rise RC structures with non-seismic details (prone to shear failure and low ultimate strength), and ensure integration between the existing frame and the seismic reinforcement materials. The present study proposes a new seismic retrofitting method using a concrete-filled tube modular frame (referred to as CFT-MF) system, which is a novel technique to overcome and improve the limitations of existing seismic strengthening methods. This CFT-MF seismic retrofitting method makes the most of the advantages of both concrete and steel pipes, thereby significantly improving constructability and increasing integration between the existing frame and the reinforcement joints. This method falls into the category of typical seismic retrofitting methods that focus on increasing strength, in which the required amount of seismic reinforcement can be easily estimated. Therefore, the method provides an easy solution to improving the strength of existing RC structures with non-seismic details that are prone to shear failure.