## 2 Overview of CFT-MF Seismic Retrofitting Method  
Figure 2 illustrates a widely used method for connecting existing frames with steel frames, which are composed of H-beams, for seismic retrofitting. Anchor bolt holes are installed in the existing structure using a hammer drill through the anchor holes on the flange of H-beams spaced at a constant distance from one another. This way, steel frames can be installed in the existing structure. This method, however, does not allow an anchor bolt to be installed in the center of the steel frame, where its web is located. Also, the presences of the flange and web interfere with the operation of hammer drills, i.e., there is less workspace. Therefore, workers have difficulty installing anchor holes in the vertical or horizontal direction. This may lead to construction defects, causing cracks to occur in concrete parts with low ductility. Reinforcement is then less likely to achieve the desired seismic retrofitting performance achieved by integrated behavior of the seismic reinforcement materials with the existing structure. Figure 3 illustrates a seismic retrofitting method using conventional CFT columns. Compared to the method using steel frames, this method can be more efficient in improving the strength and ductility performance of existing structures, but it requires welding professionals to weld the CFT column with its connection parts so as to ensure precision construction. Figure 4 illustrates the CFT-MF seismic retrofitting method proposed in the present study. This method was designed to overcome and improve the limitations of the steel frame connection method and the CFT column connection method described above; this method allows for sufficient workspace, ensures integration of the reinforcement connection, and improves constructability. The CFT-MF seismic retrofitting method is implemented by the following three systems: the reinforcement system, the connection system, and the filler system. The reinforcement system is composed of (a) existing structure, (b) cover plate, (c) CFT frame, and (d) connection frame, while the connection system is composed of (e) coupling bolt, and (f) chemical anchor. The filler system is composed of (g) high-performance mortar. Figure 2: Conventional H-beam steel frame connection method. Figure 5 presents the connection details of the proposed method. This method mainly involves two steps to integrate the existing structure with the reinforcement materials: connecting (a) the existing structure (RC columns and beams) with (E) the CFT connection frame using (H) chemical anchors and (G) epoxy resin, and then further connecting this assembly with (d) the CFT frame and (c) the cover plate using (b) coupling bolts. The detailed construction procedures, as shown in Table 1, are as follows:  
1. [label=(0)]
2. Bore anchor bolt holes in the existing structure (columns and beams). 3. Install the connection device (steel plate) using anchor bolts. 4. Install the CFT frame for seismic retrofitting. 5. Install the cover plate (steel plate) using coupling bolts. 6. Inject high-performance mortar and conduct finish work. Figure 4: Detailed illustration of CFT-MF strengthening method. Figure 3: Conventional CFT column connection method. The CFT-MF seismic retrofitting method proposed in this study is a novel technique to overcome and improve the limitations of existing seismic strengthening methods, as presented in Figures 2 and 3. The CFT-MF system makes the most of the advantages of both concrete and steel pipes, resulting in the improvement of the overall constructability and the increase of integration between the existing frame and the reinforcement joints.