## 2 Overview of the CFT Composite Strengthening System  
The reinforcing bar/chemical anchor method is widely used for the internal bonding of the existing framework and the seismic strengthening frame. The ductility of concrete at joints is insufficient, and cracking in concrete is inevitable in the event of an earthquake. Since seismic load transmission to seismic strengthening devices is blocked by cracking, it is difficult to expect effective seismic strengthening effects through the integrated behavior of the existing structure and seismic strengthening devices [3; 4]. The existing CFT reinforcement method consists of a steel tube by integrating the C-channel into the existing R/C framework through a chemical anchor method and welding another C-channel. In this case, a welding process is added, constructability is reduced, and if the surface of the existing R/C frame is not flat, it is difficult to secure joint integrity between the two C-channels. The proposed CCSS supplements and addresses the shortcomings of the existing chemical anchor method, i.e., the problem of securing joint integrity. It increases strength by directly bonding the reinforcement in the form of a frame to the existing R/C structure, as shown in Figure 3. It innovatively improves the constructability and integrity of the joints between the existing R/C framework and reinforcement. CCSS consists of a reinforcement part, a joint, and a filling part. The reinforcement part is composed of the existing R/C structure and CFT frame, and the joint is composed of chemical anchors and high-performance epoxy. The filling part is composed of non-shrink mortar. Figure 4 shows the connection details of the CCSS method. The CFT frame is bonded to the existing R/C structure (column and beam) using chemical anchors and high-performance epoxy, and non-shrink mortar is injected to integrate the seismic reinforcement with the existing R/C structure. The construction procedure shown in Table 1 is as follows. Figure 3: Detailed diagram of the CCSS strengthening system. Figure 4: Details of the connection between the CCSS system and existing column. \begin{table}
\begin{tabular}{c c} \hline \hline
**Sequence** & **Construction Procedure** \\ \hline  
Figures 6 and 7 show the reinforcement details and photographs of the R/C framework of the target building and the specimen reinforced with CCSS. The pseudo-dynamic test was performed on two specimens, including an unreinforced specimen and a CCSS-reinforced specimen for comparison. For the input ground motions used in the pseudo-dynamic test, Hachinohe (EW) exhibited the highest seismic response ductility ratio and was selected based on the research results of [PERSON] [16] to determine the seismic response ductility ratios of the 10 historical seismic waves selected using R/C buildings with less than six stories. The seismic acceleration was normalized to 200, 300, and 400 cm/s\({}^{2}\) for the Hachinohe (EW) seismic waves, and 200 cm/s\({}^{2}\) (design basis earthquake, DBE) and 300 cm/s\({}^{2}\) (maximum considered earthquake, MCE) are the accelerations of two-thirds of the 2400-year return period earthquake and the 2400-year return period earthquake corresponding to seismic zone-1 and ground type S4 in KDS 41 [15], respectively. Additionally, 400 cm/s\({}^{2}\) was selected to evaluate the seismic performance of CCSS in the event of a large earthquake. For the axial force, the axial load applied to the two actual columns (framework), i.e., 1000 kN, was divided, and an axial load of 500 kN was applied to each column.