## 5 Conclusions  
The present study proposes a new seismic retrofitting method using a concrete-filled tube modular frame (CFT-MF) system, which is a novel technique to overcome and improve the limitations of existing seismic strengthening methods. A full-scale two-story test structure modeled from existing domestic RC structures with non-seismic details was subjected to pseudo-dynamic testing. As a result, the effect of the CFT-MF system, when applied to existing RC structures, was examined and verified, especially as to its seismic retrofitting performance, i.e., restoring force characteristics, and seismic response control. In addition, based on the pseudo-dynamic testing results, a restoring force characteristics model was proposed to implement non-linear dynamic analysis of a structure retrofitted \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{3}{*}{**Displacement Point**} & \multicolumn{3}{c}{**Lateral Response Strength (kN)**} & \multirow{3}{*}{\begin{tabular}{c} **Contribution** \\ **Ratio of vs. \({}^{*}\)** \\ **(\%)** \\ \end{tabular} } \\ \cline{2-4}  & \begin{tabular}{c} **ROC Frame Strengthened** \\ **with CFT-MF System** \\ **[Vr]** \\ \end{tabular} & \
\begin{tabular}{c} **CFT-MF** \\ **System** \\ **[Vs]** \\ \end{tabular} &
\begin{tabular}{c} **Non-Reinforced** \\ **Frame** \\ **[Vs]** \\ \end{tabular} \\ \hline
10 & 486.8 & 350.7 & 136.1 & 72.0 \\ \hline
20 & 602.9 & 435.8 & 167.1 & 72.3 \\ \hline
30 & 679.6 & 499.6 & 180.1 & 73.5 \\ \hline \hline \end{tabular}  
* The contribution ratio refers to the ratio of the response strength of the CFT-MF system (V\({}_{\text{S}}\)) to the total response strength of the RC frame retrofitted with the CFT-MF system [V\({}_{\text{T}}\)].  
\end{table} 
Table 8: Contribution of each member’s load to seismic response displacement determined based on Figure 25.  
with the CFT-MF system (i.e., the test frame). Finally, based on the proposed restoring force characteristics, non-linear dynamic analysis was conducted, and the results were compared with those obtained by the pseudo-dynamic tests. The major findings of the present study are as follows. 1. The pseudo-dynamic test results of the test frame with no reinforcement applied showed that the maximum seismic response displacement was 58.7 mm (strength: 250.5 kN) at around 3.4 s when the Hachinohe wave (EW) of 200 cm/s\({}^{2}\) was applied. This also coincided with shear failure. These results indicated that the test structure was prone to shear failure and exhibited low ultimate strength, structural characteristics commonly found in domestic RC structures with non-seismic details of the 1980s. It was also found that such structures with non-seismic details may be subject to large-scale seismic damage when an earthquake with a magnitude of 200 cm/s\({}^{2}\) occurs. These data are considered important evidence to demonstrate the necessity of applying seismic retrofitting to such buildings. 2. In the test frame retrofitted with the CFT-MF system, the maximum seismic response displacement was 11.0 mm (strength: 432.2 kN) when the Hachinohe wave (EW) of 200 cm/s\({}^{2}\) was applied. Only fine flexural cracks occurred, and the degree of overall structural damage was estimated to be insignificant. At 300 cm/s\({}^{2}\), the maximum seismic response displacement was 22.4 mm (strength: 613.3 kN). The initial flexural cracks increased in number, and small shear cracks occurred compared to when the input seismic acceleration was 200 cm/s\({}^{2}\). The degree of seismic damage was evaluated to be small. When the input seismic acceleration was 400 cm/s\({}^{2}\), i.e., when a large-scale earthquake was assumed, the maximum seismic response displacement was 39.5 mm (strength: 773.9 kN). Flexural and shear cracks increased in width, and the degree of overall structural damage was estimated to be moderate. 3.