## 4 Results of Cyclic Loading Test and Discussions
### Load-Displacement Curve and Strain Behavior  
Figure 10 illustrates the hysteresis loops, and Figure 11 shows envelope curves of each specimen under cyclic loading. In the ERC specimen, the applied lateral load reached a maximum load of 322 kN, and in the negative direction, the load reached 314.2 kN at a drift ratio of 2%. After that, the load was reduced with progressing shear cracks and exhibited concrete spalling. In the first cycle of a 2.5% drift ratio in the positive direction, the maximum load was decreased to 311.4 kN and the experiment was terminated. In the initial drift ratio, the load-bearing capacity of the NoWS specimen was 1.7 times greater than that of the ERC specimen, and this gradually increased according to the drift ratio. At  
Figure 9: Location of gauges. the 2% drift ratio, when the experiment on the ERC specimen ended due to shear failure, the resistance of the NoWS specimen was 2.7 times greater than that of the ERC specimen. In the NoWS specimens, the experiment was terminated at a 2.5% drift ratio, and the load was 924.32 kN in the positive direction and 853.61 kN in the negative direction. This confirmed that the proposed seismic retrofit method effectively enhanced the strength of the existing RC structure. According to results from previous research, the load-bearing capacities of the specimen using welding were 1.9 times greater initially and 2.8 times greater when the experiment terminated than those of the existing RC structure [24]. It was demonstrated that there was no difference in structural performance between welding and bolting connections. Table 4 summarizes the experimental results. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Drift Ratio (\%)** & \multicolumn{2}{c}{**Specimens**} \\ \cline{2-3}  & **ERC** & **NoWS** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cracks which appeared in the specimen according to the drift ratio. Figure 11: Envelope curves. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multicolumn{2}{c}{**Crack Propagation**} & \multirow{2}{*}{**Observed Damage**} \\ \cline{2-2} \cline{4-4}  & ERC & NoWS & \\ \hline Lower & & & -Severe spelling of the concrete and \\ beam–column joint & & & bending of the lateral reinforcement in \\  & & & the ERC; \\  & & & -No buckling deformation or separation \\  & & & of the CFST frames occurred, and no bolt \\  & & & loosening in the NoWS. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Crack propagation. Figure 10: Load–displacement curves: (**a**) ERC; (**b**) NoWS. The strain at the end of each column measured by the cyclic loading tests is presented in Figure 12. In Figure 12, the horizontal lines show the ultimate compressive strain or yield strain. When initial cracks were formed at the end of the column, the concrete strain reached the ultimate compressive strain of 0.003. However, the NoWS specimen did not exceed the ultimate compressive strain, because the CFST frame absorbed seismic energy. The longitudinal steel and hoop in the columns of both specimens yielded drift ratios of 1.5% and 2%, respectively. In the NoWS specimen, the strain increased gradually after the yield of the rebar, but in the ERC specimen, the strain of the rebar and hoop increased rapidly in the third cycle of the 2% drift ratio. In the ERC specimen, deformation in the rebar and hoop was significant progress, because the concrete exceeded the ultimate compressive strain. The strain of the CFST was less than 10% of the yield strain of 0.001375, which was considered to cause little deformation of the CFST.