## 4 Pseudo-Dynamic Test Overview and Result Analysis
### Experiment Results and Analysis
#### 4.5.1 Crack and Failure Geometry  
(1) PD-RC  
Figure 17 shows the crack and failure situations of PD-RC after the input ground motion of 200 cm/s\({}^{2}\). According to the figure, initial bending cracks occurred at the bottom of the columns in approximately 2.08 s (displacement: 6.4 mm) for PD-RC. At 2.4 s (displacement: 16.9 mm), the bending cracks propagated, and shear cracks occurred at the top and bottom of the columns. After 3.45 s (displacement: 66.5 mm), the widths of the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimens** & **Test Methods** & **Strengthening Types** & **Earthquake Levels (cm/s\({}^{2}\))** \\ \hline PD-RC & Pseudo-dynamic & Non & 200 \\ \hline PD-LLFD-V & Pseudo-dynamic & LLFD-V typed seismic control system & 200/300/400/500 \\ \hline \multirow{3}{*}{Notation} & \multirow{3}{*}{PD} & RC & PD: Pseudo-dynamic test \\  & & LLFD-V & RC: R/C Frame without strengthening \\ \cline{1-1}  & & LLFD-V: R/C Frame strengthened using the LLFD V-typed seismic control system \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Summary of the specimen details. Figure 16: Detailed configuration of the specimens strengthened using the LLFD-V typed seismic control system (PD-LLFD-V): (**a**) specimen details; (**b**) photography. shear cracks at the bottom of the columns increased. The maximum displacement occurred at 5.35 s (displacement: 67.3 mm), and severe peeling of the concrete began, along with a serious increase in the widths of the shear cracks. Finally, shear failure occurred at the bottom of the frame on the first floor, resulting in the final collapse. These results are consistent with the results of a previous study [47] that found that significant seismic damage could occur to school buildings with non-seismic details in the event of an earthquake of 200 cm/s\({}^{2}\). These significant results highlight the need for seismic retrofitting of R/C school buildings constructed in the 1980s with non-seismic details. (2) PD-LLFD-V  
Figures 18-21 show the crack and final test situations of PD-LLFD-V after input ground motions of 200-500 cm/s\({}^{2}\). For the PD-LLFD-V specimen, fine initial bending cracks occurred at the bottom of the columns at approximately 2.28 s (displacement: 2.8 mm) under the input ground motion of 200 cm/s\({}^{2}\), as shown in Figure 18. After 3.07 s (displacement: 5.4 mm), the number of bending cracks increased, and fine cracks were observed. At 3.57 s (displacement: 9.6 mm), when the maximum response was observed, fine bending cracks also occurred. Consequently, the specimen retrofitted with the LLFD V-bracing system showed fine bending cracks under an earthquake of 200 cm/s\({}^{2}\), which caused the shear failure of the non-reinforced specimen, thereby confirming the retrofitting effect. Figure 17: Cracks and final failures of non-strengthened frame specimen (PD-FR) (200 cm/s\({}^{2}\)). Figure 19: Cracks of LLFD-strengthened frame specimen (300 cm/s\({}^{2}\)). Figure 20: Cracks of LLFD-strengthened frame specimen (400 cm/s\({}^{2}\)). As seen in Figure 19, with an input ground motion of 300 cm/s\({}^{2}\), the number of bending cracks after 2.29 s (displacement: 8.4 mm) was greater than that with 200 cm/s\({}^{2}\). After 3.59 s (displacement: 15.7 mm), when the maximum response was observed, bending cracks occurred on a small scale. Therefore, the failure mode was finally determined by bending cracks. The non-reinforced specimen shown in Figure 17 exhibited a typical shear failure frame. For the PD-LLFD-V specimens, however, the failure mode changed from shear failure to bending failure. As seen in Figure 20, with an input ground motion of 400 cm/s\({}^{2}\), a greater number of bending cracks were observed after 3.23 s (displacement: 21.6 mm) compared to those with 300 cm/s\({}^{2}\). Shear cracks also occurred at the same time, but they were insignificant. The width and number of bending cracks increased even after 3.61 s (displacement: 27.1 mm) when the maximum response occurred, but the shear cracks were insignificant.