## 3 Overview of Pseudo-Dynamic Testing and Result Analysis
### Experimental Results and Analysis
#### 3.4.1 Crack and Failure Morphology  
(1) PD-FR (with no reinforcement applied)  
In PD-FR with no reinforcement applied, fine initial flexural cracks started to occur at 2.34 s (displacement: 18 mm), when the input seismic ground motion was 200 cm/s\({}^{2}\). After 2.95 s (displacement: 45 mm), initial flexural cracks increased, and shear cracks started to occur at the lower part of the columns. After 3.17 s, these cracks at the lower part of the column started to significantly increase in width. After 3.3 s, severe concrete delamination started to occur, and at around 3.4 s, where the maximum seismic response occurred, a shear collapse was observed at the lower part of the test frame, as shown in Figure 10.  
This result was consistent with a previous study [38], which reported that school buildings with non-seismic details may be subject to large-scale seismic damage when an earthquake with a magnitude of 200 cm/s\({}^{2}\) occurred. This data is considered important evidence to demonstrate the necessity to apply seismic retrofitting to school buildings with non-seismic details of the 1980s.  
(2) PD-CFT-MF (retrofitted with CFT-MF system)  
PD-CFT-MF was the test frame that had been retrofitted with the CFT-MF system proposed in this present study. As shown in Figure 11, when the input seismic ground motion was set to 200 cm/s\({}^{2}\), small initial flexural cracks started to occur at the lower part of the columns at around 2.25 s (displacement: 5.9 mm), and after 3.1 s (displacement: 10 mm), the number of these fine initial flexural cracks increased, but the crack size was still small. After 3.59 s (displacement: 11.1 mm), where the maximum seismic response occurred, the number of these flexural cracks continued to increase, but the crack width still remained narrow.  
When the input seismic ground motion was set to 300 cm/s\({}^{2}\), shear cracks started to occur at 3.6 s, where the maximum seismic response occurred. After then, however, these cracks increased in width, but only to a limited extent. Even after the test was complete, only small shear cracks were observed at the lower part of the columns, as shown in Figure 12, in contrast to the PD-FR specimen with no reinforcement applied, which underwent shear collapse, as shown in Figure 10.  
When the input seismic ground motion was set to 400 cm/s\({}^{2}\), the magnitude of a large-scale earthquake defined in accordance with KDS 41 [36], the test specimen retrofitted with the CFT-MF system, as shown in Figure 13, exhibited more severe flexural cracks and shear cracks with a larger width at 3.63 s (displacement: 40.61 mm), where the maximum seismic response occurred, compared to when the input seismic ground motion was 300 cm/s\({}^{2}\). However, the crack size was limited to 1.0 mm or less. In short, PD-CFT, the test frame modeled from RC school buildings with non-seismic details but retrofitted with the CFT-MF seismic retrofitting method, was subject to medium-sized cracks at a seismic ground motion of 400 cm/s\({}^{2}\).  
Figure 10: Cracks and ultimate failure of test frame with no reinforcement applied (PD-FR) (200 cm/s\({}^{2}\)): (**a**) shear collapse at the final stage and (**b**) enlarged shear collapse.  
Figure 11: Cracks and ultimate state of test frame retrofitted with CFT-MF system (PD-CFT) (200 cm/s\({}^{2}\)): (**a**) flexural cracks at around 2.25 s and (**b**) flexural cracks at around 3.1 s.  
Figure 12: Cracks and ultimate state of test frame retrofitted with CFT-MF system (PD-CFT) (300 cm/s\({}^{2}\)): (**a**) small shear cracks of the right column and (**b**) small shear cracks of the left column at the final stage.