## 3 Result Analysis
### Displacement  
The midpoint and quarter\(-\)point displacement-time history curves for each test beam are shown in Figure 12. While similar to that of the A1 test beam, the mid\(-\)span displacement-time history curve of the B1-B3 test beams has a different shifting pattern. The A1 test beam shows a clear trend of increased displacement after spring back, which is the main performance. The displacement-time history curve varies significantly across the quarter\(-\)point region. This is caused by the various ways in which displacement gauges are connected. All of the test beams were damaged to varying degrees when the hammerhead came into contact with the beam body because the stress wave was carried from the top of the beam to the support. It is mainly manifested as local damage in the compression area, the development of cracks in the beam body, and the overall downward acceleration of the test beam to produce flexural deformation. After the displacement of the test beams, B1\(-\)B3  
Figure 12: Displacement–time history curves of the specimen: (**a**) A1, (**b**) B1, (**c**) B2, and (**d**) B3. reached the peak value, the value of the test beam first decreased after the peak deflection due to the pasting of the steel plate, but then a second smaller wave peak occurred. This was because after the displacement reached the first peak, the bond between the steel plate and concrete led to the cohesive failure of the concrete at the mid\(-\)span and quarter\(-\)points at the bottom of the beam so that the displacement first decreased and then increased. The second peak gradually decreased as the thickness of the bonded steel plate increased or when the U\(-\)shaped steel plate hoops were installed. This was due to the fact that increasing the thickness of the steel plate or installing U\(-\)shaped steel plate hoops reduced the cohesiveness and peeling failure of the concrete at the bottom of the beam. Although the displacement decreased after reaching the peak value, the second wave peak was not formed because the A1 test beam was not strengthened by the steel plates. The test beam elastically bounced because of the restraint provided by the steel bars inside and the steel plate at the bottom of the beam. The test beam now moves upwards. When its upward velocity is zero, the displacement-time history curve hits the rebound value. Finally, the displacement gradually approaches the residual displacement value after a short period of fluctuation. According to Figure 12, the mid\(-\)span displacement response is faster, followed by the displacement response at the quarter\(-\)point section, and the mid\(-\)span displacement increases more rapidly. This is because the time for the stress wave to reach the mid\(-\)span at the bottom of the beam is relatively short, and the mid\(-\)span damage is more severe than the quarter\(-\)point damage. This shows how the displacement response of the test beam progresses from the mid\(-\)span to both sides. The peeling of the steel plate and the change in the connection method of the displacement meter are what cause the displacement at the quarter point. Since the movement of the steel plate, not the test beam, is reflected in the data, it cannot be interpreted logically. As the steel plate is pasted, the peak mid\(-\)span displacement and residual displacement decrease significantly, while the decreasing trend is not apparent as the thickness of the steel plate increases. Because the flexural deformation caused by the quality of the test beam itself is weakened after the bottom of the beam is strengthened with the bonded steel, the difference between the residual displacement and rebound displacement decreases significantly after the steel plate is attached to the beam. The peak displacement and residual displacement values decrease with the U\(-\)shaped steel plate hoop configuration; however, it is not immediately apparent. The rate of increase in the response speed of the start displacement is almost the same.