# Novel Method for Bridge Structural Full-Field Displacement Monitoring and Damage Identification
## 2 Full-Field Displacement Monitoring of a Structure Based on Natural Texture Feature Monitoring
### Full-Field Displacement Monitoring Test of the Structure  
Figure 4 shows the size and structure of the beam used in the current test. Figure 5 shows a photograph of the test beam.  
Figure 3: Mathematical model of the relative position change of the feature points.  
The appropriate monitoring camera was selected by analyzing the measurement accuracy, field angle, focal length, and other aspects. Finally, the Fuji GFX 100 ordinary civilian camera and the Fuji GF 32-64/4 RLM WR lens were selected.  
The camera was placed 5 m from the center of the test beam during the test, as shown in Figure 6. Dial indicators were used to measure the structural deformation to provide validation data. Figure 7 shows the layout of the dial indicators; as shown, 13 dial indicators were arranged uniformly in the lower section of the test beam.  
To validate the FFDV extracted from the structure, a Leica scan station (P50 high-speed 3D laser scanner) was used to scan the structure. The scanning resolution was 0.8 mm/10 m,  
Figure 4: Illustration of the test beam with dimensions (units: mm).  
Figure 5: Photograph of the test beam.  
Figure 6: Position of measuring camera.  
Figure 7: Layout of dial indicators.  
the target acquisition accuracy was 2 mm/50 m, and the noise accuracy was 0.4 mm/10 m. Code marks with a width of 20 mm on the upper and lower gusset plates of the test beam were arranged to validate the accuracy. Figure 8 shows the location and number of the code marks. Figure 9 shows the scanned scene.  
Subsequently, the damaged member of the test beam was simulated by cutting. Table 1 lists the damage conditions for the test beams. Figure 10 shows the number and positions of the damaged members, whereas Figure 11 shows the damage to the members of the test beam.  
The loading point of the test beam was located at the middle of the span, as shown in Figure 12. The load was varied from 0 to 400 kN in 100 kN increments. Each load level was sustained for 2 min to ensure that the test beam was fully deformed, the test beam images were captured, the dial indicator data were obtained, and the 3D laser scanning of the test beam was performed. Figure 13 shows the loading system of the test beam. In the load-holding stage, images were captured every 30 s, dial indicator data were obtained every 30 s, and 3D laser scanning was conducted every 5 min.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Damage Condition** & **Member Type** & **Member No.** & **No. of Damaged Members** & **Damage Order** \\ \hline D01 & - & - & 0 & - \\ \hline D02 & Vertical member & 13–14 & 1 & 1 \\ \hline D03 &
\begin{tabular}{c} Vertical member \\ Inclined member \\ \end{tabular} & 13–14 and 12–13 & 2 & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Damage conditions.  
Figure 8: Layout diagram of test beam’s code marks.  
Figure 10: Location of damaged members.  
Figure 9: Three-dimensional laser scanning validation test.  
In the test, strong features on the chessboard were used as fixed points. Theoretically, any feature point on the chessboard can be used as a fixed point \(O\). In this study, the feature point at the center of the chessboard was selected as a fixed point. The layout of the chessboard is illustrated in Figure 14. The position of the chessboard was fixed throughout the test.  
The SIFT feature-point extraction method was used to extract the feature points of the test beam under various operating conditions. Figure 15 shows the feature-point extraction results for the test beam. The feature-extraction results for the D01 operating condition were simplified, unnecessary environmental feature points were deleted, and the main features of the structure were highlighted, as shown in Figure 16.  
Figure 11: Photographs of damaged test beam and member.  
Figure 14: Layout position of chessboard used in current study.  
Figure 12: Diagram of test beam’s loading point.  
Figure 13: Loading details of static load test.