## 3 Static Test of the Beam  
A static test has been carried out on a steel truss-concrete composite beam, to validate the proposed method, as shown in Figure 3.  
The specimen is simply supported by two hinge bearings at the both ends, as shown in Figure 3a. Two hydraulic jacks have been used to apply the two-points bending load on the specimen. Three dial meters have been placed at the quarter-span and the midspan of the specimen, to measure the structural deflection. The specimen has been loaded with a step-by-step prototype from 0 to 600 kN, with an increment of 100 kN. The loading protype is shown in Figure 4. It is worth stating that the measurement at each step has been made two minutes after the target load is reached, to allow the well-deformation of the specimen. During the test, the digital image of the specimen is also collected using Canon EOS 5 DS R low-cost digital camera; the camera and lens parameters are shown in Table 1. The spatial position of the camera in this experiment is set in the non-orthogonal projection position to simulate the normal condition in engineering practice, as aforementioned. During the whole process, the space position and azimuth of the camera should be maintained to ensure the consistency of projection centers of structure images in the whole process. In order to prevent the camera from being disturbed, shooting remote controller is used to control camera parameters and shutter shooting. Figure 2: Illustration of the edge point and noise point. Figure 4: Test loading prototype. Figure 3: The tested steel–concrete composite beam (unit: mm); (**a**) elevational view; (**b**) sectional view; (**c**) detail size. As a common practice, the system error exists in the measurement due to the physic limitation of the applied hardware. Specifically, the accuracy of the photogrammetry-based method has a stronger dependence on the capacity of hardware when compared with the traditional methods. Therefore, the calibration of photogrammetry equipment is an essential part of the measurement. Generally, the largest part of error in photogrammetric hardware originates from lens distortion [24]. On this end, the checkerboard lattice calibration method [25; 26] has been applied to calibrate the lens of photogrammetric camera. The calibration has been conducted with a total of 25 checkerboard lattice images, and the lens distortion parameters are obtained. Based on that, the photogrammetric images obtained in this paper have been corrected. The calibration process is shown in Figure 5.  
The results of camera calibration parameters are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number of Pixels** & **Size of Sensor** & **Data Interface** & **Aspect Ratio** & **Photo-Sensors** \\ \hline
50.6 million & \(36\times 24\) mm & USB 3.0 & 3:2 & CMOS \\
**Image Amplitude** & **Pixel Size** & **Lens Type** & **Focal Length** & **Lens Relative** \\  & & & **Aperture** \\
8688 \(\times\) 5792 & 4.14 \(\upmu\)m & EF 24–70 mm f/2.8 & 50 mm & F2.8–F22 \\  & & L & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parametric table of camera and lens. Figure 5: Camera calibration intersection photography. In the calibration, the distortion correction formula [27] has been used to correct the structural image element by element. As a result, the ideal image without lens distortion has been obtained, which can be further used for the extraction of structural deformation in the next. The distortion correction effect of the image is shown in Figure 6.  
In reality, the landform around the structure makes it difficult to obtain the orthogonal projection image, so that it can only be tilt photographed on both sides. According to the practical application requirements, the actual situation is simulated in the test, i.e., the camera takes pictures of the test beam at a fixed tilt angle. As shown in Figure 7a, the distance between the camera and the ground is about \(3.5\,\mathrm{m}\), and the distance between the camera and the test beam is about \(3.0\,\mathrm{m}\).