## 3 TEL-Gridshell Lab Prototype
### Measurement of Displacements  
Photogrammetric measurements were taken during the load test of the gridshell prototype, evaluating the initial and deformed positions after load application.  
A total of 145 circular coded- and dot-type paper targets of 14 bits and 150 mm diameter were used to build the 3D point cloud of the gridshell (Figure 6). These coded targets were located at all lath crossing nodes and at the midpoits of the latths between the nodes. These points were used as deformation control points after the load test. Additional  
Figure 4: Joints of the TEL-gridshell lab prototype.  
Figure 5: Interior view of the TEL-gridshell lab prototype during the load test.  
coded targets placed on the floor served as dimensional checking points to accurately measure the corresponding coordinates and to scale and level the gridshell model.  
The photogrammetric survey of the gridshell prototype was performed using a Canon EOS 550D digital camera, which had a CMOS (complementary metal oxide semiconductor) sensor of 18 megapixels resolution, 22.3 mm \(\times\) 14.9 mm dimensions, and 4.3 \(\upmu\)m \(\times\) 4.3 \(\upmu\)m pixel size. Compared to CCD sensors, CMOS has the advantage of processing pixels simultaneously rather than in pixel-by-pixel sequence, resulting in higher speed at lower cost and less space. The camera was equipped with a Canon EF 20 mm 1:2.8 lens.  
For the unloaded state, 112 photos were taken of the lower part all around the gridshell and 53 from the top using a mast with a gimbal. Regarding the final deformed state, 52 photos were taken from the lower part and 41 with the mast. The camera-gridshell distances ranged between 3.3 and 3.5 m from the top using the mast, 1.6 and 1.9 m to the long sides (without mast), and 3.4 and 3.7 m to the short sides (without mast).  
High shutter speed (1/320 s) and ISO (6400) were required with a lens aperture of f/5.6. The diffuse lighting conditions in the workspace provided adequate homogeneous brightness and colour over the entire surface of the gridshell during the photos acquisition. PhotoModeler Scanner v6 (2008) software was used to process the photos and generate accurate 3D models of the gridshell. A detailed description of the photogrammetric measurement can be found in [44].  
The absolute errors in the distances between 5 pairs of model checking points on the floor compared to the corresponding actual distances measured with a class II tape measure are shown in Table 1 for both the unloaded and loaded states. Relative errors are also provided in relation to the diagonal length of the rectangular base (8348.8 mm). The mean error of these measurements was found to be around \(\pm\)1.5 mm.  
Figure 6: TEL-gridshell lab prototype view with the photogrammetric measuring target points.  
In addition to the global measurements made by photogrammetry, three transducers (LVDTs) were used to measure the displacement of some points of special interest more precisely. Specifically, an LVDT was placed in the high area of the symmetrical axis of each of the LVL supports to record their loss of verticality (control points A and B), which was expected to be very small. A third LVDT was used to measure the vertical displacement of the central node of the structure (control point C).  
Table 2 shows the displacements of the above-mentioned deformation control points after the load test. For the central point of the gridshell (control point C), the measurements obtained by both procedures are presented. In addition, the displacements of the two lower nodes of the central transverse lath (control points D and E) are shown, as these are the points of the structure with the greatest displacement.