5 Structural Assessment of a School Building in Sankhu, Nepal Damaged Due. . . 35 5.3.1 Estimation of Global Deformation from Laser Scans As discussed in a previous section, the damage was more severe in the west end of the first story. The aligned point cloud allows the estimation of the global deformations near this end of the structure. Using Cyclone, planes were fitted to the front three bays as well as the inner western wall and floor, as illustrated in Fig. 5.5a. The angles subtended by these planes per Fig. 5.5b indicate that the structure permanently drifted towards the west. As it can be seen, the angles at the top of the columns are just slightly greater than 90ı for the left side of each bay, and slightly less than 90ı for the right side of each bay. Assuming that the west wall was initially vertical, the data indicates it rotated approximately 0.22ı out of plumb, with respect to the vertical plane. 5.3.2 Identification of Significant Surface Defects The 12 scans conducted by the Faro scanner have been used to detect and quantify the structural damage. To this end, scans were collected from the façade and one of the interior rooms that sustained significant damage during the earthquake. Figure 5.6 presents the location of the severely cracked infill walls and columns on the west side whose damage is quantified in this section. This has been achieved through the exploration of the surface geometry variation from the collected point cloud set. Once the point cloud data is organized, the normal vectors for each of the vertices are computed, based on the selected number of neighboring points. Once the surface normal is generated for each vertex, the dot product is computed with respect to the best-fit reference plane for the entire cloud and it is binned to indicate local surface perturbations. The likelihood for surface defects and/or architectural details increases, as the dot product deviates from unity. Figures 5.7 and 5.8 illustrate the location and geometry of surface defects for the selected walls which are indicated in Fig. 5.6. The light color lines in Figs. 5.7a and 5.8a indicate various surface defects, such as loss of concrete cover or cracks. For the regions of interest, the detected defects are mapped on the actual point cloud as illustrated in Figs. 5.7b and 5.8b. The quantified surface defects on the exterior wall are presented in Table 5.1. As shown in the table, the right column sustained significant spalling of the concrete cover (47.8 %) which exposed the shear and flexural reinforcement. On the other hand, the left column experienced only moderate spalling equivalent to 10 % loss of cover; however, a 37-cm (length) by 2-cm (width) crack developed. Fig. 5.6 Front view of the point cloud (the two -red and blue- rectangles indicate location of the exterior and interior walls, respectively)
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