GNGTS 2019 - Atti del 38° Convegno Nazionale

338 GNGTS 2019 S essione 2.2 The acquired dataset is composed of 965 photos around the viaduct. To georeference the 3D model, different ground points distributed on the area of the bridge in the WGS84 coordinate system were acquired using a GPS station. These points were also used for the proper scaling of the point cloud. 3dmodel reconstruction. The photographic dataset was elaborated usingAgisoft Metashape (Agisoft LLC, St. Petersburg, Russia) to obtain a 3D dense point cloud. The main features of the reconstruction process are summarized in Tab. 1. Fig. 1. Table 1 - Point cloud reconstruction features. Reconstruction process Features Photos alignment 965/965 aligned photos Sparse point cloud 776.273 points Dense Point cloud 7.679.170 points The image segmentation procedure and algorithms were applied to the point to classify the obtained point cloud in order to identify the different structural elements. 330 labelled images were used to train the neural network in the identification process. The obtained point cloud classified with ground (brown), piers (red) and deck (purple) is represented in Fig. 2b. Fig. 2 - (a) 3d point viaduct reconstructed point cloud - (b) Classified 3d point cloud. a) b) Structural analysis. The geometrical features of each structural element of the bridge, previously classified on the dense point cloud, were automatically extracted using a programmed script on a 3d CAD software Rhino (Robert McNeel & Associates, Seattle, Washington, USA) and used as visible variables input for the structural analysis. The hidden geometrical variables (such as pier wall thickness) and materials properties were assumed based on the common practice of the construction period. The seismic risk assessment on the viaduct was performed following the approach presented in (Monti and Nisticò, 2002). The checks were executed for two directions (longitudinal and orthogonal) and for two Limit States (Serviceability Limit State (SLS) and Collapse Limit State (CLS)). All checks were performed in terms of displacement at the top of the piers.