PSI - Issue 62

Matteo Castellani et al. / Procedia Structural Integrity 62 (2024) 193–200 Matteo Castellani / Structural Integrity Procedia 00 (2019) 000 – 000

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a smooth flow during infrastructure survey operations.

Fig. 1. Flowchart of the proposed approach for the integration of UAV Photogrammetry and TLS point clouds

2.1. Procedure for UAV survey The correct planning of survey operations using the drone plays the most important role in the entire photogrammetric survey process. The level of detail of the structure depends on various factors, including the flight distance from the bridge: the greater the distance, the fewer the required photographs and the lower the quality of the reconstruction; at a closer distance, on the other hand, the processing data increases, but the final result is significantly better. The quality of the photogrammetric survey is assessed by a parameter called Ground Sample Distance (GSD), representing the sampling distance of the detected object or terrain, expressed in mm/px. The GSD required for a given flight distance, relative to the surveyed object, D, can be determined as (Tu et all, 2020): = × × 100 × . (1) Here, is the real sensor width (mm), is the real focal length (mm), and is the image width (px). The other key aspect is the observance of an overlap percentage between individual frames of 80% along the x-axis and the y-axis. For box-type bridges, the flight planning considerations are to be divided into two different phases, and are aimed at the separate surveying of the superstructure and substructure. Predefined trajectories to be carried out for the different structural elements are shown in Fig. 2. The survey of the vertical elements must be conducted by means of ascending flights along the entire extension of the element, carrying out fixed rotations around it with a camera perpendicular to the axis of the element. For the superstructure, it is necessary to plan flight crawls, with camera at different inclinations, following the course of the infrastructure. 2.2. Procedure for TLS survey The crucial initial phase involves careful planning of the entire procedure, which includes evaluating access points to the bridge, determining the optimal position of the laser scanner to ensure complete coverage of the structure, and scheduling scanning sessions considering environmental conditions such as sunlight and traffic. Restricting the application of TLS survey exclusively to the substructure allows for a quick execution of the survey phase, avoiding the dangers and potential losses mentioned earlier. Ensuring complete acquisition of the bridge surface during scanning is essential to cover the entire bridge area. To achieve complete coverage and minimize shadowed or unacquired areas, it is advisable to perform multiple scans of the same structural element from different positions. In the specific case of reinforced concrete box girder bridges, it is necessary to follow a defined path as illustrated in Fig. 3. The suggested procedure involves conducting four scans for each span, two on each side, thus creating a square grid. This approach enables achieving uniform coverage of the entire box-girder structure and its supports. The indicated scans represent the minimum required for optimal results. Upon completion of the survey operations, during