GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 337 RAPID SEISMIC ASSESSMENT OF BRIDGES USING UAV AERIAL SURVEY AND PHOTOGRAMMETRY: PRELIMINARY RESULTS G. Candela 1 , C. Demartino 2 , G. Monti 1 , N. Nisticò 1 1 Department of Structural Engineering and Geotechnics, Sapienza Univ. of Rome, Rome, Italy 2 Zhejiang University – Univ. of Illinois at Urbana Champaign Institute, Zhejiang University, Haining, China Introduction. Seismic risk mitigation of infrastructures is a crucial problem that developed countries have been facing for several decades now, along with optimal maintenance and management plans. Rapid seismic risk assessment is an expedient procedure that allows to quickly judge the condition of bridges along a network system and to support the subsequent decisions of applying amelioration measures to ensure an adequate level of safety and serviceability under seismic events of varying intensity. Quickness in gathering data is essential when analyzing territorially distributed networks, both in terms of time and cost. For this reason, a rapid seismic assessment follows a large-scale approach, in order to identify the most vulnerable elements in the network with the least surveying and computational costs. Since, in most cases, blueprints and technical information are missing, the first step is that of acquiring at least some basic geometric information that allow to build a simple structural model. The capabilities of UAV (Unmanned Aerial Vehicles) aerial survey and photogrammetry to reconstruct the 3D geometry of a bridge have been shown in some recent studies ( e.g. , Marmo et al. , 2019). UAV aerial survey, combined with photogrammetry, can be used to survey bridges and gather some basic geometric data. In this study, a methodology for the rapid seismic risk assessment of a simply supported bridge is presented. An existing viaduct is considered as a case study of a bridge located in a zone with high seismic hazard. The different steps of the proposed procedure are briefly summarized as: (i) data acquisition through UAV aerial survey, (ii) photogrammetry to gather the primary geometrical information, and (iii) structural analysis and rapid seismic assessment. Photographic data acquisition: UAV aerial survey. The Annunziata viaduct (38.123295 N, 15.663884 E) is a double (one bridge per each direction) highway bridge located on the A2 “Autostrada del Mediterraneo” in the city of Reggio Calabria, Italy. The viaduct, designed in 1970 and constructed from 1978 to 1980 upon the “Annunziata” river, is a simply supported girder viaduct, with 9 short spans of 27 m and a total length of 254 m. The curvature radius is 150 m, and the medium height of the bridge is 25 m a.g.l. The viaduct deck is composed of a standard module of 29 m with 4 girders and 3 crosses in pre-stressed RC beams. The two decks are sustained by a pair of piers with a common foundation. Piers have rectangular section of 2,50 m x 1,60 m and pier cap dimensions are 8 m x 3 m. The piers’ height ranges from 16 to 26 m. The UAV aerial survey on the Annunziata Viaduct was carried out in May 2019 to acquire the viaduct geometry. The cloudy weather conditions to avoid direct sunlight in the acquired images were chosen to optimize the photographic dataset for the 3D reconstruction process. The aerial survey was performed using a commercial low-weight (700 gr) quadrotor UAV named DJI Mavic Pro (DJI, Shenzen, China). This portable UAV has a build-in GPS used both for positioning and geotagging, used to perform waypoint navigation and acquire photographic dataset according to a pre-defined flight plan. The maximum flight distance (both perpendicular and vertical) was set equal to 30 m to achieve a ground sample distance equal to a resolution GSD < 1 cm/pixel for the 3D model reconstruction (GSD = Ground Sampling Distance). Waypoint navigation with semi-circular paths was chosen to acquire the entire photographic dataset, with an overlap of 90% between each photo and 30% from one circle to the subsequent one. Following a semi-circular path near the viaduct, six different missions (three for each side) were carried out at different flight height (according to terrain slope variation) with a different camera tilt angle. The UAV route following the waypoint navigation as in the planned missions around the viaduct are shown in Fig. 1.