Photogrammetry for health monitoring of bridges : Using point clouds for deflection measurements and as-built BIM modelling

Abstract: Road and railway bridges play a crucial role for the infrastructure network in Sweden to work smoothly and keep the traffic flowing. Damage to a bridge can have catastrophic consequences if they are not corrected properly and in due time. Trafikverket in Sweden is responsible for inspection and maintenance of approximately 20 600 bridges throughout the country. This huge number of bridges require large resources in the form of machinery and experienced bridge inspectors who assess the state of the bridges on the spot. At present, the state of a bridge is to a large extent determined by a visual inspection and by manually taking measurements to assess the condition of the bridge. This approach means that the assessment of the condition of the bridge to a large extent is subjective and shifting between different cases depending on the inspector’s experience. New approaches that both could make it easier for inspectors to make more objective decisions and facilitate and reduce the risk concerning the inspection work are therefore under research. In this thesis Close Range Photogrammetry is evaluated as a mean for assessing deflection on concrete bridges and for creating as-built BIM:s for documentation and visualization of the actual condition of a bridge. To evaluate the technique both laboratory experiments and field work are conducted. Laboratory tests are conducted on concrete slabs that are subjected to pressure to inflict deflection on them. The concrete slabs are photographed using close range photogrammetric techniques for different values of deflection. The photographs are later processed into a point cloud in which measurements of deflection are taken and compared to what is measured using displacement transducers during the tests. The field work conducted is in form of photographing a railway bridge using close range photogrammetry and building a point cloud out of the photographs. This point cloud is then used as a basis for evaluating the process on how a point cloud generated through close range photogrammetry can be used to create as-built Building Information Models. Results from the laboratory experiments show that changes in deflection can be visualized by overlapping point clouds generated at different loading stages using the software Cloud Compare. The distance i.e. the deflection can then be measured in the software. The point cloud generated through the field work resulted in a as-built BIM of the railway bridge containing the basic elements. No hard conclusions can be drawn as to how well the method in this thesis can be used to measure deflection on real concrete bridges. The test basis is to small and the human factor can have affected the results. The results though show that millimeter distances can be measured in the point clouds which indicates that with the right approach, Close Range Photogrammetry can be used to measure deflections with good precision. Point clouds generated through Close Range Photogrammetry works good as a basis for creating as-built BIM:s. The colored point cloud is beneficial over other techniques that are generated in gray scale because it makes it easier to distinguish elements from each other and to detect any deficiencies. To create complete as-built BIM:s more than just a point cloud are needed as it only visualizes the shell of the captured object.