In the early days of railroads in Korea, railway bridges were constructed as steel plate-girder structures, which are vulnerable to vibration and torsion. Many of these bridges have since been replaced with concrete-slab structures, which have high stability. Nevertheless, steel railway bridges still remain all over the country, and a lot of manpower and cost is being invested in the maintenance and repair of such bridges. Moreover, there have not been experimental analyses aiming to measure the cyclic loads that occur when a train enters. To ensure bridge safety, it is necessary to periodically inspect deformations. To this end, the present study proposed a sequential photogrammetric technique for measuring the deformation of a steel railway bridge for three types of railcars. Sequential stereo images of the bridge with multiple feature points are obtained using sequential photographing cameras, to determine the ground coordinates of each point as a function of time based on the space intersection from the relative orientation with coplanarity and the scale adjustment. All of these processes are performed through automated techniques using only the cameras and the targets. With this setup, the 3-dimensional dynamic motions of the bridge due to the cyclic loading of trains could be measured. In addition, the displacements by the proposed method were compared to those obtained with the 3D Laser tracker. The horizontal displacements errors did not exceed 0.5 mm and the vertical error was within 2.3 mm in root mean square error (RMSE) at camera-to-object distances of about 9 m.
