Along with the spread of steel bridges with I-shaped girders, the use of thick steel plates has been increasing in recent years. It is feared that residual stress in the thick plate affects the girder's load-carrying capacity, yet residual stress measurement data are insufficient. Consequently, the load-carrying capacity curves used in the Japan Highway Bridge Specifications are based on old data. In this study, a test specimen that simulates the welding section between the main girder web and thick flange is elaborated; the residual stress distribution is also measured in the flange plate width and thickness direction using a mechanical cutting method. In the plate thickness direction measurement, accuracy of the measurement is sacrificed in order to propose a simple and easy measurement method. It is found that the slope of stress distribution in the plate thickness direction increases as the thickness of the plate increases. Then, considering a three-dimensional equilibrium of residual stress, the intermediate support of an existing bridge is modeled. From the elasto-plastic finite element analysis, it is confirmed that the residual stress in the thick plate does not affect the load-carrying capacity for the bending of steel girders.
