A drive-by estimation method of loose bearing based on track geometry using a measurement of unloaded track geometry

In railway steel bridges, a defect known as loose bearing has emerged as a concern. Loose bearing is a support defect in which a gap develops between the upper and lower bearing, causing the girder to flutter when a train passes. As the gap increases, fatigue cracks can form in bridge members, threatening the safety of train operations. 　In recent years, research on indirect structural health monitoring has advanced. In the railway field, methods that utilize the longitudinal level of track geometry obtained by track inspection cars operating periodically have attracted considerable attention. 　In this study, numerical analysis was conducted to investigate the influence of a loose bearing on track geometry when a track inspection car passes over a bridge with the loose bearing. The results revealed that the amplitude of the 5-m chord offset track geometry was higher at locations with loose bearing. 　However, when evaluating loose bearing from track geometry on actual railway lines, the influence of the unloaded track geometry must be removed. Therefore, this study proposes a novel estimation method for loose bearing using the “unloaded–loaded difference,” obtained by subtracting the unloaded track geometry—measured separately by a simple track-measuring device—from the track geometry measured by the track inspection car. 　The effectiveness of the proposed method was verified on an actual bridge section including 74 consecutive supports. The results demonstrated that the method could detect loose bearings at support points identified by visual inspection with an accuracy of 81.8%.