FATIGUE CRACKING IN A STEEL RAILWAY BRIDGE

The railway between Lulea, Sweden, and Narvik, Norway, was completed in 1902, and renewed in the early 1960s. Several of its old riveted bridges were replaced by welded plate girder bridges. After only a few years, cracks appeared, then gradually spread in some of the plate girders. The cracks were arrested by drilling a hole at each crack tip. In the early 1990s, a bridge replacement programme began. Five similar bridges showed 100-200mm-long fatigue cracks at the ends of vertical stiffeners. They were constructed of two parallel, simply supported welded steel I-beams, with sleepers set directly on the top flange. One bridge was selected for detailed computer analysis. It has a span length of 11.6m, and its track lies on a horizontal curve with radius 600m. Its two main girders, 1900mm apart, are straight, 1000mm deep, and connected by cross framing approximately every 2.6m. The web plate of both girders is strengthened by a vertical stiffener at every cross frame. Preliminary analysis was made on the assumption that the cracks resulted from transverse bending of the web plate. A linear static analysis was performed, by modelling the bridge using a finite element analysis program. Four alternative bracing systems were also analysed. It was found that the simplest remedy was to attach the web stiffener to the flange.