The bolted detail shown in Figure 1 below is a popular detail in bridge retrofitting. The CSA Standard S6, Canadian Highway Bridge Design Code, requires that cross-frame connection plates be connected to both flanges of bridge girders. In this detail, the stiffener also serves as a cross-frame connection plate. Both distortion-induced fatigue (resulting from secondary stresses due to deformations and out-of-plane movements) and load-induced fatigue should be considered. The bolted detail as shown does not alter the stiffener-to-web welded fatigue detail with respect to load-induced fatigue, and this welded detail remains “Category C1”. However, connecting the connection plate to the flanges (when done correctly) should improve the distortion-induced fatigue resistance substantially.


In order to avoid welded attachments in the tension flange, many older welded steel bridge girders feature cross-frame connection plates that were either cut short from, or ground to bear on, the tension flange. This outdated practice inadvertently resulted in the web taking out-of-plane stresses due to relative displacements of adjacent girders. These stress ranges, typically unaccounted for in the analyses, have been identified as the common cause of distortion-induced fatigue damage to welded bridge girders. Recent editions of CSA S6 require that cross-frames and diaphragms be connected to each flange for a minimum force of 90 kN.