QUESTION: (FALL2010) CSA Standard S6, Canadian Highway Bridge Design Code, requires that cross-frame connection plates be connected to the flanges of bridge girders. The bolted detail, as shown (in Figure 1), appears to be quite popular in rehabilitation work. I heard that this bolted detail qualifies for a “Category B“ fatigue detail, but it is not clear to me how simply bolting the stiffener to the bottom flange makes things better because the web weld is still present.
ANSWER: Where the stiffener also serves as a cross-frame connection plate, both distortion-induced fatigue 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 because 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 This outdated practice inadvertently resulted in the web taking out-of-plane stresses due to relative displacements of adjacent girders.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.