Effects of Cross-Frame Placement and Skew on Distortion-Induced Fatigue in Steel Bridges

Because of detailing practices common before the mid-1980s, many bridges are highly susceptible to distortion-induced fatigue. This research explored the influence of cross-frame placement and skew angle in bridges subject to distortion-induced fatigue. Forty high-resolution, three-dimensional finite element analyses of a bridge with multiple cross-frame and skew configurations were performed to examine the relationships between skew angle, cross-frame placement, and stresses on distortion-induced fatigue susceptibility. Bridges with skew angles of 0°, 20°, and 40° and cross-frames spaced at 4.58 m (15 ft) and 9.15 m (30 ft) were investigated. Cross-frame configurations examined included staggered perpendicular to the girder line as well as parallel to the support skew. The analyses found maximum stresses in the web-gap occurred in positive moment regions but not necessarily in regions of highest differential deflection. In configurations with cross-frames placed parallel to the skew angle, maximum stress demand was in the top web-gap but it was in the bottom web-gap region when cross-frames were staggered. Increased spacing between cross-frames correlated with slightly increased maximum web-gap stresses for parallel-to-skew cross-frame arrangements and decreased maximum web-gap stresses for perpendicular-to-girder line cross-frame arrangements. Skew angle had a minimal effect on web-gap stresses in the bridge studied.