Parametric Analysis of Cross-Frame Layout on Distortion-Induced Fatigue in Skewed Steel Bridges

The effects of skew angle, cross-frame spacing, cross-frame layout, cross-frame stiffness, and load placement on the potential for distortion-induced fatigue damage in steel bridges was investigated by performing a suite of more than 1,000 analysis jobs of high-resolution three-dimensional finite-element models. Susceptibility to fatigue damage was quantified in terms of computed stress demand in the web gap region of the girders. Bridge configurations with three different cross-frame layouts were evaluated, including configurations with cross-frames placed parallel to skew angle (skewed-parallel) and perpendicular to the girder line, both staggered (skewed-staggered) and unstaggered (skewed-unstaggered). Skew angles of configurations evaluated ranged between 0 and 50 degrees, and cross-frame spacing ranged from 2.29 to 9.14 m (7.50 to 30.0 ft). Influence and envelope surfaces were constructed to show the relationship between load placement, location of the maximum web gap stress, and the magnitude of the maximum web gap stress. It was found that maximum web gap stress always occurred when loads were positioned directly above the intersection of a cross-frame and girder web. The parametric study showed that cross-frame stiffness and spacing had a significant effect on the susceptibility to distortion-induced fatigue damage; greater cross-frame stiffness resulted in higher web gap stresses, and increased cross-frame spacing resulted in increased web gap stresses. It was also found that the bridge configuration was key to determining the location of the web gaps where damage is most likely to occur. In skewed-parallel and skewed-unstaggered layouts, maximum web gap stresses were identified in top web gaps, whereas in skewed-staggered configurations, maximum stresses occurred in bottom web gaps. It was found that in configurations with staggered cross-frames, maximum web gap stresses tended to occur in regions of support, where cross-frames are often placed back-to-back along the skewed alignment.