Abstract
This paper studies distortion-induced fatigue cracks in a 1960's era design Missouri Department of Transportation (MoDOT) welded plate girder bridge that developed cracks in the web-gap region shortly after completion of a comprehensive seismic retrofit. Finite Element analysis (FEA) was conducted in order to investigate the cause of cracking and to recommend appropriate repair measures for the bridge. The results indicated that the high stress concentrations were principally a result of replacing K-type diaphragms with stiffer cross-diaphragms as part of the seismic retrofit. Two conventional repair measures in addition to two innovative repair measures were evaluated. The first repair strategy provided a positive connection between the connection plate and the girder flange (referred to as the top-angle repair). The second repair created a 203-mm (8-in.) slot where one-sixth of the connection plate depth is cut from the web (slot-repair) resulting in a softening of the web gap.
Two additional innovative repair measures are also proposed. The first consisted of adding a vertical angle section to either side of the transverse connection plate (middle-angle repair). This repair method stiffened the web-gap region, and as a result, restrained the web-gap region from deforming out of plane. The other technique intended to reduce stress levels by softening the web-gap region as well as distributing lateral forces, transferred by cross-frames, over a wider area of the web (slot-angle repair). The proposed innovative repair measures focused on ease of installation compared to more conventional options while they also appear able to reduce the distortion-induced stresses well below the constant amplitude fatigue threshold (CAFT).