Recent changes to the AASHTO LRFD Bridge Design Specifications have significantly impacted the design of cross frames in steel girder bridges. The assigned fatigue category for fillet welded connections has increased from E to E’, greatly reducing the allowable stress range in welded cross frame members.
Load factors applied to fatigue loads have also increased, further reducing effective stress limits. As a result, designers using traditional analysis and detailing methods often find that required member and connection sizes are much larger than those commonly used in the past. This is primarily a problem in curved and skewed framing arrangements, where large forces develop in cross frame members due to differential deflections between adjacent girder lines.
AASHTO/NSBA Steel Bridge Collaboration publications provide recommendations for the analysis and detailing of steel bridges, including specific provisions for the placement and design of cross frames. When properly applied, these recommendations can reduce cross frame member forces and help to minimize the size of members and connections.
Join B&N’s Travis Butz, PE, for this presentation to review the AASHTO/NSBA recommendations and discuss their application in practical design situations.