The Boise Centre Boise, Idaho Visit Event Website
Wednesday, September 4
The Milton-Madison Bridge made history when its new steel truss superstructure was moved 55 feet onto its permanent, rehabilitated piers. The 2,428-foot-long main truss span was slid along steel rails and plates into place, making it the longest bridge slide in North America. The bridge, which connects Milton, Kentucky and Madison, Indiana, opened to traffic one week later after final inspections were completed.
The bridge is owned by the Kentucky Transportation Cabinet (KYTC) and is one of only two crossings over the Ohio River between Cincinnati, Ohio and Louisville, Kentucky, which are 100 miles apart. A requirement of the American Recovery and Reinvestment Act funding used to build the project was that the bridge needed to be replaced on its current alignment. The project was implemented as a design-build with provision for a contractor-supplied ferry service to maintain traffic during the bridge replacement. The ferry service, free to users, was to accommodate 240 vehicles per hour during daylight hours for one year. During the bidding process, the successful design-build team proposed an idea to eliminate the ferry service and instead build the new superstructure next to the existing bridge on temporary piers and slide it laterally into place onto refurbished existing piers. This allowed traffic to be maintained on the existing bridge, except for two brief closures. The 2,428-foot-long truss was pulled laterally via sliding harnesses and utilizing its permanent bearings. A 110-foot-long prestressed concrete girder approach span was also pulled laterally using temporary sliding bearings. This innovative approach generated a winning bid of $103.7 million, well below the engineer’s estimate of $122 million.
This presentation will provide an overview of the project, a description of the concepts used to maintain traffic, and the processes used for the bridge slide. While the approach span slide was on a much smaller scale than the main span slide, the unique set of challenges encountered will also be discussed.
Wednesday, September 4
Recent changes to the American Association of State Highway and Transportation Officials (AASHTO) Load Resistance Factor Design (LRFD) Bridge Design Specifications have had a significant impact on 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 of these changes, 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 occurring between adjacent girder lines.
Publications from the AASHTO/National Steel Bridge Alliance (NSBA) Steel Bridge Collaboration 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.
The presenter, Travis Butz, PE, will review the AASHTO/NSBA recommendations and discuss their application in practical design situations. Specific items of discussion will include arrangement of cross frames (including staggered and skewed alignments), cross frame configuration (X-type vs. K-type), cross frame member type (angles vs. T-sections), connection type (welded vs. bolted), and analysis methods. The discussion of analysis methods will include an overview of the capabilities of commonly used software packages, along with a discussion of finite element analysis techniques.
Travis is a member of AASHTO/NSBA Task Group 13 (Analysis of Steel Bridges) and Task Group 11 (Design) and will also discuss existing and forthcoming publications from those groups related to this topic.