Redesigning the iconic Washington Avenue Bridge over the Mississippi River in Minneapolis to carry light rail trains earned a coveted engineering award for the Metropolitan Council and AECOM.
The American Council of Engineering Companies (ACEC) of Minnesota awarded the Council and AECOM a 2013 Engineering Excellence Grand Award for retrofitting the Washington Avenue Bridge for the Green Line (Central Corridor), which is 89 percent complete and scheduled to open in mid-2014. AECOM, a leading provider of engineering, design and other related services, is also working on the Southwest LRT Project (Green Line extension), conducting preliminary engineering on the line’s western portion in Eden Prairie and Minnetonka, where several complex bridges are proposed.
The double-decker Washington Avenue Bridge, which was built in 1965, is less than one mile downriver from the Interstate 35W Bridge that collapsed Aug. 1, 2007. Only a couple of months before the disaster, the Metropolitan Council had awarded AECOM the contract to retrofit the Washington Avenue Bridge over a 1,131-foot gorge above the Mississippi. With the I-35W Bridge collapse, the retrofit took on heightened importance. Like the bridge that collapsed, the WAB -- as project staff calls it -- was fracture critical, meaning its structure lacked redundancy to prevent collapse should a single structural member fail.
“The project criteria itself was a challenge: provide for an additional 75 years of service life and meet current design specifications for the new and existing structural members,” according to AECOM.
The bridge consisted of two independent structures for eastbound and westbound traffic. The rehabilitation included the innovative design and construction of four new truss girders interlaced among the steel framing of the existing structure. Prior to construction, the bridge carried pedestrian traffic on the upper deck and four lanes of vehicular traffic on the lower deck. The retrofitted bridge has double tracks in the middle of the lower deck, with one lane each way for vehicular traffic. The upper deck retained pedestrian traffic.
“The AECOM design team, in collaboration with project partners and technical staff from the Metropolitan Council, Hennepin County and Minnesota Department of Transportation, established a retrofit and rehabilitation scheme that would preserve and strengthen the iconic structure,” AECOM said.
The innovative retrofit design included the installation of two new built-up trusses for each independent structure interlaced with the existing superstructure cross section and a new full-width composite deck. The trusses effectively converted the existing girder system into a structurally redundant eight girder and truss composite superstructure. The retrofit also included the addition of infill columns at each pier to transfer the new truss girder loads to the bridge foundations. The entire retrofit was constructed while maintaining pedestrian and vehicular traffic. The result of their innovative designs was the ability to preserve the existing bridge rather than replace the structure, resulting in cost savings to the project estimated at $80 million to $100 million and a minimum of two years in project schedule in comparison to a full bridge replacement.
Ames-McCrossan Joint Venture, the construction contractor for the Minneapolis portion of the Green Line, retrofitted the bridge for an estimated $21 million, $2 million under budget, and finished two months ahead of schedule in mid-June 2012.
About the project
The Green Line (Central Corridor Light Rail Transit Project) will link downtown St. Paul and downtown Minneapolis along Washington and University avenues via the state Capitol and the University of Minnesota. Construction began in late 2010 on the 11-mile line, and service will begin in 2014. The line will connect with the Blue Line (Hiawatha LRT) at the Metrodome station in Minneapolis and the Northstar commuter rail line at Target Field Station. The Metropolitan Council is the grantee of federal funds. The regional government agency is charged with building the line in partnership with the Minnesota Department of Transportation. The Central Corridor Management Committee, which includes commissioners from Ramsey and Hennepin counties, the mayors of St. Paul and Minneapolis and the University of Minnesota, provides advice and oversight. Funding is provided by the Federal Transit Administration, Counties Transit Improvement Board, state of Minnesota, Ramsey and Hennepin counties’ regional railroad authorities, city of St. Paul, Metropolitan Council and the Central Corridor Funders Collaborative. Redesigning the iconic Washington Avenue Bridge over the Mississippi River in Minneapolis to carry light rail trains earned a coveted engineering award for the Metropolitan Council and AECOM.
Central Corridor LRT Project