for retrofitting the Washington Avenue Bridge for the Green Line (Central Corridor), which is 89 percent complete and scheduled to open in mid-2014.
The Metropolitan Council awarded AECOM the contract to retrofit the Washington Avenue Bridge over a 1,131-foot gorge above the Mississippi. The bridge, built in 1965, lacked redundancy to prevent collapse should a single structural member fail.
The bridge consisted of two independent structures for eastbound and westbound traffic. The rehabilitation included the 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 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 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 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.