A massive flotilla will stop river and vehicle traffic for about three days this week as a 545-foot-long main span is floated into place for the new Hwy. 61 bridge over the Mississippi River in Hastings.

The huge twin-arch span has been rolled onto an eight-barge flotilla and anchored out of the navigation channel so river traffic could resume, said Tom Villar, project supervisor for the Minnesota Department of Transportation (MnDOT).

This week, tugboats will slowly guide the flotilla downstream to waiting piers and bridge structures extending from both riverbanks. The marathon move could start Wednesday but more likely will be Thursday or Friday, Villar said. The move has been delayed twice so far.

River and highway traffic under and on the old Hastings bridge will stop during the three days needed to float the span into place and hoist it onto two concrete piers flanking the main channel.

The span will fill the last gap in what will be the longest free-standing arch bridge in North America, MnDOT says. It's expected to open to four lanes of traffic by December 2013 and to last 100 years, spanning the river on an important route connecting Dakota and Washington counties.

The signature terra cotta twin arches, lit at night, will stand 98 feet tall with criss-crossing silver cables anchoring them to the 104-foot wide deck. The four-year-project is staying very close to its $120 million budget, Villar said.

Getting the 6.6 million-pound span and its skeletal steel beam deck onto the barges was no cakewalk. Four computerized flatbeds on hundreds of semitrailer-truck tires slid under protruding ends of the massive structure. It was rolled about 200 feet from its construction site on Lock and Dam Road, across a temporary causeway onto the barges.

Minnesota doesn't have boats big enough to float the span, so two ocean-going barges, measuring 260 by 72 feet and 16 feet thick, were towed upriver from the Gulf of Mexico, officials said. Six smaller barges are secured between the big barges that support the ends of the span.

The convoy will take two or three hours to float the twin-arch spectacle a half mile downstream to its final home, Villar said. The span, on giant wooden pads, will be slid off the barges onto a rail supported by river pilings between the piers. Powerful hydraulic jacks on the two piers will hoist the span into place between the piers. The whole operation will take three days, Villar said.

Thwarting gawkers

The traffic closure is needed because officials fear that gawker slowdowns to watch the main span installation could cause accidents on the old bridge, said Kirsten Klein, a MnDOT spokeswoman. Resulting traffic jams would hinder emergency vehicles from responding to fires or other incidents on either side of the river.

During the bridge closure, motorists will be detoured to the Wakota Bridge on Interstate 494, which crosses the Mississippi River between South St. Paul and Newport. Drivers to Hastings can then take Hwy. 52 and Hwy. 55.

Work on the project, built by a Lunda/Ames joint venture, started in October 2010. Its initial spring 2013 opening date was delayed by flooding last year.

After the collapse of the Interstate 35W bridge in 2007, a higher priority was given to replacing the aging Hastings bridge, now in its 60th year. Like the 35W bridge, the Hastings bridge is rated fracture-critical, meaning if one load-carrying part breaks, the whole bridge could fall. Repairs have been made to keep it safe.

The bridge was the state's first with a steel-truss style when it opened in 1952. It replaced the city's historic first span, the Hastings Spiral Bridge, built in 1895 of steel and wood. The bridge had a rare spiral roadway on the downtown end that carried horses and carriages down to the town's main street.

Jim Adams • 952-746-3283