For St. Croix Crossing engineers, some assembly now required
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After decades of debate, years of planning and two construction seasons, the new bridge planned near Stillwater is about to start its way across the St. Croix River.
Crews have been at work all winter at a plant in Cottage Grove, building dozens of giant rectangular concrete tubes, 18 feet tall and 48 feet wide. They're like open-ended boxes 10 feet thick that will line up for almost a mile across the St. Croix from Oak Park Heights in Minnesota to St. Joseph, Wis.
"We've built the piers up from the water," said Paul Kivisto, the bridge project's construction engineer. "We've built the crossbeam, and we've cast what we call pier tables. These segments will connect up to those portions of the bridge and then start taking off across the river."
Packed with concrete and reinforced steel, each of the blocks weighs about as much as a diesel locomotive.
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They start out as steel skeletons. Then workers box up the steel in temporary shells to hold the concrete. They use survey equipment to get the measurements just right on the massive pieces, which can't vary by more than about the thickness of three sheets of notebook paper.
They're cast in a giant concrete fabrication building, kept at 50 degrees so the fresh concrete doesn't freeze. When they're cured and strong enough, the bridge segments get moved outside on train-car sized carts to a giant storage yard beside the Mississippi River.
A tugboat will take them to the bridge site. They'll be barged down the Mississippi River and then up the St. Croix, about a 30-mile trip.
From the river, they'll be hoisted more than six stories into the air. The ends will be slathered with an epoxy sealant, and nearly 1,000 pieces will be winched together, end to end, with miles of steel cable that's about as thick as your thumb.
At more than 5,000 feet long, this bridge will be one of the 10 biggest in Minnesota. It just isn't practical to build something that big in place, said Manjula Louis, the project's casting engineer.
If workers had to pour each section and wait for it to cure, "you will never finish the bridge in three years," she said. "It would take probably six years."
Bridge segments will start floating up the river in about six weeks.
Work on the $600 million project started in April 2013 after Congress approved an exception to the Wild and Scenic Rivers Act allowing a new bridge.
Most of the construction so far has been on roads connecting to the bridge and five pairs of bridge piers in the river. The tallest piers stand more than 20 stories on top of underwater footings at the bottom of the river.
Once all these pieces are put together, and the approaching roads are connected, the new crossing is set to open to traffic in the fall of next year and replace the nearly century-old Stillwater Lift Bridge.
