The longest bridge of its kind in North America is edging across the Mississippi at Hastings, one girder at a time.
It was a chilly, windy morning under gray clouds as two steeple-like barge cranes swung the last of 45 massive concrete girders onto the Mississippi River piers that will support the longest free-standing, above-deck arched bridge in North America.
The 1,938-foot Hastings bridge's signature span -- a 545-foot-long steel arch -- is scheduled to be floated into place by year's end. The four-lane bridge is scheduled to open by December 2013. Work started in October 2011.
The I-beam girders placed a few days ago -- most of them 170 feet or longer -- also set a record for the longest concrete girders built for a Minnesota bridge, said Tom Villar, project manager for the Minnesota Department of Transportation. The 108-ton girders, poured and prestressed in Elk River by Cretex Concrete Products, were individually trucked 62 miles on a semi-trailer truck to the north bank piers.
Villar watched as the semi, rolling on 16 axles and 62 tires, drove very carefully from the shore across two parallel, narrow steel bridges onto the waiting 195-foot-long barge. As the laden semi inched aboard, the barge sank a few feet into the river.
About 20 men, including iron workers, carpenters, tugboat pilots and two crane operators, watched the truck as commuters flowed overhead on the old truss bridge. Each had a part to play in erecting the $120 million engineering marvel.
The two cranes lowered giant steel hooks that men attached to each end of a 160-foot-long girder.
Tommy Joyce of Wabasha, Minn., sat at the computer screen controls in the cab of a big red crane on a smaller barge flush against the semi-truck barge. He said his 170-foot tall crane can lift 300 tons. A similar crane sat nearby on another barge anchored at the other end of the truck barge. Joyce, a 14-year barge veteran, had a crane lift chart showing how far out his crane boom can lean safely from his cab depending on how heavy the load is. With the 108-ton girders, he had a narrow angle to work in before his cab would start tipping, he said.
If the wind kicks up, it can swing a loaded boom cable away, causing the cab "to tip up on its toes when it's too far out. It scares you," Joyce said. "You have to think quick. You have two choices: set the load down, or pull the boom lever up to shorten the angle and straighten the boom."
Joyce also kept an eye on his sister crane, holding up the girder's far end.
"You can't swing out and pull the other crane. I could bend or collapse its boom," he said. "When we swing, we have to swing together and keep the girder square under the boom all the time so you don't side-angle the boom."
The two crane operators gently hoisted the giant beam above the semi-truck, which then backed slowly onto shore. As soon as the beam was lowered to the barge's 52-foot-wide deck, four carpenters leaned ladders against the 8-foot-tall I-beam, climbed on top and fastened posts into beam holes so safety cables could be strung across the top of the beam.
Another semi was unloaded and left. Then, as the cranes held one beam aloft, two tug boats shoved the truck barge away to be moored. Next the tugs, motors kicking up wake, began pushing the two crane barges and their dangling I-beam into place.
The crane operators, like twin puppeteers, slowly raised and maneuvered the beam up between two piers to be fastened to their caps. On the caps, 50 feet above the Mississippi, four iron workers waited. They clipped their personal harness straps to safety cables strung across the beams and piers.
They used hand signals to guide the crane operators as they lowered each beam end onto a 1 1/2-foot-square, 8-inch-thick rubber and steel pad on a pier ledge. Each pad had two bolts jutting up to slip through predrilled holes in the bottom of the I-beam. The rubber pads allow the beams to expand or shrink a few inches on very hot or cold days.
Far below, a pair of mallards flew past the cranes, unfazed by all the activity, and skidded into the Mississippi.
Jim Adams • 952-746-3283