Illustration shows the diverging diamond interchange UDOT is building in American Fork.

Installing the bridge in American Fork requires the rigmarole of closing part of the freeway overnight, detouring traffic and removing concrete center barriers.

And that's before crews even begin moving the bridge into place — itself a process.

But it's not just any bridge. The new American Fork Main Street bridge over I-15 will be the longest and heaviest concrete bridge in the state.

The precast girders, or support beams, are 192 feet long. It will weigh 4,356 tons.

Bridge installation will occur over two nights, with the first section to be moved Friday and the second section on Sunday, said Utah Department of Transportation spokesman Scott Thompson.

The freeway in the area will be closed at 10 p.m. each night. Traffic will be diverted through on- and off-ramps of the existing American Fork Main Street interchange, and drivers are to expect heavy delays. American Fork Main Street around I-15 will also be closed during the freeway closure to accommodate the detouring freeway traffic, Thompson said.

Right now, the sections of bridge are waiting to be moved in a "bridge farm" on the southwest side of I-15. Each section will be placed on large platforms called self-propelled modular transport devices that rumble across all traffic lanes and set the bridge on columns, Thompson said.

Installation of the bridge will be such a process that UDOT is ready for spectators. People may watch on Friday night and Saturday morning. Parking will be available at the park-and-ride lot southwest of the freeway. A flagger will be posted to help spectators enter a viewing area beginning at 9:30 p.m. Friday.

"Seating will not be provided," a UDOT notice says. "Please dress warmly."

It will still take a month or so to fully secure the bridge. By December, traffic will roll over it and UDOT will begin demolishing the current American Fork Main Street bridge.

Then, construction crews will prepare to install a second bridge — which with this weekend's new bridge will complete the interchange — that will be finished by late summer 2010. The two bridges will form a "diverging diamond interchange," which requires fewer stoplights, a new concept in the United States.

While the American Fork bridge is installed, a similar process is taking place in Salt Lake City at 2300 East and I-80. Eastbound I-80 from State Street to 2300 East will be closed for 18 hours beginning Saturday.

In addition to installing the bridge, "we're also taking advantage of that closure to do touch-ups on that construction zone," said UDOT spokesman Adan Carrillo. "I believe we'll be installing overhead signs."

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