How do you build I-290 bridge with 80,000 cars a day?

The bridge-in-progress, now little more than girders resting on concrete, stretches out over a live roadway, looking from a distance like a child's half-finished Erector Set.

It's actually one part of the Illinois Tollway's expansive overhaul of the interchange at I-290 and the Elgin-O'Hare Expressway (now known as I-390). Tollway officials say the full project, when completed in 2017, will reduce travel times through that area by as much as 35 percent.

Building a bridge is a serious undertaking, especially when it's going up in an area that sees more than 80,000 vehicles drive through each day. Yes, the work is creating some traffic headaches, but even tired commuters are likely to wonder a bit when they drive by - how do they do that?

Tollway officials, despite being immersed in the minutia of construction projects every day, say even they aren't immune to that feeling.

"When I think about how something like that gets built, with lots of traffic moving right through the area, yes, it's pretty mind-boggling," Tollway Executive Director Kristi Lafleur said.

Bridges are particularly challenging because they require intricate, and potentially dangerous, work to be done while cars whiz past below, officials said.

Think about those girders, for instance. Work crews use two cranes to lift each girder into the air and then lower it onto the frame of the bridge. The cranes don't release the girder until it has been bolted into place, officials said.

After the girders are in place, protective plywood shielding is installed between them. The shielding supports workers as they pour the concrete "floor" of the bridge.

The whole process requires only short, intermittent lane closures, Lafleur said.

"We do most of the work overnight to keep traffic interruption at a minimum," she said. "But of course, night work presents its own challenges, with lighting and visibility especially."

When the bridge is completed, it will be about 2,100 feet long, with the bridge surface 34 feet above grade at its highest point. The bridge will allow traffic to move freely from 1-290 to I-390, officials said. It is expected to open to traffic next summer.

The overall I-290 interchange project comprises a variety of other improvements, including multiple ramps, drainage and noise mitigation. Tollway officials say the entire project will cost about $440 million.

The interchange overhaul is a component of an even larger tollway project, the Elgin-O'Hare Western Access Project.

For information on these and other construction projects, go to illinoistollway.com.