Is I-355 extension a miracle bridge?

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In about three months, about 53,000 cars each day will cross a massive new bridge built for the I-355 extension through Will County.

The 1.3-mile, six-lane concrete bridge stands as tall as 90 feet and is considered an engineering marvel.

But, will it be safe?

Planners said Thursday its design more than doubles minimum federal safety standards and was built to endure at least five decades of traffic.

The Des Plaines River Valley Bridge will be Illinois' fourth-largest when it opens mid-November. It will be the longest of 545 bridges in the Illinois tollway system -- a distinction now held by the aptly named Mile-Long Bridge on I-294.

The new $124 million, I-355 bridge spans the Des Plaines River and its surrounding valley where DuPage, Will and Cook counties converge near Lemont. It stretches north of Bluff Road to south of New Avenue.

Last year, Road & Bridges magazine named it one of the nation's Top 10 bridges because of its unique design challenges. The contractor had to work around several endangered species and within a limited footprint.

For starters, the bridge spans the Des Plaines river, two canals, several railroad lines, an industrial area and a forest preserve. The area that lies underneath is home to the Hine's emerald dragonfly, a federally protected endangered species. So as to not disrupt its flight pattern, the bridge rises as high as 90 feet above ground.

"It's pretty amazing to see," said Joelle McGinnis, a tollway spokeswoman. "The goal is that they'll fly under the bridge rather than over it and into traffic."

It is considered the cornerstone of the tollway authority's $730 million, 12.5-mile extension of I-355 from I-55 south to I-80.

The bridge is supported by 34 concrete piers, each of which consists of four columns that stand side by side and vary in height. Work began in March 2006. All that remains is some beam and bridge deck work, as well as finishing touches.

Though both bridges stretch over waterways, the Des Plaines River Valley Bridge has little else in common with the highway bridge that collapsed into the Mississippi River in Minneapolis.

That bridge, opened in 1967, carries 140,000 cars a day at last count, stretches 1,907 feet with eight lanes and is made of steel trusses, not concrete.