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There were about 2,500 structurally deficient bridges in Illinois at the end of last year, according to the Federal Highway Administration. But experts say that doesn’t necessarily mean the bridges will collapse.

The collapse of Minneapolis’ Interstate 35 bridge over the Mississippi River last August elevated concerns about bridge safety nationwide, said Pablo Durango-Cohen, an associate professor in the Civil and Environmental Engineering Department at Northwestern University.

With traffic now flowing over its replacement, some drivers wonder: Could the bridges I drive over collapse?

And if 2,500 bridges in Illinois are structurally deficient, isn’t that a problem?

Yes and no, the experts say.

Bridges are considered structurally deficient if “significant load-carrying elements are found to be in poor condition due to deterioration” or “the adequacy of the waterway opening provided by the bridge is insufficient to the point of causing intolerable traffic interruptions,” according to the American Association of State Highway and Transportation Officials’ report, “Bridging the Gap: Restoring and Rebuilding the Nation’s Bridges.”

However, among the hundreds of thousands bridges in the country, failures are extremely rare, he said.

Illinois puts a lot of effort into doing inspections and maintaining the bridges and the roads are generally very safe, said Mike Cima, a structural engineer in Springfield with Hampton, Lenzini and Renwick Inc., a firm that provides consulting engineering and surveying services to public agencies in Illinois.

If bridges are not safe, they will be closed. Still, infrastructure maintenance could be improved he said.

In particular, Cima said, many of the interstate bridges that were built in the ’60s and ’70s are now ready for replacement.

The state’s bridges rank sixth in the nations in terms of sufficiency, the report said.

Sufficiency is determined by the percentage of bridges that are neither structurally deficient nor functionally obsolete. Bridges are functionally obsolete if they are “built to standards that are not used today.”

A bridge that is functionally obsolete is not necessarily structurally deficient or unsafe, according to the report.

However, these bridges may have inadequate lane and shoulder widths or height clearances that are too low.

Among the biggest issues with the country’s bridges is age, the report said. Most bridges are built to last about 50 years; the average bridge age is now 43. The estimated cost to immediately modernize the bridges is $140 billion.

Though bridges are closed if they are not safe, some do have limitations. Cima said he frequently observes drivers not obeying posted load limits on bridges that are structurally deficient. It’s important not to exceed these limits, he said.

“If one falls down," he said, "there’s a reasonable chance that’s the reason.”