Prewar lift bridges were feats of engineering in their time, and they still are things of beauty and vital pieces of infrastructure. But they contain hidden dangers that, managed improperly, threaten their continued use and preservation.

In the U.S. there are approximately 100 vertical-lift bridges built for railroad use that were constructed prior to 1950. Nearly all of these bridges should be inspected for fatigue, especially those whose lift mechanisms are still operational, engineers say.

Last month, I visited the 95th Street Lift Bridge over the Calumet River on Chicago's South Side. The owner, Norfolk Southern, called in Illinois Constructors Corp. to replace a fatigued shaft for one of the cable sheaves atop the 220-foot-tall bridge. You can watch ENR's video of the project here.

A 15-ft-dia, 60,000-lb cable drum on the 100-year-old bridge had shifted about 3 inches off center, according to Rush Micheff, ICC's operations manager.

"That was inspected while we were here doing maintenance," he says.

The remedy involved blocking off the other sheaves, jacking up a 1.2-million-pound concrete counterweight, removing cables and pulling the drum, which was taken to a fabrication shop for repairs.

Vertical-lift bridges designed before 1950 may need similar repairs, says Steve Mikucki, director of mechanical engineering at Hardesty & Hanover, a New York-based bridge consultant, which was not involved with this particular repair job.

"They didn't design the shafts for fatigue, and it's common that this type of vintage of shaft will develop cracks," he says, adding, "God forbid, if you don't find it, it could crack through."

Making such repairs is not easy, either. 

Unexpected problems arose on the Chicago bridge when fabricators discovered that the original shaft was fused into the cable drum. They burned it out, bored out the drum and installed a new shaft. That procedure delayed the project by a couple of days, but the bridge remained in operation the entire time.

ICC spent months planning the job. Yet unexpected challenges, like the fused shaft, emerged, says John MacKanin, the contractor's president.

"It seemed like every day something new popped up."