Problem with a bridge? Listen to it talk
Updated: August 9, 2012 9:28AM
Bridges talk. A bridge inspector might begin his examination of a steel bridge, such as the short span that collapsed on the Northbrook-Glenview border July 4, killing two people, by listening to it ring.
“You hit it with a hammer,” said Joseph Schofer, professor of civil engineering and director of Northwestern University’s Infrastructure Technology Institute. “There’s a whole variety of technologies one can use, assessing the integrity of the bridge by the sound it makes. Acoustic emission technologies that identify the location of cracks. But the starting point is to get up close and touch the bridge. Crawl over it. Hit it with a hammer. It’s straightforward, but very important. If you have a steel bridge, you apply a load to it, listen to the characteristic sounds, get a sense of the integrity of the various components of the bridge. If you detect a crack, you use technology to zoom in on it.”
Of course different engineers use different techniques. Not everyone endorses the importance of the hammer test.
“That’s fascinating, but I don’t think it’s true,” said Ted Niemeyer, principal of Niemeyer & Associates, a McHenry County railroad engineering firm. “There’s a lot more to it. Yeah, at certain conditions you’ll tap it with a hammer to listen to the sound. But in general most of your steel bridge inspection is heavily visual.”
Inspections are so regular, a rail bridge collapse like the one Wednesday is a rare event. “It’s not very often we lose one,” Schofer said.
When a bridge topples, it usually is not the bridge itself failing on its own, but something catastrophic happening to it.
The Federal Railroad Administration reported that 11 of the 19 rail bridge collapses it studied were due to either a barge hitting bridge supports or a flood undermining it.
“In all my years working for railroads, I cannot really think of any bridge collapses that weren’t caused by outside forces,” said Niemeyer. “Barges hitting a pier, a semi-truck ramming the supports under a bridge.”
While what happened July 4 seems apparent — the train derailed, maybe due to heat buckling the rails, and 28 coal cars, weighing up to 100 tons apiece, jackknifed together on the span, collapsing it onto Shermer Road — the cause won’t be known definitively for a long time. The Federal Railroad Administration’s goal is to complete investigations within nine months, but it could take a year, or more, to sort out the rubble.
One reason suspicion initially fell to the bridge itself is that our nation’s bridges are so notoriously under-maintained. But that is more a problem with highway bridges. The tendency is to lump highway and train bridges together, but they are different animals.
“There is a tremendous difference between a highway bridge and a railroad bridge,” Niemeyer said.
For one, highway bridges are in worse shape. They have the corrosive problem of salt to contend with, while rail bridges don’t. There are also a lot more of them — 600,000 highway vs, about 100,000 rail bridges.
Another possible factor — score one for the Tea Party — is that while highway bridges are maintained by governmental bodies, railroad bridges are maintained by the individual railroads that own them, and private enterprise just does a better job. “Even when railroads were struggling, they always had very strong bridge programs, even before regulation,” Niemeyer said.
The government did study whether it should more closely oversee railroad bridges, but in a 2000 report concluded, “the expense of such an action to the railroad industry and to the Federal government is not justified.” So rather than inspect the bridges, it inspects the railroad inspection programs to see that they meet standards. The FRA can levy fines and if it finds a bridge is a danger, it can order it shut down, though it has only done so three times in its 56-year history.
Even if it wasn’t the bridge’s fault, the fact remains that the train seems to have been derailed by a hot day. That doesn’t seem right, a reminder of the importance of maintaining our nation’s sagging infrastructure.
“When was the last time you saw a politician cut a ribbon at a maintenance project?” asked Bill Kwasny, a Wisconsin geotechnical engineer. “Nobody cares about maintenance. How many people check their roofs or their tire pressure? Maintenance is a non-glamorous issue, but if you don’t do the maintenance, it doesn’t last.”
“Maintenance is not as glamorous as original construction,” said Charles Dowding, a Northwestern professor of civil and environmental engineering. “As our infrastructure ages, we need to pay more and more careful attention to the state of service. We need to be mindful that all things have a replacement life. Everything we own, including our own bodies, are subject to the ravages of nature and use.”
If we take better care of our vital infrastructure, it will take better care of us.