After the I-35W bridge collapsed in Minnesota last year, terms to describe bridge conditions — such as "structurally deficient" and "functionally obsolete" — peppered news stories. The I-35 bridge had been classified as structurally deficient, but what does that really mean? Are structurally deficient bridges unsafe? And how often are bridges inspected? Here, an explanation of bridge lingo and what it reveals about safety.
When is a bridged deemed to be "structurally deficient"?
Bridges are classified as structurally deficient bridge when inspectors find trouble spots that need to be monitored and/or repaired.
What is a "functionally obsolete" bridge?
That refers to bridges that were built to design standards no longer in use today.
Do either of these classifications mean a bridge is unsafe?
No. Neither classification suggests a bridge is unsafe or likely to collapse. Both suggest that the bridge must be monitored, inspected and maintained. In some cases, bridges may have weight or speed restrictions placed on them until they can be repaired or replaced.
How are these classifications determined?
Bridges are rated using a scale of zero to 9; a zero rating means the bridge should be shut down, while a 9 rating means it is in perfect condition. At the time that the I-35 bridge in Minneapolis collapsed, it was rated 4, a classification that meant the state could continue to operate the bridge without load restrictions.
How often are bridges inspected?
Under federal law, all bridges are inspected every two years. Standards for inspections, frequency of inspections, and training and standards for inspectors would all be increased under legislation that recently passed the House and is headed to the Senate.
Is it rare to find a crack in a steel bridge?
Not according to Wisconsin bridge inspector Joel Alsum. He says it is common to find cracks in steel bridge structures in secondary members such as girders, plates and beams.
"Often people will hear 'crack on a bridge' and think it's ready to fall down. But on a secondary member, it's very common. And on a steel bridge, probably about 90 percent have some kind of crack on them," Alsum says.
What are common causes of structural deficiencies?
Water, salt, stress and corrosion can make a bridge structurally deficient. So can age. As a bridge ages, corrosion and decay can reduce its ability to support vehicles, according to the Department of Transportation.
How much would it cost to fix these bridges?
If the U.S. were to repair every single bridge labeled as structurally deficient or functionally obsolete, it would cost at least $140 billion, according to a recently released report from the American Association of State Highway and Transportation Officials. But that's in today's dollars, and highway officials caution that the price tag is rapidly rising. The cost of concrete, steel, asphalt and diesel fuel for heavy equipment have all been spiraling upward, forcing some states to delay repairs.
How do we pay for bridge maintenance and repairs?
At the gas pump, mostly. The federal gas tax — which is 18.4 cents per gallon on regular gas and 24.4 cents per gallon on diesel — goes directly into the federal highway trust fund; highway and bridge projects are paid out of that fund.
But the fund may soon run out of money. Four dollar-a-gallon gasoline has led to an unprecedented drop in vehicle miles driven in the first six months of this year, so Americans are buying less gas and paying less in motor fuel taxes. States often match federal transportation funds with their own, which they, too, usually collect at the gas pump. It's a double whammy that could leave some states short on road and bridge repair funds next year.
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