Des Moines Register
08-03-07
Deficient bridges can still be safe for traffic
Answers to some of the most common questions about Iowa's bridge safety:
Q: Who is responsible for making sure Iowa's bridges are safe?
A: About 4,000 bridges of 24,800 are maintained by the state. City and county
governments are individually responsible for the rest.
The Iowa Department of Transportation uses teams of people to inspect bridges
every two years, according to spokeswoman Dena Gray-Fisher. Those with
deficiencies are put on monitoring or repair plans. Any bridge that is considered
unsafe is put out of service.
Q: How can the public tell truly unsafe bridges from those that need work?
A: The safety of a bridge is determined by a number of variables using
inspections and different technology, according to Brent Phares, associate
director of Iowa State University's Bridge Engineering Center. The variables
include where a bridge is located, the amount of traffic it experiences, age,
damage to its substructure and other factors, such as recent flooding.
"Dangerousness isn't a statistic kept in the national bridge database," Phares
said.
The U.S. Department of Transportation's database labels bridges "functionally
obsolete" or "structurally deficient," but those labeled as such may still be safe to
drive over, he said.
Gray-Fisher said most bridge failures in Iowa are related to being hit by vehicles.
Q: What's the difference between a bridge that is labeled "structurally deficient"
and one that is "functionally obsolete"?
A: According to the U.S. DOT, "functionally obsolete" refers to bridges that do not
have the lane widths, shoulder widths or vertical clearances adequate to serve
traffic demand, or the bridge may not be able to handle occasional roadway
flooding.
"Structurally deficient" refers to bridges needing significant maintenance
attention, rehabilitation or replacement.
A structurally deficient bridge is closed or restricted to some vehicles because of
its deteriorated structural components. While not necessarily unsafe, these
bridges must have limits for speed and weight.
The Register analysis of the national database found 29 percent of all bridges in
Iowa are functionally obsolete or structurally deficient, or they rated 50 percent or
below on the bridge sufficiency scale.
"But Iowa ranks fifth in the nation in the number of bridges, so we always rank
fairly high when you compare things nationally," Gray-Fisher said. "The national
average is 24.5 percent."
Q: Can construction actually harm a deficient bridge?
A: "Not in and of itself," Phares said. "Usually the idea with construction is that
you're improving the quality of the bridge."
Of the Minnesota collapse, he said: "It's very, very early to say what happened.
There are a number of potential causes. The bridge was considered "fatigue
sensitive." That means if one part of the bridge failed, there was a good chance
the entire structure could fail, he said.
Q: What happened in Minnesota?
A: No one knows yet what caused the Minnesota collapse.
The bridge, opened in 1967, was a steel-arch structure that carried 140,000 cars
a day at last count in 2002.
The national bridge database, updated in June, shows the I-35W bridge in
Minneapolis was rated at 50 percent. A sufficiency score of 80 percent or less
indicates some rehabilitation on a bridge may be needed; a 50 percent score or
less indicates it may need to be replaced. The bridge was also characterized as
structurally deficient.
The last time the bridge was inspected, its superstructure was rated a 4 - or poor
- on a 9-point scale, with 0 being "failed" and 9 being "excellent."
Its deck was rated a 5, or fair, and the substructure - including piers, abutments,
footings and other components - was rated a 6, or satisfactory.
According to an article in the Minneapolis Star-Tribune, a research report on the
bridge found that it was unlikely to experience any fatigue cracking in the trusses
supporting its deck.
The report, prepared by the University of Minnesota's Center for Transportation
Studies, evaluated both the main trusses and the floor truss of the bridge.
The report found the bridge's deck "has not experienced fatigue cracking, but it
has many poor fatigue details on the main truss and floor truss system," the StarTribune reported.
Q: What's the priority for repairing bridges in Iowa?
A: The DOT prioritizes projects a number of ways, but those with emergency or
urgent needs always are considered first, Gray-Fisher said.
Right now, reconstruction of two aging but major river-crossing bridges that are
handling far too much traffic for their original design are being delayed by lack of
funding.
The Julien Dubuque bridge at Dubuque and an Interstate Highway 74 bridge in
the Quad Cities "definitely need to be replaced," she said. But the price tag - $1
billion for the Julien Dubuque Bridge - is cost-prohibitive.
Some federal money has been provided to replace the I-74 bridge, but thus far
not enough. That bridge has one of the worst rankings of all bridges in the state.
Gray-Fisher did not have specific numbers available for that project.
- Lee Rood and Paula Lavigne