Des Moines Register
08-04-07
DOT checks Iowa steel-truss spans
By LISA ROSSI and PAULA LAVIGNE
REGISTER STAFF WRITERS
Iowa Department of Transportation bridge inspection teams scrambled Friday to
examine three of the state's most structurally sensitive bridges in the aftermath of
the collapse of the Interstate Highway 35W bridge in Minneapolis.
Meanwhile, federal and state transportation engineers are trying to decide
whether more rigorous inspections of some the nation's bridges should be
required following the tragic failure on Wednesday evening of Minneapolis'
busiest bridge, a span that carried an average of 141,000 vehicles a day over the
Mississippi River.
The Minneapolis bridge and the three Iowa bridges that were scrutinized by DOT
inspectors Friday all are steel-truss spans. The design incorporates a framework
of steel trusses to support the road on top of a span.
U.S. Transportation Secretary Mary Peters recommended this week that all
states immediately inspect steel-truss bridges. Peters also called for her
department's inspector general to conduct an extensive review of the federal
bridge inspection program.
Iowa DOT inspectors spent Friday analyzing parts of the Interstate Highway 74
suspension bridge between Bettendorf and Moline, Ill., and the Iowa Highway 9
bridge in the northeast Iowa community of Lansing. Both bridges carry cars and
trucks over the Mississippi River.
DOT crews also conducted an extensive inspection of the Iowa Highway 926
bridge over the Des Moines River in Fort Dodge - the most similar to the
Minnesota bridge.
DOT officials said late Friday that they would not have the results of the
inspections until at least next week.
"The thing about the bridge in Minneapolis, it gives you pause when something
happens and you don't know why," said Bob Younie, a DOT engineer who
participated in the Fort Dodge inspection. "And so it just makes sense to come
out here and look at a bridge somewhat similar to make sure there's nothing
going on that we don't know about."
The Fort Dodge bridge, which runs along Avenue B, was built in 1935 and carries
about 5,000 vehicles a day over the Des Moines River.
The Bettendorf and Lansing bridges are built using partial steel-truss designs, a
DOT spokeswoman said.
There are 740 steel-truss bridges nationwide, and nine are in Iowa, a Des
Moines Register analysis of federal bridge data shows.
The analysis showed that inspectors have rated six steel-truss bridges in Iowa as
structurally deficient, with similarly poor sufficiency ratings.
The Register analysis also found:
- The Jefferson Street viaduct in Ottumwa, another steel-truss bridge over the
Des Moines River, has ratings similar to the Fort Dodge bridge.
- One steel-truss bridge in Des Moines County was built in 1896 and carries
about 2,000 vehicles a day.
- Six other steel-truss bridges are scattered in rural areas across the state and
carry fewer than 50 cars per day, meaning they pose far less risk to drivers.
- The only steel-truss bridge in top shape was the Sumpter Avenue bridge, built
in 2002 over the Middle Raccoon River in Carroll County.
In July 2005, DOT inspectors found the Fort Dodge bridge to be outdated but
ranked it 69.5 percent on a sufficiency scale. Bridges ranked below 50 percent
on the scale are generally considered in need of repair or replacement.
Inspectors considered the bridge's structure to be in fair condition and its support
structure to be satisfactory, 2006 federal bridge data shows.
On Friday afternoon, with the rumble of traffic overhead, DOT inspectors
scrutinized the Fort Dodge structure for cracks and flaws. The inspection, like the
other two Friday, was ordered by Gov. Chet Culver and the Federal Highway
Administration.
Inspectors said the Fort Dodge bridge and parts of the Bettendorf and Lansing
bridges were "not structurally redundant." That means that if one part of a bridge
fails, the entire bridge could collapse, said Younie, the engineer who heads the
DOT's maintenance office.
DOT officials on Friday said they did not know the total number of Iowa bridges
that were not structurally redundant.
However, they insisted that if any bridge were considered unsafe, it would be
closed.
Terry Wipf, director of Iowa State University's Bridge Engineering Center,
said most Iowa bridges have structural redundancy.
That way, if a girder or other support member fails, other parts of the bridge can
absorb the load and prevent the bridge from collapsing.
"Structural redundancy - it should give you added safety in case something
catastrophic goes wrong," Wipf said. "Most bridges are built with that."
Preliminary inspection results from the three bridges will be given to the DOT's
chief engineer early next week.
However, DOT spokeswoman Dena Gray-Fisher said she did not know when
results will be made public, because some analysis will be required first.
Inspectors at the Fort Dodge bridge will continue next week to comb through the
complicated steel support system to look for cracks, missing bolts or other
problems. The inspectors will also look for erosion in the soil around the concrete
supports lodged in the river.
Two teams of three inspectors conducted the Fort Dodge review Friday. Two
workers also operated equipment. Another person directed traffic as a Fort
Dodge Fire Department truck sat near the river in case of injuries.
The inspection was originally scheduled for later this summer. The last major
repairs on the span were performed in 1977, Younie said.
In Iowa, state bridge inspections are done by six different teams of about three
people. They review the state's approximately 4,000 bridges on a two-year
schedule.
DOT officials said an inspection will be conducted in the next few weeks of
another bridge, the U.S. Highway 275, or South Omaha, bridge, which falls under
the joint jurisdiction of Iowa and Nebraska.
Reporter Lisa Rossi can be reached at (515) 232-2383 or lrossi@dmreg.com