MnDOT feared cracking in bridge but opted against making repairs
Structural
deficiencies in the Interstate 35W bridge that collapsed Wednesday were
so serious that the Minnesota Department of Transportation last winter
considered bolting steel plates to its supports to prevent cracking in
fatigued metal, according to documents and interviews with agency
officials.The department went so far as to ask contractors for
advice on the best way to approach such a task, which could have been
opened for bids later this year.
MnDOT considered the steel
plating at the recommendation of consulting engineers who told the
agency that there were two ways to keep the bridge safe: Make repairs
throughout the 40-year-old steel arched bridge or inspect it closely
enough to find flaws that might become cracks and then bolt the steel
plating only on those sections.
Fears about bridge safety fueled
emotional debate within the agency, according to a construction
industry source. But on the I-35W bridge, transportation officials
opted against making the repairs.
Officials were concerned that
drilling thousands of tiny bolt holes would weaken the bridge. Instead,
MnDOT launched an inspection that was interrupted this summer by
unrelated work on the bridge’s concrete driving surface.
“We
chose the inspection route. In May we began inspections,” Dan Dorgan,
the state’s top bridge engineer, said. “We thought we had done all we
could, but obviously something went terribly wrong.”
Dorgan said
there was enough money in the agency’s budget to pay for construction
work on the underside of the bridge. But he and Gov. Tim Pawlenty
acknowledged that transportation officials will face tough questions
about the state’s upkeep of the bridge, which has had known
deficiencies since 1990.
“We will absolutely get to the bottom of
this,” Pawlenty said. “There were a lot of decisions made, a lot of
judgment calls made, and they’re all going to have to be critically
reviewed.”
Pawlenty said an independent consultant will be hired
to scrutinize MnDOT inspection practices meant to safeguard the state’s
13,026 bridges. In the case of the I-35W bridge, MnDOT inspections
convinced officials that the bridge wouldn’t need to be replaced or
overhauled until 2020, the governor said.
Was there an internal debate?
State
and federal officials said it was too early to speculate on what caused
the eight-lane bridge to collapse during Wednesday’s evening rush hour,
but Dorgan said the focus of the investigation is on the bridge’s
superstructure, or steel underside.
He said inspectors have
long been on the lookout for metal fatigue and cracking in the bridge
because it was designed before engineers learned about dangers to
bridges from fatigue cracking.
“Up until the late 1960s, it was
thought that fatigue was not a phenomenon you would see in bridges.
Unfortunately that was a wrong assumption,” Dorgan said.
According
to a source with knowledge of the state and federal investigations,
MnDOT is focused on the east side of the northbound section of the
bridge past the Washington Avenue entrance as the likely spot where the
bridge first gave way.
Bob McFarlin, assistant to the
commissioner at MnDOT, dismissed speculation that the collapse was
somehow linked to corrosion from de-icing chemicals automatically
dispensed on the bridge.
The National Transportation Safety
Board is conducting the official investigation and Minnesota has hired
its own forensic engineering firm to conduct a parallel study.
A
construction industry official who met with MnDOT about shortcomings on
the I-35W bridge told the Star Tribune that there have been ongoing
concerns among some MnDOT employees about the safety of this and other
similar bridges.
“There were people over there that were deathly
afraid that this kind of tragedy was going to be visited on us,” the
industry official said. “There were people in the department that were
screaming to have these replaced.”MnDOT has been trying to move these
‘fracture critical’ bridges up in their [budget] sequencing so
something like this wouldn’t happen,” the source said.
“The
Lexington Bridge [I-35E over the Mississippi River], that was a
fracture-critical bridge. MnDOT moved that up pretty aggressively. What
was happening on the Lexington Bridge was crack migration in the steel
in the I-beam.”
Dorgan said no open dissension existed.
“It
was talked about whether replacement was needed, whether we could keep
it in service,” Dorgan said. “That was the whole point of those
studies. There was engineering discussion of that, but I’m not aware of
anyone who was rankled, or a heated discussion.”If there was a strong
opposition, it was not voiced,” he said.
MnDOT said Thursday that
about 8 percent of all bridges in Minnesota, including the I-35W
bridge, have been listed by the federal government as “structurally
deficient,” compared with 13 percent nationally. The label doesn’t
necessarily mean a bridge is unsafe, but in the case of the 1,907-foot
bridge, inspections were increased from once every two years to once
every year, officials said.
According to findings from the most
recent inspection in June 2006, inspectors noted various cases of
corrosion and cracking, but found no evidence of growth in pre-existing
cracks, Dorgan said. Another inspection began early this year but was
put on hold when work began on a $9 million contract to patch and
improve the bridge’s driving surface. Dorgan said he has seen no link
between the surface work and the collapse.
“We considered the bridge fit for service,” he said.
Still,
as recently as December, MnDOT indicated a desire to reinforce the
bridge by 2008 with steel plates. According to a newsletter distributed
in January 2007 by the Minnesota chapter of the Associated General
Contractors, MnDOT was intending to take bids in late 2007 on a project
that would “retrofit some of the chord members on the steel deck truss
of [the I-35W bridge over the Mississippi River].”The Department is
looking for feedback and advice from contractors regarding the project
staging and constructability,” the newsletter said.
But Dorgan
told the Star Tribune Thursday that plans changed. “We decided to
handle it with inspections instead,” he said. Gary Peterson, MnDOT’s
assistant bridge engineer, said plating would have required drilling
thousands of holes in the bridge.
“If you take a look at drilling
all of those holes in a bridge that is already fracture critical you
could initiate flaws that might initiate a fracture,” Peterson said.
The
option to monitor through inspection was one of two suggestions given
to the department in 2006 by URS Corp., a San Francisco-based
construction management consultant.
Some close observers of MnDOT
continued to speculate Thursday that the decision to monitor instead of
fix deficiencies in the bridge was driven by financial concerns. Dave
Semerad, CEO of the Minnesota chapter of the Associated General
Contractors, said everything MnDOT does is based on cost-benefit
analysis.
“Let’s face it. They don’t have any money,” Semerad
said. “At the end of the day, that’s the issue. This is indicative of a
long-term pattern.”
Asked whether a lack of money was behind
MnDOT’s decision not to reinforce the bridge, MnDOT Metro District
Engineer Khani Sahebjam said: “No, we would never do that because of
money.”
Laurie Blake • 612-673-1711 lblake@startribune.com Paul McEnroe • 612-673-1745 pmac@startribune.com Pat Doyle • 651-222-1210 pdoyle@startribune.com Tony Kennedy • 612-590-5973 tonyk@startribune.com
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