If you have been following the Minnesota bridge collapse, you may find the
following article interesting. These are speculations right now. But I suspect
they are based on the shape of the steel debris. The extra weight due to
maintenance equipment and material could have resulted in concentrated loading
on some of the gusset plates. It is a standard practice in USA to keep some
lanes open on roads and bridges during repair. The combined loading may not
have been taken into consideration by those who let out the contract to repair.
In the refinery and chemicals business, it is a standard practice to look at
both operations and maintenace while designing the plants.
Dilip Deka
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Potential Flaw Seen in Design of Fallen Bridge
By MONICA DAVEY and MATTHEW L. WALD
Published: August 9, 2007
MINNEAPOLIS, Aug. 8 Investigators have found what may be a design flaw in
the bridge that collapsed here a week ago, in the steel parts that connect
girders, raising safety concerns for other bridges around the country, federal
officials said on Wednesday.
Ruth Fremson/The New York Times
Work continued on Wednesday in removing wreckage from the collapsed
Minneapolis bridge.
The Federal Highway Administration swiftly responded by urging all states to
take extra care with how much weight they place on bridges of any design when
sending construction crews to work on them. Crews were doing work on the deck
of the Interstate 35W bridge here when it gave way, hurling rush-hour traffic
into the Mississippi River and killing at least five people.
The National Transportation Safety Boards investigation is months from
completion, and officials in Washington said they were still working to confirm
the design flaw in the so-called gusset plates and what, if any, role they had
in the collapse.
Still, in making public their suspicion about a flaw, the investigators were
signaling they considered it a potentially crucial discovery and also a safety
concern for other bridges. Gusset plates are used in the construction of many
bridges, not just those with a similar design to the one here.
Given the questions being raised by the N.T.S.B., it is vital that states
remain mindful of the extra weight construction projects place on bridges,
Secretary of Transportation Mary E. Peters said in a statement issued late
Wednesday.
Since the collapse, the concern among investigators has focused on fracture
critical bridges, which can collapse if even a single part fails. But neither
the safety board nor the federal Department of Transportation on Wednesday
singled out any particular design of bridge in raising its new concerns about
gusset plates and the weight of construction equipment.
Concerns about the plates emerged not from the waters of the Mississippi
River here, where workers have only begun to remove cars and the wreckage with
cranes, but from scrutiny of the vast design records related to the steel truss
bridge.
In Minneapolis, state transportation department officials seemed surprised by
the sudden focus on the bridges gusset plates, which are the steel connectors
used to hold together the girders on the truss of a bridge. On this bridge,
completed in 1967, there would have been hundreds of them, officials here said.
Gary Peterson, the states assistant bridge engineer, said he knew of no
questions that had ever been raised about the gusset plates, no unique
qualities to distinguish them from those on other bridges, no inkling of any
problem during decades of inspections of the bridge.
I dont know what this could be, Mr. Peterson said. Im frankly surprised
at this point. I cant even begin to speculate.
If those who designed the bridge in 1964 miscalculated the loads and used
metal parts that were too weak for the job, it would recast the national debate
that has emerged since the collapse a week ago, about whether enough attention
has been paid to maintenance, and raises the possibility that the bridge was
structurally deficient from the day it opened. It does not explain, however,
why the bridge stood for 40 years before collapsing.
In an announcement, the safety board said its investigators were verifying
the loads and stresses on the plates as well as checking what they were made
of and how strong they were.
State authorities here said the plates were made of steel, and were, in most
such bridges, shaped like squares, five feet by five feet, and a half inch
thick. Such plates are common in bridges as a way to attach several girders
together, said Jan Achenbach, an expert in testing metals at the Northwestern
University Center for Quality Engineering and Failure Prevention.
A consultant hired by the State of Minnesota in the days after the collapse
to conduct an investigation of what had gone wrong, even as the national safety
board did its work, first discovered the potential flaw, the board said.
Representatives at Wiss, Janney, Elstner Associates Inc., the consulting firm,
could not be reached late Wednesday.
Federal authorities said one added stress on the gusset plates may have been
the weight of construction equipment and nearly 100 tons of gravel on the
bridge, where maintenance work was proceeding when the collapse occurred. A
construction crew had removed part of the deck with 45-pound jackhammers, in
preparation for replacing the two-inch top layer, and that may also have
altered the stresses on the bridge, some experts said.
The chairman of the National Transportation Safety Board, Mark V. Rosenker,
said on Sunday that investigators were calculating the stresses generated on
each girder and other bridge components from the construction equipment and
materials.
While cautioning states on Wednesday about the weight of construction
equipment and materials, the federal transportation department did not
immediately issue any broader warnings about gusset plates. Brian Turmail, a
spokesman for the Transportation Department, said on Wednesday evening that his
agency was conducting additional analysis to determine whether we need to ask
the states to do checks of their designs.
If there was a design error in the 1960s, failure to identify it before the
bridge collapse indicates a problem with the federal inspection program, said
Thomas M. Downs, who was the associate administrator of the Federal Highway
Administration from 1978 to 1980.
Here, state officials were racing to respond to the new concerns about a
design flaw, but said they had no details. Were going to leave that to the
N.T.S.B., said Bob McFarlin, assistant to the commissioner of the Minnesota
Department of Transportation.
Of a potential design flaw, Brian McClung, the spokesman for Gov. Tim
Pawlenty, said the states transportation department will be looking into
every single issue and possibility raised by the N.T.S.B. or the parallel
investigation ordered by Governor Pawlenty, including this one.
Mr. Peterson said that concerns about gusset plates might normally focus on
questions of corrosion over time, but that he had never heard of a question
over the original design or metal make up of a plate here. Had ultrasonic
testing of the plates shown signs of corrosion or cracking, that would be a
concern, he said. But in the case of the I-35W bridge, Mr. Peterson said he
recalled no gusset plate issues at all.
When the bridge was built, in the 1960s, its hundreds of gusset plates were
attached with rivets, though bridge designers here switched to bolts, a
stronger option, in the 1970s.
Bolts are better, Mr. Peterson said, but we wouldnt consider anything
wrong with rivets.
Monica Davey reported from Minneapolis, and Matthew L. Wald from Washington.
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