FYI...

It makes one wonder how many more of these under sized gusset plates
are out there waiting to be found....

TMT

Design flaw cited in bridge collapse By FREDERIC J. FROMMER,
Associated Press Writer

Steel plates connecting beams in the Interstate 35W bridge in
Minneapolis were too thin by half and fractured, "the critical factor"
in the collapse that killed 13 people and injured 145, the National
Transportation Safety Board said Tuesday.

The connectors, called gusset plates, were roughly half the 1-inch
thickness they should have been because of a design error, NTSB
Chairman Mark Rosenker said. Investigators found 16 fractured gusset
plates from the bridge's center span.

"It is the undersizing of the design which we believe is the critical
factor here. It is the critical factor that began the process of this
collapse. That's what failed," Rosenker said at a news conference.

What caused the bridge to collapse during rush-hour traffic in the
early evening of Aug. 1 -- "the straw that broke the camel's back," as
Rosenker put it -- was not yet known, he said. A final report by the
NTSB was expected this fall.

The Minneapolis span was a steel-deck truss bridge that opened in
1967. Rosenker said it wasn't clear how the design flaw made it into
the bridge because investigators couldn't find the design
calculations.

The bridge was called "fracture critical," or lacking redundancies,
meaning that a failure of any number of structural elements would
cause the entire bridge to collapse.

Rosenker said the safety board had no evidence that the deficiencies
in the Minneapolis bridge design "are widespread or go beyond this
bridge."

However, the NTSB couldn't discount the possibility of similar errors
in other like bridges, he said, and cautioned that states and
contractors should look at the original design calculations for such
bridges before they undertake "future operational changes." The NTSB
issued a safety recommendation to the Transportation Department's
Federal Highway Administration suggesting that the agency require
bridge owners to do so.

Transportation Secretary Mary Peters called on states to calculate how
changes in bridge weight, capacity or evolving bridge conditions will
affect gusset plates.

"With a few calculations, we can help reassure travelers that our
bridges remain safe," Peters said in a statement.

Rosenker noted that structural weight had been added to the
Minneapolis bridge in two major renovations, in the 1970s and 1990s.

"When they added the weight they didn't realize they were bringing the
margins of safety down to where they didn't exist anymore," he said.

Rosenker said that construction materials on the bridge the day it
collapsed, which were part of a resurfacing project, added about 300
tons and were on the same side where failure of the bridge began.

Asked if the construction was the tipping point, Rosenker said, "I'm
not ruling it in, and I'm not ruling it out." That will be left to the
final report to determine, he said.

Abolhassan Astaneh-Asl, a professor of civil and environmental
engineering at the University of California at Berkeley and an expert
on gussets, spent 10 days in Minneapolis after the collapse gathering
information for his own research.

"I concluded that the construction load may have been the last straw,"
he said.

Last August, Peters advised states to consider the additional stress
placed on bridges during construction projects.

Rosenker said there was little chance that bridge inspectors would
have noticed undersized gusset plates.

"No one recognized that you could undersize a gusset plate," Rosenker
said.

"In the history of this organization, we have not seen anything like
this before," he said, adding that gusset plates are supposed to be
stronger than the beams they connect. "It was a shock, if you will, to
the investigating team."

Dan Dorgan, the state bridge engineer in Minnesota, said that the
assumption is that gussets wouldn't be the weakest point in the
design.

"And, in fact, computer programs that generally model these bridges
typically do not take gusset plates into consideration," Dorgan said.
"They mainly look at the main members."

The Minneapolis bridge was deemed "structurally deficient" by the
federal government as far back as 1990, and the state's maintenance of
the structure has been questioned. But Rosenker said the NTSB
investigation has found no evidence that cracking, corrosion or other
wear "played any role in the collapse of the bridge."

Investigators also found no flaws in the steel and concrete material
used in the bridge.

The bridge was originally designed by Sverdrup & Parcel, a company
acquired in 1999 by Jacobs Engineering Group Inc. of Pasadena, Calif.
A message left by The Associated Press with Jacobs Engineering wasn't
immediately returned.

Minnesota Gov. Tim Pawlenty noted that he had warned others not to
jump to conclusions about the collapse. Pawlenty and his
transportation commissioner, Lt. Gov. Carol Molnau, have been sharply
criticized over state funding for infrastructure and maintenance.

"While the NTSB investigation is not complete, the focus of the
investigation appears headed in a direction different than many of the
political claims that have been made," Pawlenty said.

Minnesota is reviewing 23 state bridges with truss designs to make
sure their current load ratings fit with the gusset design. Officials
hope to complete the state reviews by June at a cost of $500,000.
There are another 36 such bridges controlled by municipal governments
in Minnesota.

Late last year, President Bush signed a massive spending bill that
included $195 million to help replace the bridge. That came on top of
the $178.5 million the federal government had already given Minnesota
for the project.

___

Associated Press reporters Brian Bakst in St. Paul and Steve Karnowski
in Minneapolis contributed to this story.