Was bridge collapse due to design flaw?


Richard Moore

Original source URL:

August 9, 2007

Potential Flaw Seen in Design of Fallen Bridge

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.

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 Board¹s 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.

³Given 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,² 
Secretary of Transportation Mary E. Peters said in a statement issued late 

Since the collapse, the concern among investigators has focused on ³fracture 
critical² 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 bridge¹s 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 state¹s 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.

³I don¹t know what this could be,² Mr. Peterson said. ³I¹m frankly surprised at 
this point. I can¹t even begin to speculate.²

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 ³verifying the 
loads and stresses² 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 

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 
³conducting additional analysis to determine whether we need to ask the states 
to do checks of their designs.²

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. ³We¹re going to leave that to the N.T.S.B.,²
said Bob McFarlin, assistant to the commissioner of the Minnesota Department of 

Of a potential design flaw, Brian McClung, the spokesman for Gov. Tim Pawlenty, 
said the state¹s transportation department ³will 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.²

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 ³no 
gusset plate issues at all.²

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.

³Bolts are better,² Mr. Peterson said, ³but we wouldn¹t consider anything wrong 
with rivets.²

Monica Davey reported from Minneapolis, and Matthew L. Wald from Washington.

Copyright 2007 The New York Times Company

Posting archives: http://cyberjournal.org/show_archives/?lists=newslog
Escaping the Matrix website: http://escapingthematrix.org/
cyberjournal website: http://cyberjournal.org

Community Democracy Framework: 

Moderator: •••@••.•••  (comments welcome)