Slowly it’s coming out… today it’s the New York Times:

Potential flaw seen in design of 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 Board’s 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 bridge’s 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 state’s 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 don’t know what this could be,” Mr. Peterson said. “I’m frankly surprised at this point. I can’t 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. “We’re 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 state’s 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 wouldn’t consider anything wrong with rivets.”

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

2 Responses to “Potential flaw in I-35 bridge”

  1. Richard Says:

    The speculation about the cause of the bridge collapse avoids an important issue. In the background of almost all photos of the scene is a concrete bridge across the same river. It did not fail. This concrete arch design was first used by the Romans and there are examples of 2000+ year old Roman bridges still surviving today. Why are we using metal truss designs that are inferior now?


  2. Carol A. Overland Says:

    Are you referring to the 10th Ave. bridge or the Stone Arch bridge? The Stone Arch bridge has been around Mpls at least twice as long as me, but is now a foot/bike bridge, no car traffic, and that might have been a structural based change… don’t know…

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