Minneapolis Bridge had design flaw

By P.J. Huffstutter, Los Angeles Times Staff Writer
January 16, 2008
CHICAGO -- Federal investigators announced Tuesday that a "serious design error" was a key factor behind last summer's deadly collapse of a Minnesota bridge, but also said that the mistake would not likely have been discovered during routine state inspections.

National Transportation Safety Board Chairman Mark V. Rosenker said the Interstate 35W bridge had been built with gusset plates -- the steel parts that connect the girders, which support the bridge -- that were too thin to hold up the bridge with increased traffic and additional weight of infrastructure improvements.


The Aug. 1 collapse killed 13 people and injured more than 100.

Investigators have found 16 fractured gusset plates from the center section of the steel-deck truss bridge, Rosenker said Tuesday at a news conference.

"Basically, those 16 gusset plates were too thin to provide the margin of safety expected in a properly designed bridge such as this," Rosenker said. "These gusset plates were roughly half the thickness that would be required -- half an inch thick rather than an inch thick."

Safety board officials said their investigation was far from over, and the exact cause of the collapse was still unknown. But investigators don't think that deficiencies in materials, maintenance or inspections played a significant role.

A final NTSB report on the rush-hour tragedy is expected by the end of the year.

In the days immediately after the bridge collapse, the NTSB had raised concerns about the plates.

The board's finding, which suggests that the 40-year-old bridge was structurally unsound from its debut, could alter national debate over maintaining the transportation infrastructure.

It is unknown how or why "there was a breakdown in the design review procedures that allowed a serious design error to be incorporated into the construction of the I-35W bridge," Rosenker said.

Investigators haven't located the original designer's calculations, he said, so "we cannot determine whether the error was a calculation error, a drafting error, or some other error in the design process."

Inspectors don't look for design problems that could date back decades, Rosenker said.

"The National Bridge Inspection Standards are aimed at detecting conditions such as cracks or corrosion that degrade the strength of the existing structure," he said. "They do not, and are not intended to, address errors in the original design."

The bridge was designed by Sverdrup & Parcel, a Missouri-based civil engineering firm that also designed and oversaw construction of the Louisiana Superdome in New Orleans and the Chesapeake Bay Bridge-Tunnel, which was once designated one of the Seven Engineering Wonders of the Modern World.

The company was later bought by Jacobs Engineering Group Inc. in Pasadena, which did not return calls for comment Tuesday.

Experts had warned for years that the I-35W bridge needed expensive repairs. In 1990, a federal inspection declared the bridge "deficient," noting a history of fractures along the plates that hold together its structural arches.

After opening to traffic in 1967, the I-35W bridge -- like many others -- was modified. Such modifications, which included thickening the driving deck, probably added strain to the bridge's weaker spots. And over the decades, traffic increased on the downtown bridge.

At the time that the bridge gave way, dropping traffic into the Mississippi River, maintenance crews were using tons of equipment and construction material on the bridge's deck.

The safety board urged that all states and contractors take extra care with how much weight they place on steel-deck truss bridges -- of which about 465 remain nationally -- and reexamine the original designs before adding to or otherwise modifying them.

In Minnesota, 23 state bridges and 36 bridges under the jurisdiction of local governments have similar steel-deck truss designs.

State investigators had been reviewing seven of the largest.

"Now we're going to expand that to all of the state bridges and try to work with the local governments, to see if they're looking at the same things," said GOP Gov. Tim Pawlenty.

The $500,000 review is expected to be complete by June, he said.

Rebuilding of the I-35W bridge, estimated at $400 million, is underway.

The 10-lane bridge is to open this year.

That project has sparked federal and state political infighting, while weary commuters have been forced onto crowded surface streets.

And victims and their families have called for new state laws to compensate them