NTSB points to design flaw in Minneapolis bridge collapse

WASHINGTON — The fracture of poorly designed gusset plates connecting beams on the Interstate 35W bridge that collapsed in August in Minneapolis contributed significantly to the deadly disaster, the head of the National Transportation Safety Board said Tuesday.

In a news conference in Washington, NTSB Chairman Mark V. Rosenker emphasized that the NTSB has yet to determine the "probable cause" of the collapse that killed 13 people and injured 144 others. However, he characterized his report Tuesday as a "significant development" toward the agency's conclusions.

Rosenker zeroed in on 16 gusset plates that he said investigators found to be fractured. These 16 were paired in eight "nodes" concentrated in the bridge's main center span, he said.

The plates were a half-inch thick, and they should have been an inch thick, he said.

With emphasis in his voice, Rosenker said: "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."

He labeled the under-sizing "an error in calculation" by the bridge's designers.

The bridge was designed by Sverdrup & Parcel, a company acquired in 1999 by Jacobs Engineering Group Inc. of Pasadena, Calif.

This finding by the NTSB was "a shock to the investigative team. We've never seen anything like this," he said, adding that gusset plates are usually the "more robust" than the ones he spoke of Tuesday.

Rosenker said his department found nothing to suggest that the deficiencies in the bridge's steel-deck truss design, is replicated anywhere in the United States. However, he said governments and contractors should review the nation's more than 450 bridges similar to the one that collapsed and act accordingly.

The NTSB is making this recommendation, Rosenker said, "to ensure that the original design calculations for other bridges of this type have been made correctly, before any planned modification or operational changes are accomplished ... and before any additional stresses are placed on them."

Washington state has 26 steel truss bridges, including the Interstate 5 Ship Canal Bridge and the Aurora Bridge in Seattle.

Each is inspected at least once every two years, and each gusset plate is examined at arm's length or closer, based on national standards, said Harvey Coffman, bridge preservation engineer of the Washington State Department of Transportation. The Ship Canal Bridge underwent its regularly scheduled inspection shortly after the Minneapolis collapse, and the DOT reviewed its records for other bridges at that time.

Coffman said there have been no major modifications that would increase the load on the Ship Canal Bridge trusses, a possible contributing problem in Minneapolis. The I-5 Seattle bridge was built in 1962, while the Aurora Bridge opened in 1932. Both have undergone seismic strengthening.

The DOT is following the Minnesota investigation, and will take further precautions here if needed, or if requested by the federal government, he said.

Coffman described this state's bridge decks and trusses as "in pretty good condition." Gusset plates tend to be 5/8 to 3/4 inches thick, or thicker, he said.

In the NTSB report, Rosenker also spoke in detail about a "significant amount of weight" that was added to the 40-year-old bridge during two major upgrades. The bridge's weight was increased by a thickening of the pavement, installing rails on the sides and adding a median strip.

The weight from those upgrades brought the "margins of safety down to where they didn't exist anymore."

As for the repaving equipment and material that was on a concentrated part of the bridge that fateful day, Rosenker said those many tons "may or may not" have played a role in the collapse. That aspect of the investigation continues, he said.

In Minnesota, Gov. Tim Pawlenty said Tuesday's report showed that "some individuals have leapt to premature conclusions ... it is our hope that at least now people will reserve further judgment until the investigation is complete."

After reviewing the NTSB report, Pawlenty said he had ordered the state transportation officials to recalculate load capacity for the 23 state bridges and 36 local bridges with a design similar to the I-35W bridge. Recalculations are already under way for seven state trunk highway bridges and 16 more are scheduled. Pawlenty said those recalculations will cost about $500,000 and should be complete by June.

Minneapolis Mayor R.T. Rybak said Tuesday afternoon that many questions remain and "we owe it to the victims and their families to answer the questions of why this bridge fell and to make the needed investments so that disasters like this never happen again."

The NTSB's final determination of "probable cause" of the bridge collapse is expected by the end of the year.

Material from Seattle Times staff reporter Mike Lindblom is included in this article.