|An investigator stood on a gusset, a plate used to connect trusses on the collapsed Interstate 35W bridge in Minneapolis, in this Aug. 9 photo. Undersized gusset plates in the bridge were "the critical factor" in the bridge collapse last year that killed 13 people and injured 100, the National Transportation Safety Board said yesterday. (JIM MONE/Associated Press/FILE)|
States cautioned about repair work on truss bridges
Warning follows report on collapse of Minn. span
WASHINGTON - Undersized gusset plates in the Interstate 35W bridge in Minneapolis were "the critical factor" in the bridge collapse last year that killed 13 people and injured 100, the National Transportation Safety Board said yesterday.
Mark Rosenker, chairman, said the plates, which connected steel beams, were roughly half the thickness they should have been because of a design error. Investigators found 16 fractured gusset plates from the bridge's center span, he 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," Rosenker said.
The Minneapolis structure 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 calculations.
Once they made it into the completed bridge, he said, there was little chance they would be noticed by inspectors.
There are about 465 other steel-deck truss bridges around the country. Rosenker said the safety board had no evidence that the deficiencies in the Minneapolis bridge design "are widespread or go beyond this bridge."
But he cautioned that states and contractors should look at the original design calculations for such bridges before they undertake "future operational changes."
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.
In his update yesterday, Rosenker also noted structural weight had been added to the bridge in two major renovations, as well as with construction materials that were on the bridge the day it collapsed as part of a resurfacing project.
The findings are consistent with what the NTSB said about a week after the Aug. 1 collapse, in which the bridge plunged into the Mississippi River. At the time, the NTSB said it had found issues with the bridge's gusset plates, but expected a full investigation to take more than a year.
Transportation Secretary Mary Peters was expected to issue an advisory urging states to check the gusset plates when modifications are made to a bridge, such as changes to its weight, said a federal official with knowledge of the plans. The official spoke on the condition of anonymity because Peters had not yet made the announcement.
Currently, such calculations are done for the entire bridge, but not down to the gusset plates, the official said.
In August, Peters advised states to consider the additional stress placed on bridges during construction projects. An 18-person crew was working on the bridge when it collapsed.
Nearly three months later, she told a gathering in Washington of a "working theory" of a poorly designed gusset plate and a heavy load of construction materials.