News

Bridge rated in 2005 'structurally deficient'

MINNEAPOLIS - The highway bridge that collapsed into the Mississippi River on Wednesday was rated as "structurally deficient" two years ago and possibly in need of replacement.

That rating was contained in the U.S. Department of Transportation's National Bridge Inventory database.

Jeanne Aamodt, a spokeswoman for the Minnesota Department of Transportation, said the department was aware of the 2005 assessment of the bridge. "We've seen it, and we are very familiar with it," she said.

Aamodt said the department plans its bridge repairs using information from the Bridge Inventory database.

Many other bridges nationwide carry the same designation that the Interstate 35W bridge received, Aamodt said.

Aamodt declined to say what the agency was going to do to address the deficiencies found in 2005 and referred further questions to Dan Dorgan, state bridge engineer and director of the bridge office. Dorgan wasn't available for comment.

The deficiency rating is derived from a complex formula that evaluates many factors and condenses them into an overall score. A score of 80 percent or less indicates some rehabilitation might be needed; a 50 percent score or less indicates replacement might be in order.

The I-35W bridge was rated at 50 percent. The rating data was provided to the Star Tribune by the National Institute of Computer Assisted Reporting.

The inventory data also summarize the bridge's status as "structurally deficient." Bridge components are ranked on a scale of 0 to 9, with 0 being "failed" and 9 being "excellent."

In 2005, the bridge's superstructure - meaning the physical conditions of all structural members - was rated at 4, records show. The bridge's deck was rated 5, and the substructure, comprised of the piers, abutments, footings and other components, was rated 6.

In 2001, a research report on the bridge had found that it was unlikely to experience any fatigue cracking in the trusses supporting its deck. The paper, prepared by the University of Minnesota's Center for Information Studies, evaluated both the main trusses and the floor truss of the bridge.

The report by the late Robert Dexter and others concluded that the bridge's deck "has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system."

The authors said their research helped determine that "fatigue cracking of the deck truss is not likely." They added that the bridge shouldn't have any problems with fatigue cracking "in the forseeable future" and that there was no need to "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project."

However, the report noted "many poor fatigue details" and said certain members of the main truss should be inspected every two years, as was being done at the time. In addition, the report said, certain sections of the floor trusses had high stress areas that should be inspected every six months.

Gov. Tim Pawlenty said Wednesday night that the bridge was inspected in 2005 and 2006, and that no structural deficiencies were identified.

Workers had been engaged since early May on a resurfacing project on the bridge that was to conclude in September. It included the installation of guard rails and lighting.

The span carried the name Bridge 9340 in the books of the state Department of Transportation, although it originally was dubbed the St. Anthony Bridge.

The steel-arch structure, opened in 1967, carried 140,000 cars a day at last count, in 2002, its 1,907 feet spanning eight lanes. More on inspections

The Minneapolis bridge had been inspected annually since 1993, and the structure had been thought to be in good enough shape to last until 2020, when it was due for either a major overhaul or replacement.

The most visible threat to a bridge is usually corrosion. But metal fatigue - the weakening of steel by repeated weight of heavy trucks bouncing across the bridge surface - is harder to see. Bridges in northern climates are particularly vulnerable to metal fatigue, because steel becomes more brittle, and prone to crack, when it is cold.

New York Times News Service

  Comments