Tacoma Narrows Bridge Collapse Video of 1940
Minnesota had one of the nation's better bridge-safety records, according to a 2006 report from the U.S. Department of Transportation, which found that 3 percent of Minnesota bridges were ranked as "structurally deficient" - a better record than all but 10 states. By contrast, 23 percent of Rhode Island's bridges were ranked as structurally deficient, 17 percent of Michigan's and 15 percent of Pennsylvania's."
Inspections, reports raise questions about bridge's safety
BY CHRISTOPHER SNOWBECK, MARY JO WEBSTER and DENNIS LIEN
Article Last Updated: 08/02/2007 04:09:49 AM CDT
Bridge inspectors had noted structural problems over the years in the Interstate 35W bridge over the Mississippi River that collapsed Wednesday evening, but it was unclear whether obvious warning signs had been ignored.
Gov. Tim Pawlenty said Wednesday night that no structural deficiencies were found during bridge inspections in 2005 and 2006. The bridge deck was scheduled to be replaced in 2020 at the earliest, Pawlenty said, and legislators offered a similar assessment.
But public reports on the bridge raised questions about its safety.
In 2005, inspectors from the Minnesota Department of Transportation deemed the bridge "structurally deficient," in data submitted to the Federal Highway Administration's National Bridge
Inspectors gave the bridge a sufficiency rating of 50 percent on a scale of 0 to 100 percent. A rating of 50 percent or lower means the bridge might need to be replaced.
The condition of I-beams, girders and other components called "structural members" was rated four on a scale of 0 to nine. A rating of 0 means failed, nine means excellent. Inspectors look at these for signs of distress such as cracking, deterioration, section loss and malfunction and misalignment of bearings. The deck received a rating of five and the substructure rating was six.
Components that have a tension element are supposed to receive extra attention, including a separate inspection every two years, because failure of these "would probably cause
a portion of or the entire bridge to collapse," according to the National Bridge Inspection Standards. The last such inspection, according to the FHWA data, was in June 2003.
No further details were immediately available.
Minnesota had one of the nation's better bridge-safety records, according to a 2006 report from the U.S. Department of Transportation, which found that 3 percent of Minnesota bridges were ranked as "structurally deficient" - a better record than all but 10 states. By contrast, 23 percent of Rhode Island's bridges were ranked as structurally deficient, 17 percent of Michigan's and 15 percent of Pennsylvania's.
Federal law requires all bridges to be inspected once a year, unless the state commissioner of transportation authorizes a two-year interval. The I-35W bridge was scheduled for annual inspections. Data on any inspections conducted in 2006 were not readily available Wednesday.
If the bridge had a troubled reputation, it was news to legislators.
"The fact MnDOT did not bring that forward and request funding tells me engineers did not have any reason to be concerned about its safety,'' Rep. Melissa Hortman, DFL-Brooklyn Park, said.
"If they know there was any risk with this bridge, that would be their highest priority with spending dollars,'' she said.
"We have never sensed there were any troubles with this particular bridge,'' said Rep. Frank Hornstein, DFL-Minneapolis.
The I-35W bridge apparently is what state transportation officials consider a "fracture critical" bridge, meaning it has at least one critical tension member whose failure would be expected to result in a collapse of the bridge.
The bridge that collapsed supported four lanes in each direction, with average daily traffic of 141,000 vehicles in each direction, according to the FHWA.
Fatigue cracking had not occurred in the deck truss itself, according to a March 2001 report from civil engineers at the University of Minnesota. But there were "many poor fatigue details" on the main truss and floor truss systems. Even so, the engineers concluded that fatigue cracking of the deck truss was not likely.
"Replacement of this bridge, and the associated very high cost, may be deferred," the engineers wrote, according to a copy of the report on the Web site of the Minnesota Department of Transportation.
Still, engineers said the fatigue cracking was a serious issue due to the lack of redundancy in the main truss system. Only two planes supported eight lanes of traffic, they wrote.
"The truss is determinate and the joints are theoretically pinned," the report states. "Therefore, if one member were severed by a fatigue crack, the plane of the main truss would, theoretically, collapse."