Structural fatigue, and the lack of a backup system in the event of a failure, may have been factors in the collapse of the Interstate 35-West bridge in Minnesota last night, experts told ABC NEWS.com.
National transportation officials said it's too early to know exactly what caused the bridge to give way in Minneapolis last night. But state reports from 2001 and 2005 indicated there were fatigue cracks in the bridge's trusses, and that the bridge had no secondary system to bear the weight of traffic in the event of an unexpected failure.
The bridge "exhibited several fatigue problems, primarily due to unanticipated out-of-plane distortion of the girders. Concern about fatigue cracking in the deck truss is heightened by a lack of redundancy in the main truss system,'' a 2001 report by the Minnesota transportation department found.
"Structural fatigue and fatigue cracks" could have contributed to the collapse, Roberto Ballarini, a structural engineer and head of the civil engineering department at the University of Minnesota, said.
"Degradations in the materials or structural overload could also lead to collapse," Ballarini added.
The 1,900 foot bridge is supported by two arching superstructures called trusses.
The bridge had been classified as "structurally deficient," but that determination meant only that it needed to be maintained and not torn down, Minnesota Gov. Tim Pawlenty said in a press conference.
If one of the 10 pins connecting each truss to the roadbed, or deck, failed, the entire bridge would collapse, said David Billington, a professor of structural engineering at Princeton University.
"The pins carry vertical loads down. … If one pin fails, the whole structure fails," Billington said. "There are two trusses. If one truss goes there is no way the bridge can stand up. The deck cannot be supported on only one side."
Billington said the design of the bridge was "not generically bad but susceptible" to total failure if one part fails. There is, however, "no history of this kind of structure failing," he said.
Corrosion too could be a factor, said John M Hooks, director of the Bridge Management Information Systems Laboratory at the Department of Transportation Turner-Fairbank Highway Research Center.
"In a metal structure, [collapse] could be caused by fatigue and multiple occurrences of stress or corrosion," Hooks said. "Most bridges are built with a lot of redundancy in them and the members are pretty thick."
White House Press Secretary Tony Snow said the bridge, part of Interstate 35-West spanning the Mississippi River, rated 50 on a scale of 120 for structural stability.
"This doesn't mean there was a risk of failure, but if an inspection report identifies deficiencies, the state is responsible for taking corrective actions," he said.
Some 70,000 to 80,000 bridges nationwide are also rated "structurally deficient," Pawlenty said.
Billington said many bridges across the United States need regular maintenance, and he compared the "structurally deficient" rating to a grade of "C- or D."
Mark Rosenker, chairman of the National Transportation Safety Board, said it was "clearly much too early to know what happened."
The federal government's investigation would entail reassembling parts of the bridge, he said.
Four people have been confirmed dead as a result of Wednesday evening's rush-hour collapse. As many as 30 more people are missing.