Minneapolis — The metal plates that held the girders together on a failed 1960s-era interstate bridge were originally attached with rivets, old technology that is more likely to slip than the bolts used in bridges today.
Some of the plates, or gussets, also may have been weakened by welding work over the years, and some of them may have been too thin or too small, engineering experts said Thursday.
The National Transportation Safety Board cited a “design issue” with the bridge’s gussets.
Engineers say the plates are an obvious place to start looking but that a number of other factors might have contributed to the Aug. 1 collapse that killed at least eight people, with at least five still missing.
On Thursday, NTSB officials said “people have run maybe a little bit too far” with the statement on the gussets.
“Simply by finding a piece of metal that’s been sheared or twisted doesn’t necessarily mean it’s a critical piece of the puzzle,” said Bruce Magladry, director of the NTSB’s Office of Highway Safety. “We see a lot of steel that’s damaged because of the bridge collapse. What we need to ferret out is what’s an initial cause of damage versus what’s a secondary cause.”
Engineering experts said failure of the plates, which usually sandwich the bridge’s steel beams where they intersect, in a critical spot could have brought down the whole bridge, although no one has pinpointed a gusset as the cause of the failure.
“What they’ll be looking for is to see whether one of the gusset plates may have fractured,” said W. Gene Corley, a forensic engineer with the Skokie, Ill.-based engineering firm CTL Group.
The bridge’s builders in the 1960s riveted the plates together, which required many more holes than bolts would have.
More holes weaken steel, said Abolhassan Astaneh-Asl, a professor of structural engineering at the University of California at Berkeley, who compared them to Swiss cheese.
The rivets also tend to slip more than bolts and can lead to more cracking, Corley said.
Bolts are preferred in modern bridge construction and were used in more recent repairs.
Welding work on some gussets — at temperatures of 2,600 degrees or more — could also have caused tiny cracks to form as superheated steel cooled, possibly developing fatigue cracks.
Various problems in the bridge may simply have added up over the years and created stresses that the designers never contemplated, Astaneh-Asl said. For instance, at least one expansion joint locked up, possibly pulling one of the bridge’s piers out of alignment and leading to undetermined pressures on other parts of the bridge.
The bridge was one of Minnesota’s busiest, carrying 140,000 vehicles a day. Mark Hallenbeck, director of the Washington State Transportation Center at the University of Washington, said that’s drastically higher than designers would have considered in 1965.
The traffic would have contributed to fatigue, but its weight is “pretty insignificant” next to the weight of the bridge itself, Corley said.
