Investigators in Minnesota have raised concerns about a structural part used in bridges all over the country, including 177 in Oregon. The possible design flaw came up in the investigation of deadly collapse of a Minneapolis bridge.
Federal transportation officials are calling on states to be aware of the stress placed on what are known as “gusset plates.” Colin Fogarty reports.
Gusset plates may not be a household term. But they're everywhere — including houses.
Imagine an A-frame roof. Those diagonal boards that meet at the peak are connected with a metal plate bolting them together.
On a steel deck truss bridge — the kind that collapsed in Minnesota — the steel girders that fit together to hold up the bridge are connected with gusset plates. Oregon State University structural engineer Chris Higgins says gussets plates are the most efficient way to connect two parts, or members as he calls them.
Chris Higgins: “A gusset plate is a steel plate that's used to connect different members within a truss. And a truss is a system that's made up of individual components and it's the plate that joins those individual components together.”
Gusset plates are what concern investigators in Minnesota. U.S. Secretary of Transportation Mary Peters issued a statement cautioning states to “carefully consider the additional weight placed on bridges during construction or repair projects”.
Peters said the cause of Minnesota bridge collapse is not yet known. But “the stress on the gusset plates may have been a factor”. The girders of steel truss bridges are held together with gusset plates.
Colin Fogarty: “I'm standing under the Marquam Bridge, which cuts through Portland on I-5. It's a bridge with a similar design — called a steel deck truss bridge — as the one that collapsed in Minnesota. And you can see just by looking at it, if there's a problem with those gusset plates, then there's a problem with the whole bridge. And you can imagine the calculus that would go through your mind driving over the Marquam bridge as so many thousands people do every single day, if the gusset plates failed in Minneapolis, should I be on this bridge at all?”
Oregon transportation officials say categorically, yes.
Pat Cooney: “I know a lot of people are probably concerned about what they're hearing. But we believe our bridges in Oregon are safe.”
ODOT spokesman Pat Cooney says 177 bridges in Oregon have steel parts connected with gusset plates. But he says any bridge that's considered unsafe is closed.
Cooney's agency is in the middle of a massive re-inspection program ordered by Governor Ted Kulongoski. He says ODOT bridge inspectors are looking at gusset plates in the same way they're looking at all the other parts.
Pat Cooney: “In fact many of them are even surprised today that the someone might think that they were the gussets because a gusset failure will not bring down a bridge. No bridge is designed so that if one piece fails it will come apart.”
In fact, OSU researcher Chris Higgins says there are few inexpensive alternatives to gusset plates.
Chris Higgins: “I think it would be very difficult to make a steel truss bridge without gusset plates. This would be the first gusset plate issue on such a major bridge that I'm aware of. There are a lots of possible failure modes within a gusset plate. But I'm not aware of large numbers of these in the past.”
In the meantime, ODOT plans to present Governor Ted Kulongoski with a preliminary report Friday on Oregon bridges and how the agency plans to inspect the 43 or so that resemble the one that collapsed in Minnesota.
Ted Kulongoski: “I am as interested as every citizen is to find out exactly what happened to that bridge and see if a similar happenstance could occur here. But I'm going over the Sellwood Bridge tonight.”
Portland's Sellwood Bridge — which is owned by Multnomah County — is slated for either major repair or replacement.