Collapsed Minneapolis Bridge Was Under Repair
RENEE MONTAGNE, host:
The bridge in Minneapolis was built 40 years ago. It was a steel and concrete arch bridge like many in the country that are carrying more and more traffic every day.
Engineers will have a huge job recovering pieces of the span from the Mississippi River and reconstructing what happen that caused this collapse.
NPR's science correspondent Christopher Joyce joins us now to talk about what causes bridges generally to go down. Good morning.
CHRISTOPHER JOYCE: Good morning.
MONTAGNE: How unusual is it for a bridge of this, you know, size and strength to collapse like this and apparently without any warning?
JOYCE: It's rather unusual, actually, for a bridge to collapse like this. Most of the collapses that have taken place over the last 10 years in a study done recently by engineers, about 85 percent of them were from some external cause like a flood that undercuts the pilings or a barge or a ship hitting them. Of the 15 percent or so that are considered to be sort of manmade, if you will, most of the time, it was lack of maintenance, lack of proper maintenance that caused a collapse like this. There have been 37 collapses due to lack of maintenance over this 1989 to 2000 period. In a situation like this, a bridge in the Mississippi River, they're vulnerable to, for example, scouring, which is the turbulence of the water at the base of the pilings; that's a constant cause of concern. It's obviously too soon to tell right now if that was the cause here.
One item of note that has arisen is that this bridge was evaluated in 2001 by the University of Minnesota for the state. And engineers at that time were concerned about signs of fatigue in a steel truss; this is a part that holds the bridge deck up. But they didn't find any cracks then and they deemed it was safe.
MONTAGNE: And the governor of Minnesota is quoted as saying this bridge was inspected last year and the year before. Is that normal? These sort of once a year inspections?
JOYCE: Generally, it's once every two years. It depends on if there's a problem. If they see a problem, they'll come back more often. There's something called a national bridge inspection program, and they run these inspections. It was started in 1968; it was the year after of tragic collapse of the Silver Bridge over the Ohio River. That was also at rush hour and it killed 46 people at that time.
But since then there's a set of inspection standards. And you know, it's all done by the book. It starts with the visual inspection usually every two years, and that can lead more in-depth inspections. They'll look at the things like the integrity of the concrete and the steel. They'll look at the welds that hold the structural elements together. They'll look for fatigue cracks often, especially with heavy traffic, especially trucks.
These bridges start to show fatigue and the cracks are what you want to avoid. All of this information goes into something called the national bridge inventory, which is a huge - 600,000 bridges in the country, really. Bridges, culverts and tunnels.
MONTAGNE: And of all of those bridges, what's the status, if we know? I mean I guess I'm thinking is this possibly something that could happen again?
JOYCE: These happen on occasion. I couldn't tell you exactly how often. But frankly, the latest inventory is showing that the bridges in the country are in worse shape than you might imagine.
In 2005, in fact, they found that 156,000 bridges were structurally deficient or, as they said, functionally obsolete. Many of these are old bridges. But that's 25 to 30 percent of the bridges in the country. And the Federal Highway Administration says that this number is likely to grow larger because bridges are getting older. It's not that they're going to fall down necessarily, but of course if you look to the past, they sometimes do.
MONTAGNE: Now, this bridge was being worked on, although, you know, nobody knows why, at this point in time, why it collapsed. But is there anything to be learned from these previous collapses, which will help figure out what happened here?
JOYCE: Well, truck traffic and constant - overloading of bridges that were not designed for the amount of traffic that they're seeing is sometimes viewed as a cause for cracking and for weakening of bridges. They're going to be looking at things like the chemistry even of the steel to see whether or not it's been changed. They'll look at the welds, as I said. I mean this bridge was built in 1967 and it apparently has not had serious problems in the past, but it gets a lot of traffic. It's sitting in the Mississippi River. And that's very turbulent water. So there are a lot of possibilities. It's going to take a long time though.
MONTAGNE: Chris, thanks very much.
JOYCE: You're welcome.
MONTAGNE: NPR science correspondent Christopher Joyce.