STEVE INSKEEP, host:

It's MORNING EDITION from NPR News. I'm Steve Inskeep.

RENEE MONTAGNE, host:

And I'm Renee Montagne.

One moment Aaron Dougren(ph) was among those stuck in traffic in Minneapolis; the next moment everything changed.

Mr. AARON DOUGREN: I felt a rumble and I saw the bridge just go. The bridge broke right behind my front tire and my pickup fell down, probably 10 or 15 feet, onto the concrete below.

MONTAGNE: Aaron Dougren speaking to the St. Paul Pioneer Press. He's among the survivors of yesterday's bridge collapse.

INSKEEP: Let's talk more about what happened with Steve Olson. He teaches bridge engineering at the University of Minnesota in a campus building near the collapsed bridge. Mr. Olsen, welcome to the program.

Prof. STEVEN OLSON (Mechanical Engineering, University of Minnesota): Good morning.

INSKEEP: What type of bridge was this?

Prof. OLSEN: This was a truss bridge - a deck truss bridge where the roadway sits on top of the trusses.

INSKEEP: Trusses meaning?

Prof. OLSEN: Usually they are depicted by triangular frameworks of steel.

INSKEEP: So you've got steel supports underneath that bridge. And when you see all that twisted steel, as so many of us have seen on television in the last several hours, what's the first thing you want to know?

Prof. OLSEN: Well, I want to know what happened, of course, like anyone does. I think it will take a long time to get sorted out. I think the river span is a much more complicated bridge than the approach spans on each end of the bridge. And there's very capable people at the FHWA and the Transportation Safety Board to investigate the matter.

INSKEEP: We should mention, I suppose, that getting a bridge across any river can be challenging, and getting a bridge across the Mississippi River historically has been something that has been quite a challenge for engineers for quite a long time.

Prof. OLSEN: Correct. Correct. Yeah, the bridge just upstream from this one is a railroad bridge constructed in 1883, one of the earliest bridges over the Mississippi.

INSKEEP: What makes it a challenge to go over that river?

Prof. OLSEN: The currents. The stone below the river is eroding slightly, until they built the falls and the concrete works at St. Anthony Falls to protect the falls.

INSKEEP: And so what do engineers do when putting a bridge across a situation like that to make sure that a disaster like this doesn't happen?

Prof. OLSEN: Well, one of the things they did is they put the primary supports for the bridge on opposite banks of the river so it was much less susceptible to river actions.

INSKEEP: Oh, meaning that if the current press on one side, you'd have more support elsewhere? Is that what you're saying?

Prof. OLSEN: Well, there were no - there are no piers in the river proper.

INSKEEP: Oh, they're outside of the current entirely is what you're saying.

Prof. OLSEN: Correct.

INSKEEP: Well, so we don't have to think quite as much about that. What about the issue of weather in Minnesota. Of course, it's summertime. But this is a bridge that's been through decades and decades of cold winters and lots of use of salt and everything else.

Prof. OLSEN: Well, when we design bridges in Minnesota, we typically design for a larger temperature range. We assume it gets much colder than it does in, say, Alabama or Florida. And we assume it gets warm in the summertime like it's been recently here.

INSKEEP: And so what do you do?

Prof. OLSEN: You have bearings designed into your bridge to accommodate larger movements because your bridges grow and shrink with temperature. And you have joints to accommodate interface pieces between different parts of the bridge.

INSKEEP: So without getting into the details or speculating too much, we can assume that a bridge like this would have been designed to deal with the climate and deal with the situation and everything else.

Prof. OLSEN: Yeah. Yes, of course. So, about 80 years ago, there was no national highway design spec. This bridge was designed 40 years ago when bridges were anticipated to have a 50-year lifespan, you know, prior to any national bridge inspection program getting started, which was started in the early 1970s.

INSKEEP: The fact that this bridge was inspected a couple of times in recent years, how could - well, does that raise any questions in your mind?

Prof. OLSEN: No. No. The frequency with which this bridge was evaluated raises no concern in my mind.

INSKEEP: Okay. Well, Mr. Olsen, thanks very much.

Prof. OLSEN: Yes. You're welcome.

INSKEEP: Steve Olsen is group manager of the Bridge Division for the engineering firm HNTB, and he teaches bridge engineering at the University of Minnesota.

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