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For more now on the questions that investigators will examine in Minneapolis and other areas where bridges are now under closer inspection, we turn to Joe Martin. He's a professor in the Civil Architecture and Environmental Engineering Department at Drexel University. Professor Martin, thanks so much for being with us.

JOE MARTIN: It's my pleasure.

NORRIS: Now, I want to ask you about something that we keep hearing over and over again in the wake of the collapse: that the bridge that collapsed in Minneapolis was built in the late '60s, at a time when engineers did not fully understand fatigue cracking in steel. Could you help us understand that and how that issue might factor into this investigation?

MARTIN: Well, I wouldn't say we didn't understand fatigue cracking. There's been some major advances in metallurgy since then. But the traffic on the bridge has been a lot higher than we would have expected. And so we're - every couple of thousand cycles is a lower allowable stress. And so we're way off the chart, I guess you'd say, from what we would have expected when it was designed in the '60s.

NORRIS: This question of capacity, is this something that investigators and inspectors and engineers will be looking at across the country? It seems that there are several older bridges that are carrying much more traffic than they were built to accommodate. A system that used to accommodate 65 million cars across the country in the mid-'50s now handles almost four times that much traffic.

MARTIN: Americans love their interstates. We did not anticipate, when we put them up - keep in mind, it was a railroad-based country then, and suburban sprawl had just got started, okay? And so we had not anticipated across the board, just about every superhighway has far, far more traffic on it.

NORRIS: These were boom years, as you say, in terms of infrastructure across the country. A lot of bridges were built during this period. What kinds of questions will bridge inspectors and engineers be exploring as they look at aging bridges across the country?

MARTIN: Well, the first thing we want to do is we want to be going over every inspection report. We have some labels which are not really appropriate. "Structural deficiency" is pretty bad. It's a catchall phrase, which we should not have invented, which meant that at some point in the inspection, it showed that some load-bearing member is starting to deteriorate, okay? And that deteriorate from what it was originally that doesn't have anything directly to do with what it should end up at. And so anything that has that kind of an inspection report will be - all the reports will be revisited in order to have a good picture of what we're looking for. And the other thing that might be happening is, as I was pointing out, that to figure out exactly how many cycles of loading they've had from whatever kind of records we could do, and comparing that to what they were expected to do.

NORRIS: Help me understand that when you say cycle of loading. What is that?

MARTIN: Well, every time that a bridge expands and contracts - now this is Minnesota, it goes from minus 60 degrees in the winter to 120 in the summer, sometimes. Every one of those expansion and contractions and every expansion and contraction as a load goes over the bridge and then lets it elastically rebound is going to take some of the structural members from being in compression and some to being in tension. And the number of cycles or repetition of that - if there was any possible - potential of corrosion or initiation of a hairline crack, will make them expand.

NORRIS: We've heard transportation officials in Minnesota pan the idea of that the construction on that bridge may have contributed to the collapse. How does that strike you?

MARTIN: I have a hard time seeing how construction on the deck, which is just sitting on top of the main bridge would have an effect on this, so we're open to make sure we close that loop. But it's one of the ones that, I think, is going to be put to bed first. So I would be inclined to agree with them.

NORRIS: Professor Martin, thank you very much for speaking with us. That was Joe Martin. He's a professor in the Civil Architecture and Environmental Engineering Department at Drexel University.

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