We took the question "why did the bridge collapse" to an expert who offered some insight about bridge construction and this collapse.
For 25 years Bob Beckel has been with Ed Kraemer & Sons Company constructing bridges in Minnesota.
We wanted to know how he sees the destruction of the I-35W bridge, so we took him up in Sky 11 for a bird's eye look at the damage.
'"The green iron you see there," Beckel explained, "are members of the truss."
Beckel had not seen the bridge from this perspective, and as soon as we had him over the site of the wreckage he honed in on the south side of the bridge. He figured government investigators would look there too,
"They'll look for how it listed, where it shifted to," he said. "That will probably help them hone in on exactly where the failure started."
When you look at the collapse scene, the deck on the south side landed at an angle,
and a twisted mass of iron lie underneath. Compare that to the center span, which lies relatively neatly in the river. It appears to have dropped straight down. On the north end you see a nice clean break in the pavement.
To an engineer and bridge builder like Bob Beckel the key to it all is at the far south side in the tangled mass of iron.
"This just tells me the failure had to have started toward that side of the bridge,"
There are basically three parts of this type of bridge. At the bottom is the concrete sub-structure. In the middle is all that metal which makes up the truss. At the top is the concrete deck you drive on.
Beckel doesn't pretend to know the precise cause of the collapse — but he's confident he knows what it's not.
"Was it the substructure," we asked
"No, not here," he said.
"It's not the deck?"
"It's not the deck, no."
"It's the truss?"
"The truss itself. Yes."
From these pictures, this bridge builder puts the failure at the south side because of the twisted metal and the way that the road landed angled to the east. What's more, he says because it landed so far to the east, about 50-feet off its original course, he suspects the metal piece that went bad was one time completely vertical.
The security video of the collapse reinforces much of his theory. It shows the start of the collapse at the south end of the bridge. He finds the whole thing distressing.
"I'm sure it makes people wonder 'do we have more of this somewhere?'" he said. "I'm fairly confident that you're seeing an isolated case. An anomaly."
Bob Beckel says he's worked with the people who do inspections in Minnesota for a quarter-century and he considers them among the best in the nation.
While we were working with him Friday on this story the N.T.S.B. Held a news conference saying that they were becoming very interested in the south side of the bridge along the eastern edge.
(Copyright 2007 by KARE. All Rights Reserved.)