2007-04-30

A Quick Materials Science Rant


I was reading the coverage of the San Francisco-Oakland Bay Bridge overpass collapse, and listening to the story on NPR. Basically, a gasoline tanker truck took an overpass too fast, overturned, caught on fire, and collapsed the overpass. Now, everything coming from the Bay Bridge and heading to the east (highway 24 out through the tunnel) and southeast (highway 580 in Oakland and out to Livermore) now has to be routed through surface streets. As Perlick wrote:

Obviously, I'm not there, but I wrote Christy last night and said I'm not moving back til that's fixed because that's the ramp that goes from the Bay Bridge to my place in Oakland. Pretty ridiculously spectacular pictures, though. Who needs terrorists when you have a bonehead truck driver with a tank of fuel?

But here's my pet peeve--all of coverage talks about the steel "melting" in a fire--this was also true for a lot of the World Trade Center attack coverage. The steel doesn't melt (i.e., turn to a liquid). When it heats up, it softens and loses its structural integrity--it basically turns to taffy. So the structure pulls apart either from its dead load (i.e., its own weight) or live load (stuff on it). That's why fireproofing is so important for steel structures, and part of the reason the World Trade Center towers went down was that the airplane impact blew the fireproofing off the structure. It just annoys me that some of the most erudite news outlets get this wrong.

There's a big difference between steel and wood behavior in a fire. As mentioned above, steel softens and deforms. In contrast, wood chars on the outside, which insulates the remaining wood, and often keeps it from collapsing. There's a classic photo used in architecture and engineering textbooks, showing a steel beam that had collapsed like a wet noodle, draping over a wooden beam at a right angle, that is still in place (Google Images hasn't turned it up yet). I'm not saying that this is always the case (depends on the intensity and length of the fire, size of the members, etc.); also, I'm definitely not saying we want to build skyscrapers out of wood.

There. Got my morning materials science ranting out of the way.

1 Comments:

At 4:23 PM, Blogger Catherine said...

It was all staged by the Illuminati - they wanted to provide "evidence" that large volumes of burning fuel really can bring a steel-based structure down, when all good conspiracy theorists know that wasn't what happened on 9/11.

Am I a bad person if I sort of wish I'd been there to see it? Not too close, of course.

Anyway, I was wondering if you were being too technical, but upon reflection, you're right - "soften" is every bit as easy to parse and understand as "melt", but much more accurate, so there's really no excuse.

And thanks for the postcard! :)

 

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