Sunday Conspiracy Blogging

By linking to this Robert Fisk article about questioning the 9/11 explanations, LitBrit produced quite a bit of commenter angst over at the House that Klein Built. As an engineer, I find the discussions of 9/11 steel collapse fascinating, and at the risk of revealing ignorance within my own field I want to wade in a bit here.

Fisk’s version of the “Steel shouldn’t have failed” complaint:

If it is true, for example, that kerosene burns at 820C under optimum conditions, how come the steel beams of the twin towers – whose melting point is supposed to be about 1,480C – would snap through at the same time?

Apparently he’s mixing in issues about how fast the various buildings collapsed, but I’m setting that aside to focus on the tempature-induced steel collapse. The accepted explanation for this is laid out by Popular Mechanics:

“Steel loses about 50 percent of its strength at 1100°F,” notes senior engineer Farid Alfawak-hiri of the American Institute of Steel Construction. “And at 1800° it is probably at less than 10 percent.” NIST also believes that a great deal of the spray-on fireproofing insulation was likely knocked off the steel beams that were in the path of the crashing jets, leaving the metal more vulnerable to the heat.

And PM quotes other experts who explain that the kerosene fire would have spread to the office finishes, which burned as hot as 1800°F. All this makes intuitive sense, and I trust AISC — they wrote my steel manual. What I don’t understand is how the steel “snapped.” Also from the PM article:

“I have never seen melted steel in a building fire,” says retired New York deputy fire chief Vincent Dunn, author of The Collapse Of Burning Buildings: A Guide To Fireground Safety. “But I’ve seen a lot of twisted, warped, bent and sagging steel. What happens is that the steel tries to expand at both ends, but when it can no longer expand, it sags and the surrounding concrete cracks.”

This is exactly as I’ve been taught. A Temperature change loads a steel member, reducing it’s capacity until it fails. It’s important to understand that engineers use “fail” in a very particular way, and it is distinct from “fracture.” Here’s the stress-strain curve for steel at different temperatures, taken from this article:

Steel Stress-Strain Curves

Stress just means force per area and strain is basically elongation. So, as you load a member more, it deforms more. Note that, at every temperature level there is a point where increasing the load is not necessary to cause the steel to continue elongating. A member has “failed” when it has passed the initial near-linear portion of the stress-strain curve, and begun deflecting at great rates. The steel warps and bends, and is structurally useless. Eventually it will fracture, but it’s rare; long before a beam actually cracks through, its connections to adjacent members will break.

Connections are always the weak point of a structure. They experience higher forces since loads flow across them in three dimensions. Welding can weaken steel, lowing its capacity. Lines of bolts reduce the cross-sectional area of a member, creating a failure plane.

So: it would be extremely surprising if steel members snapped through as a result of temperature increase and excessive load transferred from neighboring columns. I’m betting people throwing “snap” around are referring to connections failing, causing steel elements to break apart from each other. Which, since connections are weak, occur at lower loadings than you might expect by just calculating the capacity of a system.

Of course, this doesn’t address the fact that fireproofing should have caused the collapse to take longer and I don’t know that a plane crash could have scraped off enough of it to make a huge difference. But I’m hardly knowledgeable about plane crashes or fireproofing, so on this I trust the experts.

However, I do have an alternative explanation for how the fireproofing was overwhelmed and the steel collapsed:

Evil female construction workers removing your fireproofing.

(God this image is horrifying. I mean, of course women do construction in a low-cut micro-dress with sky-high heals, but that tool belt is just not going to cut it.)

5 comments to Sunday Conspiracy Blogging

  • Thank you for this–I actually learned something that I can take to the next argument, as opposed to feeling sick to my stomach for setting myself up for a day’s worth of ad hominem attacks.

    That fireproofing question–was it all dislodged by the plane so completely that the steel would heat up and fail so quickly?–is one of the ones our architect had. It amazes me that some people dismiss the quite reasonable questions like that, calling the person a “loony crackpot” and similar. It also amazes me that seemingly well-educated people can’t discuss and disagree without name-calling. I won’t play the woman card, but I must admit, it’s tempting to do so in light of the absence of such attacks on male writers, of which there are a majority at Ezra’s (only one other female, Kathy G., blogged there for the first time this week during his vacation; I don’t know if she’s going to be a permanent addition, but I hope she is).

    Anyway, peace to you this Monday!

  • Pr

    Good work! I’ll have to make a note of this post.

    Further, here are two more responses to the Fisk article:

  • I’m glad I could contribute — I enjoy the rare opportunities I get to actually use my education on this blog. I’m not sure why this topic provokes so much certainty and anger. Sure we can all read the PM article and the claims of experts, but so much of the proof relies on knowledge and experience that isn’t widespread.

    I’m sure you’re more likely to be accused of looniness because of the whole woman thing. Whether that was at work here? Who knows, some commenters are just punks. I’m sorry it got frustrating.

    And apologies to Pr — your post got auto-moderated because of the multiple links, not anything about the content.

  • Great article, lots of intersting things to digest. Very informative

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