About Failure

During a discussion on the WELL, a pointer came up to this article from the New Yorker about the Citicorp Building in New York. Seems that the engineer who designed the unusual structure of the Citicorp Building discovered, after it had been completed, that a last-minute change had been made without his being notified, a change that under ordinary circumstances would have made no practical difference, but in the case of this building’s unusual design meant that the whole thing could have toppled over in a severe wind — a level of wind that weather records showed hit Manhattan about every 16 years.

It’s a fascinating article, so it’s petty of me to pick nits, but as a technical editor I’m going to pick one anyway. The writer at one point refers to “the word ‘failure’ being a euphemism for the Citicorp tower’s falling down”. This is quite a bit of overstatement. The word “failure” is standard engineering talk, and not so much a euphemism as a matter of practicality.

For one thing, any time you design anything — say, a bridge — you figure out two numbers: how much weight or other kind of force that it needs to be able to resist at the worst (the plausible worst, not the once-in-a-millenium freak-accident worst), and how much force will cause it to fail.

In a complicated structure like a building, you make this calculation over and over again in countless different ways; since a structure is only as strong as its weakest part, you need to know how strong every last part is in relation to how much force might be put on it.

So you need a couple of short words you can use over and over to label those two ideas. Engineers use “design” and “failure” — the “design load” is how much force we’ve calculated the structure needs to withstand, and the “failure load” is how much force we’ve calculated will cause the structure to fail. You have to know both numbers, and in fact building codes typically set a “safety factor” so that the failure load has to be two or three or five or however many times the design load.

The other thing is that failure does not just mean toppling, it can mean deformation. In the case of the Citicorp Building, since the parts that would fail first would have been bolts, toppling might well be how it would fail; but ordinarily when you’re building something out of steel the point of failure is the point at which the steel bends out of shape, not at which it collapses.

In fact, if your structure is made out of a combination of materials — like concrete with steel reinforcements — it’s required practice to design it so that the steel will fall first. That way the bridge starts sagging noticeably long before it collapses. It seems like it would be the case that the more steel reinforcement, the better, but in fact the result of too much steel is a beam that gives no warning when its weakest part — which is now the concrete — is about to fail. Steel fails by deforming and concrete fails by shattering to pieces, so you don’t put so much steel in the beam that the concrete will fail first.

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