A Staff Report from the Straight Dope Science Advisory Board

Do tall buildings sway in the wind?

Dear Straight Dope:

Is it true that tall buildings sway in the wind? Almost everyone believes this about the Empire State building in New York. I think it's an urban myth. Do you know the truth?

George, George, George, the answer is right under your nose, figuratively speaking. The tallest free-standing building in the Western Hemisphere is probably visible outside your window. I'll wait while you go look. For the rest of you Americans, the building in question is the CN Tower, in downtown Toronto. Back with us George? Saw it? Yes? Does it sway? You betcha.

From the official statistics, available at the CN Tower website (www.cntower.ca):

In a 120 KPH breeze, there is this much sway:

Antenna mast (highest part): 1.07 meters (about 3 1/2 feet for you Yanks)

SkyPod (Upper observation deck): .46 meters (1 1/2 feet, about)

Lookout (Open observation deck): 22.9 centimeters (9 inches)

It has to move. The only alternative would be to engineer these parts to withstand the tremendous forces (wind, gravity, shear, compression and precipitation) and you'd still have one inescapable fact: these things are held up in the sky on a stick. Remember moving things with a lever in grade school? The longer the lever, the more the mechanical advantage. The Skypod is on a lever 1100 meters long - how much force to make it topple if the lever were one stiff piece? That's why there's some "give" in it - to destroy the mechanical advantage and keep the tourists alive, happy and spending money. That's what keeps Toronto green.

Now let's turn to the Empire State Building. This is from their official website (www.esbnyc.com):

Does the Empire State Building move or sway?

The Empire State Building does not sway, it gives. With a wind of 110 miles an hour, the Building gives 1.48 inches. Movement off center is never greater than one quarter inch, thus measurable movement is only one half inch, one quarter inch on either side.

I just report this. I don't understand how there can be an inch and a half movement, yet farther down it says the movement off-center is never greater than a quarter inch on either side of center. Never mind. Seems clear enough that the ESB does have some "give" in it, just as the CN Tower does, although since the ESB is less slender and thus more resistant to shear, the amount of lateral movement is less. But this one's no urban myth.

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