Question:
How fast and hard must the wind be to knock down the Sears' Tower?---

Replies:
Real fast! Usually buildings are engineered with safety
margins of something like a factor of 3-5, so at a rough
guess I would say a wind 5 times faster than the fastest ever
known in Chicago (say maybe 600 miles/hour) would be
required to do some structural damage to the building.
Of course, the windows would probably get knocked out
before that...
Arthur Smith
Question!!! Let us try and estimate the answer.
The force of the wind is proportional to the air density time the square of
the velocity times the surface area of the building (all in a consistent wet
of units). I do not know the constant of proportionality, but surely it is
less than 1.

The force needed to tear the building apart is some typical shear stress
times something like the cross-sectional area of the building. The building
material is probably reinforced concrete, so look up the maximum shear stress
for steel, and use, say 1% of the cross-sectional area near the top.
Let me know what you get.

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