Quote:
Originally Posted by ZNPaaneah
But, why is it that the 58 floors below the fire offered absolutely no resistance to the fall? Why didn't it slow it down a little?
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Let me try to address this. The supporting pillars on each floor are designed to withstand all live (people, furniture) and dead (structure) loads on all the floors above it. Hence, going from top to bottom, the supporting structure grows in size as the loads increase. Practically, these incremental increases in load bearing capacity, may change 5 or 10 floors at a time, depending on the structural engineers.
These loads are basically static in nature, discounting a couple tons of weight from occupants coming and going. The only dynamic loads designed for would be movement from the earth and weather. Probably the worst of these in NYC would be hurricane force wind gusts, which would load one side of the base of the building and unload the other. Hence all skyscrapers must accommodate worst case wind gusts, and they can be significant on such a huge structure.
The tragedy of 9/11 presented entirely new dynamic loading. Once the structures at the crash site (perhaps a half dozen floors) buckled due to the intense heat, effectively dropping the available yield strength of the steel, dynamic loads due to gravity were introduced onto otherwise undamaged pillars beneath them.
The simple rule in dynamic loading says that the added forces of a moving load, at minimum, doubles the load. Thus a load which falls from only 1 mm, and never has opportunity to accelerate, STILL doubles the load. On top of this doubling, we must add the dynamic component of the weight falling roughly 10 feet at a time.
Think about hammering a nail into a 2 by 4. Lay a 20 oz framing hammer on a nail and nothing happens. Up it to a 16 lb sledge, and still we see only an indent in the wood fibers. Now swing away, and you get a feel for how much multiplication of load forces which existed as each successive level in the Trade Towers gave way.
Did they slow things down a little? Yes, a little, almost imperceptively.