All of the descriptions I have seen on TV and read in the paper seem to indicate that the two 110-story towers collapsed.
I once read that, in 1945, a B-29 crashed into the side of the Empire State Building (completed in 1930) and, while it did considerable damage to that part of the building, the rest of the Empire State Building survived.
So my question is: Was the framework of the WTC buildings somewhat more brittle than the ESB, or was the airplane’s mass and speed sufficient to cause even a sturdy steel framework to topple?
believe the WTC was as strong if not stronger than the Empire State Building.
The reason the WTC collapsed and the ESB did not was due to the melting steel caused by the high temperatures induced by the fires.
The steel used to build the WTC has a coating on it that protects the steel from high temperatures for an x numbers of hours. I think around two.
This is why the buildings did not collapse immediately.
The Empire State Building, a structure from the 1930s, was constructed of iron and limestone (from good ol’ Indiana). The mass of the building is HUGE! The WTC towers were constructed in the modern skyscraper fasion: A steel framework on which is hung a glass skin. The floors are concrete, but the rest is fairly light. The impact of the B-25 in 1945 was absorbed by the sheer mass of the ESB. The twin towers survived the initial impacts because they were flexible (the designer was quoted as saying he designed the buildings to withstand 200 mph winds). Eyewitness accounts indicate that the first tower swayed six feet or more when it was hit. The collapse was a result of the fire weakened steel columns(remember that the entire top of the building was held up by many fewer columns, taking into account those cut by the plane)buckling and allowing the concrete floors to pancake into lower floors…dominos. The insulated steel beams are designed to hold their shape for 1 to 2 hours. Allowing evacuation. I think the buildings performed admirably.
I don’t know how the damage would have varied between the two structure types given the same type of crash, but structurally the outer shell of the WTC bore a lot of the weight of it’s structure (that’s why they’re so monolithic) whereas the internal skeleton of the ESB bore the weight of IT’s structure. On the WAG side, another difference may be that since a building like the ESB tapers as you go up, there would have been less weight bearing down on the weakened parts. So I don’t think it’s as much a matter of brittleness as much as it is a matter of construction style.
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- Does anybody know what the “outer” floor dimensions of the WTC were? I can find the floors/height all over, but was curious as to the aspect ratio and can’t find anywhere now that gives the base width. - MC
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Quite probably but this is not my area of expertise. Another thing is that the ESB wasn’t hit nearly as hard.
My guess is that you got this from the BBC. I have seen this sort of thing kicking around for the last couple of days. They try hard but they are wrong on a few counts. They say “Fire Reached 800 deg. C-Hot enough to melt steel supports”. One problem is that steel doesn’t melt until at least 1500 deg. C. See below.
The steel is encased in concrete which is a relatively good insulator. Even without the concrete the structure would have withstood for some time until the structural beams had time to come up to a critical temperature.
The steel did not melt. Steel behaves somewhat like taffy when the temperature is increased. It will go “soft” and lose structural integrity long before the melting temperature is reached. Since the steel was insulated by the surrounding concrete its internal temperature would have been significantly lower than that of the fire. The building collapsed not from the initial shock or other mechanical pressure (winds etc.) but from the temperature as has been mentioned. The WTC took the initial impact of the plane(which is made of relatively flimsy aluminum). The working theory is that the impact caused the sprinkler system to fail allowing the fire to burn out of control forming its own furnace. This heated the structure to a high enough temperature to reach a plastic deformation failure mode in steel. From that point the awesome mass of the structure was enough to bring it down.
For a good graphic and explanation see the USA Today site on this. USA Today lists the temperature as "2000 deg. F which is right around 1000 deg. C–still well under the melting point of steel.
BTW the rough floor dimensions are given on the USA Today Site.
I’m a Mech E, but had enough materials science classes to almost be a Met E. I agree with everything you have said in here, but the fire could have been hotter, depending on the drafting involved. I’ve got to 2000 F in a wood fireplace by using a small bellows (my Omega thermocouple meter wouldn’t go any higher than that, so it may have been hotter).
The differences in architechture have been addressed in this and at least one other thread. And it’s been noted that the hijacked jetliners were loaded with fuel for transcontinental flights, thus making them effective flying molotov cocktails, with the ensuing fire contributing to the collapse.
Another factor, which is slight compared to others, is that, compared to a 757 or a 767, a B-25 is a small airplane.
It’s velocity was probably quite a bit slower, too. Doesn’t the force increase as a square of the speed? (Complete layman here.)
The kinetic energy does: KE = 1/2(mass)(velocity[sup]2[/sup])