On 20/09/18 12:06, z9wahqvh wrote:
posit a mechanism for building collapse that [..] has never been seen before or since, despite the fact that planes do crash into buildings and fires do happen in buildings pretty frequently.
Planes. A B-25 crashed into the Empire State building in the 1940's, but that was about 10 tons flying at about 150 mph, versus 150 tons at 500 mph. A cargo 747 crashed into a 10 story apartment building in Amsterdam in 1992 [1]. It was flying straight down when it hit. IIrc 40-odd people were killed. The building was made of reinforced concrete, and the part which was hit was demolished down to the ground by the impact, but the fire was put out quickly and most of the rest of the building was OK. An Air France Concord crashed into the ground then hit a three-story hotel, killing 4 on the ground and all on the plane. Then there was the Pentagon, and there are are some cases of large aircraft hitting houses and light aircraft crashing into various buildings. But apart from WTC there have been no incidents where a large aircraft has crashed into a high-rise, skyscraper or large steel framed building, either before or since. [1] I was in Amsterdam and heard the crash - we were warned on short-wave radio to stay inside because there might have been nerve gas in the plane. The ordinary radio said all was well. Didn't know whether to be scared or not. Fires. WTC1+2 were built of sprayed fire resistant material coated steel (SFRM/S) structure to a lightweight-framed-tube design. Very few buildings like them were ever built, the lightweight-framed-tube design went out of fashion in the late 70's/ 80's. SFRM/S is still used sometimes though, if you want cheap - although if eg the chance of war damage is high then it would be excluded. That's partly why the Burj Khalifa is built of reinforced concrete. In most cases of fires in high-rise buildings, the building is made of reinforced concrete, not SFRM/S. The concrete protects the steel much better against heat than SFRM. In general SFRM/S buildings do not collapse in actual fires, though if the fire lasts for a long time in one place they can be badly damaged, come close to collapse, or even collapse. Most fires are progressive, and in high-rises very often spread is upward by external means, the flames outside the building setting the downwind parts of higher stories aflame rather then whole floors being on fire. Most fires are fought by firemen, who tend to start at the bottom and work up - so while a fire may last several hours, an individual part of the building is unlikely to be on fire the whole of that time. Also, WTC had no sprinklers. Modern buildings have them as standard, and many are refitted with them. Even when they fail to control the spread of a fire, they lower the temperature. Apart from WTC: There are no other cases of large fires in high rises where the fire spread so quickly. There are no other cases of fires in SFRM/S buildings which lasted for more than four hours in one place. There are few other cases of fires in SFRM/S high-rises in which an entire floor was on fire. Most often only one side of the building burns, the fire spreading up the outside of the windward side. Of these few cases, One Meridian Plaza springs to mind - the SFRM there was rated for four hours, not the usual three. The fire lasted for 11 hours, though not at full intensity in any one place - it covered nine floors, and was least fought by firefighters. Even so, the building came very close to collapsing, and later had to be demolished (by disassembly, not explosives). I know of no other cases where there was a large fire in a SFRM/S high-rise and no firemen to fight it. I know of no other cases of large fires in high rises which were preceded by such extensive mechanical damage. I expect there are examples of the last two, in war situations or the like. If anyone knows of any could they let me know please. WTC1+2. WTC1+2 collapsed because they were built of unusual materials (sprayed fire resistant material/steel), and to an unusual design (lightweight framed-tube). One might expect the method of collapse to be unusual. If they had been made of reinforced concrete, to a traditional design, they wouldn't have collapsed. They wouldn't have been so tall though, and they would have been more expensive to build. If they had been made to a traditional steel-framed design, even with SFRM/S, it is unlikely that they would have collapsed, and certainly not in that way. But then they would have used twice as much steel, and been twice as expensive. WTC7. WTC7 fell because there were no firemen to put the blaze out. The mechanical damage might have provided a point of first failure, but it wasn't really significant, the building would have collapsed anyway. There probably weren't enough firemen in Manhattan to fight the initial five major fires in WTC7, even if they hadn't been busy with WTC1+2 - and then there weren't enough firemen as they were either dead or busy rescuing people elsewhere. The rescue firemen stayed until 2:30 or so, but there never was any significant attempt to actually fight the fires in WTC7. They were too big, there weren't enough firemen, plus the water mains had been destroyed by the collapse of WTC1+2. There wasn't anything the fire department could do to fight the fires. So the fires went unfought and merged into a single blaze - and guess what, a building which was rated for 3 hours structural fire resistance lasted on fire for 7 hours hours before collapsing. - Peter Fairbrother