On 20/09/18 03:35, juan wrote:
so yes, a controlled demolition.
If you are talking about QTC1 and WTC2, while at first glance the collapses may look like explosive demolitions, if you look closely they are very different. (I think you mean explosive demolition, or perhaps building implosion - a controlled demolition is just a demolition which is done to a plan, manual demolitions are supposed to be controlled too. "Controlled Demolitions" are also a large firm) First, let's look at some real explosive demolitions of large-ish structures. There are many ways to do explosive demolitions and implosions, and they are often computer-modelled to a fare-thee-well these days, but here's a start. If you have a tallish large building and you want it to implode into it's own footprint, in the the modern classic implosion method, you start by weakening it with pneumatic hammers and chisels. In the actual explosion you might first cut up two sides, but leaving some horizontal strength so that as they collapse downwards they pull the other two sides inwards: then once those walls are in motion, cut the bottoms of them - but leaving the rest of them uncut so they have enough strength to lean inwards as a whole. In general, you leave some strength in the bits which are falling, so they can drag not-yet-falling bits with them. Another common technique is where a large structure is first weakened with high explosives and/or linear charges to cut some steel and concrete columns and beams; then a few milliseconds or seconds later low explosives are used to move the remaining structure away and to eg ensure the cut ends of the columns move in the direction you want them to. From outside you hear first the cutting charges, crack-crack-crack, then the shifting charges, boom-boom-boom. Though they tend to let them all off at once nowadays, which I think is a pity. The cutting charges can be distributed through the building, but mostly they are at the bottom - shifting charges are almost entirely at the bottom. There is a variation on this theme where the building is divided into two or three horizontal parts, and modern classic is used on each part. This is done for safety, the preweakening with chisels etc which is normally done being judged too dangerous (or too time-consuming). Also it can limit spread. One characteristic of the modern classic method is that the bottoms of the buildings are always cut, which means that the lower stories begin to fall at the same time as the upper stories. There are other methods, but the only one I know of where the bottom stories don't start to fall at the same time as the upper stories is what I might call the chop-it-in-half method. It tends to be used on 10-15 story buildings with one longer side. It does have a few advantages; it uses less explosives, it is quick and simple to set up, and it is fairly easy to ensure that most of the rubble falls to one side. You chop off 5 stories by blowing a vertical slice in one long side of the building 5 stories down from the top - this then collapses, and the top 5 stories fall as a lump to that side. You can do this twice in one go, so chopping off ten stories. You tend to be left with a pile of rubble that's five stories high, but that can be dealt with in non-explosive ways (and the rubble has to be dealt with anyway). So, what do we see in the WTC1-2 collapses? In WTC2 there are fires and damage about 20 stories down from the top. The fire seems to cover an entire floor. One corner of the building's shell is badly damaged. The top 20 stories tilt and drop a little, giving off dust/smoke, and then fall vertically down on through the remaining stories in a ball of dust or smoke. The bottom stories remain intact and in place until the falling top stories hit them. Some vertical core structure remains for a few seconds. (the dust and smoke as the top tilts is not the result of explosives, too slow, wrong velocity profile. It's just shake) In some videos we can see the dust and smoke coming out of the building in puffs, floor by floor, though it doesn't come out from all sides simultaneously. This may look superficially like the result of highly-timed [2] and controlled sequence of explosions, but it comes out too slowly to be the result of explosives and is almost certainly caused by the floors collapsing under the weight of the top 20 stories falling on them, pushing out the air which was in them, along with concrete dust, paper, etc into the ball of dust. (the concrete floors of the top 20 stories weighed 12,500 tons - each lower floor was rated for a static load of 1,300 tons, and would have had absolutely no chance whatsoever of stopping the moving weight from above) In WTC1 there is a fire and damage about ten stories down. The fire seems to cover more than one story. The top ten stories then twist slightly, and fall. The building collapses in a ball of dust and smoke, from the top downwards. It lasted longer than WTC2 probably because the weight above the fire was only about half the weight at WTC2. Again some core remains temporarily. So, none of the usual overall implosion or explosive demolition techniques were used on WTC. Having a shortish top part fall down onto a taller bottom part and then breaking it is occasionally used for chimneys or some brick-built structures, but not for large or steel framed buildings. There is no evidence from the videos of the use of explosives. Nor do we hear any detonations. So, if there were any explosives, where were they placed? How were they placed, it would take months with full access to prepare the WTC for demolition? What sort of explosives? When did they go off? What were they supposed to *do*? If a traditional demolition or implosion, they didn't work - there wasn't one. If they were supposed to weaken the lower part of the building, why? 12,500 tons moving vs 1,300 tons static, no chance whatsoever. No point. Any competent structural engineer would have told them that [1], and they would have needed one to successfully use any explosives. If they were supposed to weaken the floors which were on fire, well either they guessed which floors would be hit pretty accurately or they weakened maybe 20 floors in total. And again, to absolutely no point. The impact and fire would start the job. And gravity, mass and momentum would finish it. -- Peter Fairbrother [2] almost impossibly timed, each would have to be set off individually in individual sections, you would have to know which corner of the top lump was falling first, and it isn't a technique used in demolitions anyway. [1] It has been said that OBL hired an engineer before 9/11, who told him the towers would collapse if hit as they were.