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To start this topic of mine, I will go with something topical and much refuted; the collapse of the World Trade Centre Buildings 1 and 2.

Basic Design




Each building had to be able to withstand hurricane force winds and as such had to be designed to withstand 2kPa (about 5000t) of horizontal force. To allow each tower to withstand this wind load, the architects selected a lightweight “perimeter tube” design made up of 244 exterior columns of 36 cm square steel box sections on 100cm apart. This allowed windows more than one-half meter wide. Inside was a 27m × 40m core, designed to support the weight of the tower. It also housed the elevators and internal staircases. Joists 80cm tall connected the core to the perimeter at each story. Concrete slabs were poured over these joists to form the floors. In essence, the building was a hollow construction that was about 95% air, which is why the rubble after the collapse was only a few stories high.

The floor trusses themselves were designed with office space in mind, spanning the full 18m from the core to the perimeter, with a depth of only about 70cm - allowing more storeys. This prevented load bearing walls and columns from becoming obstructions on the basic floor space.


Impact

The buildings did not fall because they were hit by planes. As stated above, they could take 5000 tonnes of horizontal force, the impact of the planes gave just over 1/50 of this. Because of the redundant (it's ability to shift loads to load bearing elements in the event of the failure of others) nature of the design, both buildings stayed standing after the initial impact.

But these planes weren't fired out of a sling shot, they weren't empty husks of dead load. At least a third of their weight was pure, flammable jet fuel. And each plane broke through onto at least six seperate floors, which would have created a violent inferno across at least half of the office floor space of each. The impact itself also damaged the fire protection on the inner core columns, not to mention destroying serveral floor trusses and as much as a third of the perimeter columns on one side.

Fire

The temparature of the fires caused were normal, around about 800C. Not high enough to come close to melting steel. But it would have softened at around 450C and loses half of it's load-bearing strength at 650C. But I'm getting ahead of myself.

The fire protection on the columns would have held off the fire for a number of hours, allowing most people to escape the building. That's what it was designed for. What the designers didn't count on was a blast as large as an exploding plane occuring within an office, they also don't count on a fire being created across such a large area of floor space over a number of floors. Thus the initial blast removed the protection on the columns and then there was already a powerful office fire there when they were open to the elements.

And so with fires raging at temparatures of around 800C at the open steel reinforcement of columns on the inside and outside of the structure, it was only a matter of time before the steel began to soften and lose a large proportion of its strenth.

Collapse

Each floor was designed to take 700Pa and so would have been able to take 1,300 tonnes in excess of its own weight. The fire would have been causing distortions in all surrounding steel sections, the interior and exterior columns and the trusses themselves. The outer perimeter columns around the impact hole were already taking excess weight from those destroyed by the blast. Collapse was inevtable.

There are a number of reasons why one the storeys could have failed. The buckling of a remaining floor truss, or column. Any of the joists holding the floor together could have failed, it doesn't matter which. Because once one floor fell, the floor below had take the weight and gained momentum of all the floors above. Which was impossible.

If all the floors below the impact hole had suddenly disappeared, the remaining floors below would have taken nine seconds to hit the ground. As it was, the resistance from the building caused it to take two seconds longer. The building did not topple because there were no lateral loads acting on it, only gravity. And a 500,000 tonne building can only really fall one way.


I will make no claims as to who was behind it or why. Or who had what to gain or what people used the disaster to accomplish. I will simply give the engineering facts of the collapse. I am open to any and all discussion about how Buildings 1 & 2 fell (perhaps a little digression on building 7). Leave all your whos and whys at the door on this one guys.

Well I wouldn't say shoddy (they were one the first buildings to be designed to survive an impact of an airplane (I think a 707)) And I think was Gypsy Board not plasterboard and yes reinforced concrete would have been a much better protection but what I'm trying to say is the buildings (as in the structual parts of the building were built very well just the materials and the techniques designed to protect it were (I hate to say) shoddy.

I would say shoddy. Had they been proposed in London, they would NOT have been built. They were not safe enough. They were designed to withstand the largest plane of the day, moving very slowly with most of its fuel burnt. They should have been designed to withstand a larger plane, since it's bloody obvious they're always going to build larger ones.

Man you just keep saying london but back to my point were any buildings in london designed to take a hit from jetliner or are there any tall buildings in london? (I really don't know) so what would london know of buildings being hit by jetliners. (no offense ment) and also they as you said designed it to be hit by a smaller plane right? well they survived the impact so they were actually slighty overbuilt. main problem with them is fireproofing which I admit wasn't the best but the overall structure took and survied a hit but all that fuel got hot enough to get steel not melted but at something like 50% strength and thats the problem not the structual integerty.
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Yes, I keep saying London. That's because the safety regulations here are so much more stringent. And yes, there ARE some tall buildings in London. Nowhere near as tall as in New York, but that's mainly because the ground wouldn't take it. Just because there's very little chance of an aeroplane hitting a building here does not mean that engineers (the same the world over, y'know) do't know how to deal with it.

The structure "survived a hit", did it? Rubbish. If it survived the hit, the buildings would still be standing. Going back to the London building regulations, if they had been built, they woud have been to a higher standard, and would stil be standing. This is my point. Evidently, considering that - we know more than you about buildings being hit by planes.

I would like to know why the fuel exploded twice...It woul;d ignite correct? But it would take long for almost all the fuel to be ignite, so why did each plain explode more than once?

Seriouisly if there is a specific dynamic to this I'd like to know.

I'm also wondering why the building fell almost perfectly straight down...

Because demoltion teams are trained to make that happen, just blowing a hole in a building at ant point doesn't cause straight fall. It takes years of training to understand exactly how to make a building go down as clean as that one did...So to me it looks either VERY well planned or the worlds most extrodinary coincidence...TWICE.

Oh and the one building that was admittedly demolished already had the demolition equipment in it...which seem a little pre-planned...

To me I guess the one thing I've seen missing is enough explination about what happened.

Nice bit of info at the top, by the way.

They survived the hit but then the fuel got fires going and huge amounts of heat weakend the steel thats what killed the towers. not the actual hit, if the planes had been the same weight but without fuel the towers would still be here

Puffs of Smoke Prove Controlled Demolition

The planes struck the buildings and took out dozens of their perimeter columns from several floors and they continued to stand. The above image shows two of the puffs of debris emerging from the tower (I'm not sure which one), each is the width of, at most, two windows. Meaning that three perimeter columns have just been demolished, tops. What possible effect would this have on an already doomed, collapsing building?

But maybe the explosions are from the inner core and have just reached the perimeter? Okay, so that would mean an explosion strong enough to take out a reinforced concrete column has travelled through 60 feet of office spac, but only has enough force to take out two windows? Common sense should cover that one.

So what are they?

Okay, you have 100,000 tonnes of rubble falling onto remaining floors. Everything is shaking. The perimeter columns and those of the inner core and being pounded again and again as subsequent floors give way. This compresses them at such a rate that the air within rooms and hallways becomes compressed - not having time to creep out underneath doors etc. This compressed air, mixed with the increased pressure imposed on the windows frames means that the glass just gives way and the air simply pops out of the window, along with any dust and small rubble caused by the constant structural movement.