In reply to Sircumfrins:
> Now, because the towers have collapsed an explanation to why the buildings collapsed needs to be found.
> This is where alot of assumptions are made which I believe are flawed.
Quite.
> I don't believe that the fires were intense enough to weaken the steel.
Well that settles that then. I'd suggest some research might be more appropriate than relying on gut feelings in this matter but I'd be wasting my time.
> The reasons I say this is because people were standing in the gaping hole and if the fires were that intense they would have died.
They did die.
Clearly the fire was not uniformly distributed, that does not mean it was not intense enough locally to weaken the structure.
> The firefighters clearly stated that they could subdue the fires with 1 or 2 lines. There was more smoke pouring out the building than flames.
People make mistakes. Also, it's possible they could subdue the fires they could access, they may not have been the fires doing the harm or they may have been remnants of the fire that had already done the harm. Given the situation I'm inclined toward believing this is a mistake or a misquote. Either way it's indicative of little.
> If the steel was weakened then it wouldn't be uniform (as obviously there is heat variation)
Correct
> ...and so the collapse would have started at the area of least resistance and continued falling in that direction.
Also sort of correct. Imagine the central tube fails unevenly, the upper building lists toward the failure applying intolerable stresses to the other side of the tube which subsequently also fails. At this point the upper building is essentially unsupported and rotating slowly (toppling). The key thing is it's rotating slowly and the torque that initiated the rotation has been removed, it's accelerating rapidly downwards. As the mass of the falling upper structure impacts the lower structure the forces are enormous and the structure is destroyed, the collapse continues rapidly downwards. The upper section is still rotating but in the time it takes to fall it's not actually rotated very much at all. Tall thin buildings become basically a conical heap of wreckage.
> This would have mean't that a section of the building would have tipped towards this side and (perhaps) collapsed. This would have been a partial collapse.
Almost right.
I imagine you don't care and there's nothing I can do to convince you of this but I can propose a simple experiment that might help.
Stand on an empty coke tin, standing straight on flat ground it'll support your weight. Tap the side with a stick and that side will buckle, you will fall 'through the structure', 'into the footprint', however you want to think of it almost un-opposed by the residual resistance of the damaged can. To all intents and purposes you 'freefall'. Now check out the shape of the can, did it topple or did it collapse axially? You might need to repeat the experiment a few times, it doesn't work perfectly every time not least because it's hard to balance and tap. You tap one side, it fails on one side initially yet you don't topple like a tree.
> MG: Your points 2 & 3 are conjecture...you have to admit this. You've assumed the fire was hot enough and therefore the collapse was inevitable.
It's easy to think of steel as something solid, rigid, impervious. That's understandable but it's not the reality, that's only the reality at the pressures and temperatures we normally apply to steel. Heat it up and (or) apply greater pressures then it's better thought of as something more akin to a thermoplastic. Soft and malleable.
Understand this and the 'kerosene can't melt steel' argument becomes irrelevant.
> You simply can't get me to believe that the failure of a few floors, of the upper third of these buildings, would fall through the lower two third of the undamaged building...at near free fall speed.
You can take a horse to water...
> Why would the lower two thirds of the building suddenly not have the capacity to hold the upper on third of the building up? What had it been doing for the last 30 years?
> To believe the upper third of each of the buildings could crush through the intact two thirds of the lower building is absolutely ludicrous!
Put a large hammer on your hand. Your hand supports it easily, right. Now drop the large hammer 10m onto your hand.... Yeah, the white bits are bone. But your hand can support the hammer, right?
To believe you could drop quarter of a million tons even an inch onto an existing tower structure and expect either part to survive in any meaningful way is what's ludicrous.
Please do do the can experiment, it's really quite enlightening. Sky scrapers are basically big tin cans, adequately strong while intact. For the avoidance of lawsuits: Please don't do the hammer experiment.
jk