Questions about the collapse and then the global collapse

Undesired Walrus

Penultimate Amazing
Undesired Walrus
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Apr 10, 2007
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Hey.
When the towers were hit, destroying those floors and so forth, I believe the towers ridistrubuted the weight from the top section to around the point of impact right?

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So, if the fires had burnt out instantly, would the building have survived? Or is it that the fires spread out over all the floor trusses that were undamaged, making ridistrubution impossible?
 
Undesired Walrus said:
Hey.
When the towers were hit, destroying those floors and so forth, I believe the towers ridistrubuted the weight from the top section to around the point of impact right?

http://static.stripgenerator.com/generated/anonymous/strip/2007/05/13/untitled-23.png

So, if the fires had burnt out instantly, would the building have survived? Or is it that the fires spread out over all the floor trusses that were undamaged, making ridistrubution impossible?
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I always get a bit leery when people say 'redistributed' as it conjures up an image in my mind of some kind of intelligent re-routing of the loads away from the damaged section.

Of course what happened is that something which was supported by 'x' number of columns was suddenly only supported by 'y' number of supports and that those remaining supports had to bear a greater load.

The design of the structure allowed for this, up to a point. But it still meant that the remaining structure was stressed closer to it's maximum ability.

So, add to that mix the effect of fire and the damage to the fireproofing material on the remaining structure and you start to have a scenario where catastrophic failure can occur.

And it did.
 
The NIST report describes in detail how they predict loads were "redistributed," both as a function of core vs. perimeter columns, damaged core vs. undamaged core members, and actual suspension via the hat truss. They also show how this redistribution changed over time with the effects of the fires and consequential additional loads thanks to the first heated and then cooled floors.

If you accept the NIST models -- which are not without some points of debate but widely held to be quite plausible -- then you will conclude that, had the fires been extinguished quickly, the Towers would not have fallen. They would have been damaged beyond any hope of repair and highly vulnerable to wind or reignition, but there would have been no collapse.

Since the Towers each stood for a significant period after impact, had there been no fire, it is a given that they would have stood longer still.
 
This seems to be a very popular line of questioning with some twoofers, they seem really puzzled as to what happened after the towers started to collapse.
 
R.Mackey said:
The NIST report describes in detail how they predict loads were "redistributed," both as a function of core vs. perimeter columns, damaged core vs. undamaged core members, and actual suspension via the hat truss. They also show how this redistribution changed over time with the effects of the fires and consequential additional loads thanks to the first heated and then cooled floors.
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I think it is also important to point out that the geometry of the structure was changing post-impact as members were increasingly loaded. Some of this was due to the fire changing the response of the materials to load, and some was due to over-loading, but once the structural members began to be transversely loaded (loading that isn't along the long axis) due to deformation, the initial design analysis of the tower's structure went out the window.
 
R.Mackey said:
If you accept the NIST models -- which are not without some points of debate but widely held to be quite plausible -- then you will conclude that, had the fires been extinguished quickly, the Towers would not have fallen. They would have been damaged beyond any hope of repair and highly vulnerable to wind or reignition, but there would have been no collapse.

Since the Towers each stood for a significant period after impact, had there been no fire, it is a given that they would have stood longer still.
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I think it's clear they would have stayed up for some time longer, but it's hard to say how long. As mentioned, wind loading (and probably other factors) would be a constant, and constantly changing, problem. It's possible that an usually strong gust of wind, or a gust from a different direction, might have added extra load to one element that was already near failure. Failure of that component could then lead to more failures, and a possible collapse.

But of course, figuring out probabilites for these events is probably near impossible.
 
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Bear in mind the OP proposes the hypothetical in which the fires are instantly extinguished, in other words you're talking purely a kinetic impact damage model, no fire, no heat.

-Gumboot
 
The building probably would have survived so long as there were no large wind events.

Fyi, for a building that is only partially enclosed (like after the plane hit), the pressure from wind is actually much higher than if it is completly enclosed.
 
Just to clarify that further: we design buildings with the wind loads hitting the facades. Big holes mean they hit other bits, and we may not have designed the buildings for this. We also assume that wind loads will be fairly uniformly distributed, which again may not be the case if you've got it whistling in a whopping huge hole.
 
So.. How did the damage from the hole benefit the collapse? Would the same same quantity of fuel and fire alone, without a hole, not be enough to collapse the building?

I must make it clear I am sounding like a truther.. apologies.. Im just interested
 
Architect said:
However Edinburgh University and Ove Arup disagree with you, and argue that fire alone could have done it.
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really? with no fuel source other than office contents? or are they assuming 10000 gallons of fuel exploded in the building and started the fire?
 
No, that's the interesting thing. They both believe that any major fire might have resulted in structural failure. It's not a widely held point of view, but it comes from 2 well respected sources. What amazes me is that the CTers have never really grasped it.
 
Architect said:
No, that's the interesting thing. They both believe that any major fire might have resulted in structural failure. It's not a widely held point of view, but it comes from 2 well respected sources. What amazes me is that the CTers have never really grasped it.
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What about the 1975 fire in Tower 1? Do they compare the severity of their hypothetical fire to this historical example? According to High Rise: Fire & Life Safety, "The heat damaged structural supports for the 12th floor and caused the failure of the exterior windows on the east wall of the building." (p. 37) This sounds like a pretty serious fire to me.
 
It's been a wee while since I read up on that one, but in actual fact I think that the Arup and Edinburgh argument centres around a fire over a much larger proprtion of the floor plate. I'll find you a link.
 
Wow, i had never heard that, but suspected it. The floor sag from heat would be the key factor, especially if localized to one side of the building. I'm guessing they believe uneven stress on the core may have been enough to cause a collapse?
 
Architect said:
It's been a wee while since I read up on that one, but in actual fact I think that the Arup and Edinburgh argument centres around a fire over a much larger proprtion of the floor plate. I'll find you a link.
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Interesting. The 1975 fire was mostly limited to 1/4 of the 11th floor with some limited vertical spreading (9th thru 16th floors had some damage). Later on (p. 47), O'Hagan (the author) says "If the alarm had been delayed an additional 30 to 60 minutes, it is reasonable to expect that the entire east half of the building could have been involved." Perhaps this is the sort of fire that Arup & Edinburgh were contemplating, rather than one comparable to the actual fire.
 
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