Some Calculations requested regarding why the towers upper portion did not slide off

[Newtons Bit]

To answer your question, I have trouble seeing how a structure with vertical support columns can fail "floor by floor" because the vertical impact force of a falling "upper block" will be absorbed by the columns as a whole - not on a "floor by floor", progressive basis. But any examples would be appreciated.

Follow the link in my signature. I calculated the resistance of an entire floors worth of columns. It's less than 10% of the total energy provided by a traditional upper block striking the lower block. The incidental resistance of the columns on the floors below can be calculated as well with the same procedure. The crux here is two things: one, what is the effect of vibration, it is significant, and the columns below can only be loaded to a certain degree before they too buckle.

 

[Furcifer]

Yah, I hate that term as well. As I have stated before, a pancake collapse is a term for what the collapse looks like AFTERWARDS. The floors remain intact and pile up like a stack of silver dollar pancakes at IHOP. This is common in smaller, concrete and steel reinforced buildings. For those of us familiar with this we know how to distinguish between a pancake collapse and pancake theory.

I'm looking at a "pile driver onto a small stack of pancakes collpase, that ultimately destroys the pancakes in the process" collapse. In this, it would seem that the small pile of pancakes may have surfed the wave just in front of the pile driver. Does this clarify things?

 

[lozenge124]

Well, let's see. The Pancake Theory, as described by Fema, is a description of how collapse STARTED. Particularly, that the thermal expansion of the floors caused them to disconnect from their connections.

Er, a floor's connections fail and the floor thus begins to fall - the collapse has initiated yet there has been no collision yet with the lower floor, so there has been no pancaking yet. "Pancaking" requires more than 1-floor, in this theory it refers to the failure mechanism AFTER initiation as the initial floor pancakes onto the next causing it to fail, and so on in cascade fashion. Pancaking is not a description of how the collapse started but how it progressed once started.

 

[lozenge124]

Follow the link in my signature. I calculated the resistance of an entire floors worth of columns. It's less than 10% of the total energy provided by a traditional upper block striking the lower block. The incidental resistance of the columns on the floors below can be calculated as well with the same procedure. The crux here is two things: one, what is the effect of vibration, it is significant, and the columns below can only be loaded to a certain degree before they too buckle.

I've seen your calculations and have also read Greening's "ENERGY TRANSFER IN THE WTC COLLAPSE". But they fail to convince me because they do not explain how the energy of the upper block impact is concentrated in the columns directly below the impact point without being transferred further on down. If you push down on a straw until it fails for example, it won't buckle directly below the top - in fact you cannot tell where it will buckle. Same if you drop a massive weight on a crane, you don't expect each "floor" or section to fail one after the other with no damage further down - the whole structure will absorb the impact as a whole because the support columns are in the same vertical axis as gravity.

I agree with Gordon Ross on this one:

Dr. Bazant argues that all of the potential energy associated with the fall of the upper section of the tower through two storey heights [the storey where the failure occurred and the uppermost storey in the lower section] would be concentrated into the destruction of the uppermost storey of the lower section. This energy would overwhelm the ability of the columns of the uppermost storey to absorb energy and collapse would progress through this storey. The available energy would then be concentrated into the next storey down and the tower would collapse one storey at a time to ground level.

The first error which Dr. Bazant has made is his assumption that all of the available energy would be utilised exclusively in the destruction of the uppermost storey of the lower section. This is physically impossible under any and all circumstances

Energy availability is a necessary condition for a crush down scenario, but not a sufficient one - the energy still has to be directed in the right directions to the right spots.

 

[Furcifer]

NO. A pancake collapse is a form of progressive collapse. I mean by definition that is. I don't mean to be difficult, but this is why people get confused. (and confuse me)

What if you consider the steel in the WTC was tapered, getting thicker as you go down. Does this change how you see the failure of the floors directly beneath the falling upper section? I imagine it like a pyramid getting crushed.

I don't agree with Gordo on that for one key reason. I think the lower section was more compromised by the falling debris than he assumes. This to me means the relative strengths of the upper and lower sections favour the upper section. At least for a period of time denoted by the freestanding core.

 

[lozenge124]

It would in the static case, but not in the dynamic case lozenge. Once the upper section started moving you need to consider how the energy is being transmitted componentwise.

I agree, but I think the only way to do that is through a FEA model and visualizations.
NIST didn't provide this, the Bazant & Greening style models simply assume that "energy is dissipated only at the crushing front" before developing their math; so what source do I go to to "consider how the energy is being transmitted componentwise"?

 

[Furcifer]

I agree, but I think the only way to do that is through a FEA model and visualizations.
NIST didn't provide this, the Bazant & Greening style models simply assume that "energy is dissipated only at the crushing front" before developing their math; so what source do I go to to "consider how the energy is being transmitted componentwise"?

NewtonsBit.

Since this was not an ideal collision (we have millions of small collisons happening at an angle) I'm visualizing everything crumbling in the core, while the exterior falls away after being detached from the truss connections. I say you end up with the upper section basically crushing the core, the floors taking care of themselves in a, dare i say pancake collapse, and the exterior basically falling away due to gravity. I think this also accounts for the speed of the collapse as well.

 

[Corsair 115]

Do you have any examples of such a total progressive collapse?

I could counter that by asking if you have any other examples of major office towers which were struck by large passenger jetliners moving at a high rate of speed and carrying a large fuel load.

To answer your question, I have trouble seeing how a structure with vertical support columns can fail "floor by floor" because the vertical impact force of a falling "upper block" will be absorbed by the columns as a whole - not on a "floor by floor", progressive basis. But any examples would be appreciated.

Given the sheer amount of mass which began moving in each WTC collapse, I cannot see how anyone would expect anything but a complete and total collapse.

 

[Furcifer]

I pushed this model of the collapse over at PhysOrg, but was shut down due to two things. I think this model ends up with more than 80% of the debris outside the footprint. This goes against what the big boys are saying. Who am I to argue this without making some serious calculations of my own ( I would have to work backwards from the conical pile of debris distribution from the FEMA report). More importantly, they say the inertial mass of the floors would slow the floor collapse (pancake) to such an extent that the upper section would "catch up" and ultimately become part of the crushing zone B. Until I get off my butt and prove otherwise I'm trumped on this.

 

[Furcifer]

Given the sheer amount of mass which began moving in each WTC collapse, I cannot see how anyone would expect anything but a complete and total collapse.

Yep, and Greening showed this rather conclusively IMO. The design of the building also meant that the support for the upper section was prettty much the core. If we extend that to the picture lozenge posted above, we see why it was "total" and not "partial".

Incidentally, I hate working on those suspended patios like we see in that picture, they make me nervous. And that picture didn't help either!

 

[Sword_Of_Truth]

The problem is, sir, there is no evidence to support the position.

Just a reminder; the person saying this claims that death rays from space vaporized the World Trade Center.

 

[lozenge124]

NewtonsBit.

Since this was not an ideal collision (we have millions of small collisons happening at an angle) I'm visualizing everything crumbling in the core, while the exterior falls away after being detached from the truss connections. I say you end up with the upper section basically crushing the core, the floors taking care of themselves in a, dare i say pancake collapse, and the exterior basically falling away due to gravity. I think this also accounts for the speed of the collapse as well.

Well that's great, but again, an FEA model is needed to see if your intuition is right. (I thought you boys were all about peer reviewed papers, not just personal opinions...)

On a side note, we had a debate at physorg about whether the perimeter floors would just "fall over" absent the floors, because this seems to be a common misconception of OCTers - that without the floors and core, the perimeter structure of the WTC would have just collapsed under it's own weight, as you appear to believe.

This section from the book "City in the Sky" makes clear that this was not the case at all, the outer walls.made of the perimeter columns held together by the spandrels was the main support structure of the WTC.
[QUOTE="City in the Sky" (Glanz/Lipton)]Like the horizontal lengths of wood that connect the staves, or vertical pieces, of a picket fence, deep steel plates called spandrels would run from one perimeter column to the next to give the building stiffness and let it resist the lateral force of the wind. Traditional buildings relied on the combined strength of their perimeter columns as well as everything inside them to resist the wind - from stiff masonry stair shafts, to the zigzagging steel cross-bracing and riveted connections of structural skeletons, to the great mass of the stone facades. There was a certain inefficiency here: using the relatively narrow interior of the building for stiffness was something like trying to resist a sideways push with feet spread. Much more efficient. In a building, less stell would be required for the same stiffness if the perimeter became the main element, rather than sharing the duty with the interior.

Such buildings would be akin to stiff tubes, filled with anything the owner wanted to put inside. Even after all the masonry and cross-bracing was stripped away, calculations at the Skilling firm showed, the structure would be stiffer than a traditional structure anyway, more than ten times stiffer.[/QUOTE] (p.120-121)

 

[Newtons Bit]

If you push down on a straw until it fails for example, it won't buckle directly below the top - in fact you cannot tell where it will buckle.

You don't understand how structures work. There is not 1 straw. There is 110 straws with 111 fingers. The straws vary from being large at the bottom to being smaller at the top. Each finger represents a floor, and this floor braces the straws against buckling. There is no buckling length over several floors. Each finger is a floor, it laterally braces the straw against buckling. The smallest straw will buckle first as it has the highest slenderness ratio. The force which causes the top-most straw to buckle is the same force the causes elastic strain energy in all the straws below. This buckling force is the peak force that the upper straw can transmit, and hence, this is the peak force that the straws below are subjected to. Ross has calculated the energy the lower columns absord (inaccurately I might add), and it is less than the energy required to completly fail the top most column.

It is rather obvious that the energy absorbed by the lower structure is small. In addition, and why I say that Ross calculates this inaccurately, is this impact causes a vibration through the lower structure. This vibration lowers the structural capacity of the lower members as the collapse continues. I can't quantify this number, but it's important to note that the way Ross calculated the energy of the lower structure is somewhat favorable to collapse prevention.

 

[Corsair 115]

This section from the book "City in the Sky" makes clear that this was not the case at all, the outer walls.made of the perimeter columns held together by the spandrels was the main support structure of the WTC.
(p.120-121)

I note that the book excerpt you posted does not explicitly state the claim you made here:

On a side note, we had a debate at physorg about whether the perimeter floors would just "fall over" absent the floors, because this seems to be a common misconception of OCTers - that without the floors and core, the perimeter structure of the WTC would have just collapsed under it's own weight, as you appear to believe.

It merely talks about the WTC structure being much stiffer than other buildings; it says nothing about the outer columns being able to stand on their own.

 

[lozenge124]

I could counter that by asking if you have any other examples of major office towers which were struck by large passenger jetliners moving at a high rate of speed and carrying a large fuel load.

Given the sheer amount of mass which began moving in each WTC collapse, I cannot see how anyone would expect anything but a complete and total collapse.

Here's a challenge: build a vertical structure such that when you impact it from above with a sledge hammer, it collapses "floor by floor" with no visible damage below the impacted "floor".

No angled cuts allowed in the structure, and no cheating with wedges or anything (the sledge hammer has to come down vertically).
Can you see why this is problematic?

Jim Hoffman has a more involved challenge too if you are interested:
http://911research.wtc7.net/wtc/analysis/challenge.html

 

[lozenge124]

I note that the book excerpt you posted does not explicitly state the claim you made here:

It merely talks about the WTC structure being much stiffer than other buildings; it says nothing about the outer columns being able to stand on their own.

So a stiff tube "filled with anything the owner wanted to put inside", "more than ten times stiffer" "than a traditional structure", would just fall apart with nothing inside?
Try reading again.
btw. are you saying you believe the perimeter columns would just peel off absent the floors then?

I can see how the WTC collapse is less problematic for those with these views, because all you have to do is collapse the floors by causing the truss connections to fail and the whole thing just comes down by itself.

 

[Furcifer]

Well that's great, but again, an FEA model is needed to see if your intuition is right. (I thought you boys were all about peer reviewed papers, not just personal opinions...)

On a side note, we had a debate at physorg about whether the perimeter floors would just "fall over" absent the floors, because this seems to be a common misconception of OCTers - that without the floors and core, the perimeter structure of the WTC would have just collapsed under it's own weight, as you appear to believe.

This section from the book "City in the Sky" makes clear that this was not the case at all, the outer walls.made of the perimeter columns held together by the spandrels was the main support structure of the WTC.
(p.120-121)

All I got is my opinion and I always try to make that perfectly clear. I'm lazy so I'm waiting for someone else to prove me right. Who wants to be the reviewee when it's so easy to be the reviewer.

I lurked for a bit while this discussion was going on over at PhysOrg. I missed the conclusion of it, so I am at a little of a disadvantage here. I think in the static state they were very strong, but they relied on the truss connection to do so. Once they were severed and in the dynamic all bets were off. While the spandrel cross bracing was important in the static, this would not be the case in the dynamic. Once you have lateral forces acting on the exterior you have a lot of weight acting on the connections between columns. Not to mention the top section was pushing it out of the way.

10 times stiffer than a traditional structure? Well yah, it had an exoskelton, most building just have what we call a "curtain". The WTC exterior was anything but a curtain that's for sure. However, stiffer does differ from strength. I like that, it's like "a pints a pound the world around" but for structural engineers.

 

[WildCat]

Here's a challenge: build a vertical structure such that when you impact it from above with a sledge hammer, it collapses "floor by floor" with no visible damage below the impacted "floor".

Oh dear Great Flying Spaghetti Monster...

 

[Newtons Bit]

Here's a challenge: build a vertical structure such that when you impact it from above with a sledge hammer, it collapses "floor by floor" with no visible damage below the impacted "floor".

[qimg]http://pharaqey.org/temp/PVC2.jpg[/qimg]

[qimg]http://pharaqey.org/temp/PVC3.jpg[/qimg]

No angled cuts allowed in the structure, and no cheating with wedges or anything (the sledge hammer has to come down vertically).
Can you see why this is problematic?

Jim Hoffman has a more involved challenge too if you are interested:
http://911research.wtc7.net/wtc/analysis/challenge.html

It's been done. I recommend you look at pictures of the collapse on 9/11. Snide comments aside, the problem with your picture, and why you have difficulty understanding this, is that your tower is is very slender. The WTC was not of similar proportions. Depth and Width are very important, as the floor slab laterally braces the one floor of columns. I can prove your "challenge" on paper, but hey, it's been done by people like Bazant. You may find problems in his last paper because they don't show crush up before crush down, but he still models an even that is almost exactly what you are looking for in your challenge.

 

[WildCat]

Here's a challenge:

Here's a challenge: Get a truther to publish a paper in a respected science journal. Will that ever happen? Do you ever wonder why it hasn't happened? Or never will happen?