Video Nullfies Pancake/CD Theory

Originally posted by exponent

an effective FoS of around 0.3 is needed to make progressive axial collapse in less than 20 seconds possible.

You seem to keep making this mistake. The towers did not collapse because there was too much energy. It's not a question of the amount of energy, it's a question of the forces and the geometry.

You in turn keep saying this, and all I have to do is quote Bazant ("what matters is energy, not strength nor stiffness") and point you to the fact that force is energy per distance. I have also provided a calculation based on the sum of forces alone, comparing the top's stiction to its sliding friction and leaving the mass of theintact 95/80 stories out of the equation. You can't just keep saying that this is false and wrong and meaningless, yet not propose a different calculation that is more meaningful, more correct and still describes what is globally going on in a structure that falls through itself within seconds.

There's no trivial way to summarise this as an 'effective factor of safety'. Please stop using it, it's meaningless.

I understand why it makes you cringe. Partly, because it is not very scientific and partly because it brings matters to a head. I am fully aware this "effective FoS" thing is my own invention to describe a phenomenon that doesn't occur very often - something being somewhat stable and static but be unstoppable once gone kinetic - and therefor has not its own SI unit yet.

The best and most physically scientific approach would be to look at it as a compression (in the sense of a volume change under an axial force), however, my proposal was not very welcome - again, most probably because it brings matters to a head and sheds some light upon the question under which pressure/tension the towers stood.

Let me clarify right from the start, please. I would have a hard time explaining what I mean even in my mother's tongue, it's even harder in English, so bear with me if I got the terminology wrong, you may want to help me. What I mean is this: when I stack my Jenga blocks on top of each other, I will make sure they are well-aligned. That makes them stable. When there is an offset, the area each block stands on - the contact face - becomes smaller, so less force is needed to dislocate the center of mass beyond the contact face to cause the block's toppling (and less height can be achieved). So, 5 blocks well-aligned are under less "inner tension" than 5 blocks alternately offset half an inch left and right - although both stacks have the same height, one is less stable and prone to collapse than the other.

The isolated and iterative analyses are techniques used in FEAs to simplify the problems so they can be resolved by computers. I don't see you doing an FEA or p-delta calculations. These are what are needed in order to support your point.

I understand your critique. Would you give me a helping hand with the p-delta? I'm not too good with trigonometry and rheology. Let's say each tower had a sailing area of 64*400 meters and withstood winds with 260km/h. So, in a sense, they were built like vertical cantilevers. This lateral force was transferred by momenta to act vertically into the ground. Can we make any deductions from the fact the towers did not fail and buckle under the pressure of the first caribbean hurricane? It may be another conspiracy nut rumour, but I heard the towers sometimes swayed so hard on windy days that pencils rolled off the tables; elsewhere it was said that the dampers on the truss system were in place so they do sway to a certain degree (so that some of the kinetic energy is converted to heat, so to say).

There are several problems with a FEM. Firstly, I couldn't afford the software. Secondly, we would need the blueprints. Thirdly, there would have to be a trade-off between transparency and complexity of the model. And fourthly, I don't have the computing power for this sort of stuff, I even had to limit my processors to 90% so they don't overheat when rendering a POV-Ray scene.

I would prefer a most simple model with a simple core, simple slabs and a simple perimeter wall that actually explains, not obscures, the collapse mechanism for one simple reason. I believe - on the grounds of experience and my understanding of the findings of Newton, Galileo and Archimedes - that the towers did not collapse accidentally, but being a good scientist, I know I could be wrong. In the case that I am wrong, it would be one hell of a favor to architecture students around the world to show what mistake they want to avoid at all costs. So either way, much would be gained with a model as simple as possible.

but you seem so confident there's something dodgy to find that you miss out all of the important knowledge in your rush to the answer.

I am quite confident, therefor I welcome any new knowledge -- but it has become sparse.

Originally posted by Akareyon
You in turn keep saying this, and all I have to do is quote Bazant ("what matters is energy, not strength nor stiffness") and point you to the fact that force is energy per distance.

I'm certainly not trying to imply that we should ignore energy, my statement was meant to be combined with the second sentence you quoted so I will address just that.

I understand why it makes you cringe. Partly, because it is not very scientific and partly because it brings matters to a head. I am fully aware this "effective FoS" thing is my own invention to describe a phenomenon that doesn't occur very often - something being somewhat stable and static but be unstoppable once gone kinetic - and therefor has not its own SI unit yet.

The problem is that it cannot be given a unit as it represents several independent variables. Typically when factors of safety are discussed they are combined with a particular type of loading. For example, for columns it is typically axial load, wheras for trusses it is often mid-span deflection. In this case your factor of safety depends not only on a specific characterisation of collapse ('ROOSD' vs 'Bazant' for example) but also the location of that collapse and its magnitude (how much of a floor or how many floors collapse).

This is why I take issue with its use, because while you may be able to come up with a steel cross sectional area that would be able to resist a single storey collapse purely in elastic strain, you have only inspected that particular design. Changing column weight by a significant amount will change every floor of the building and may reduce its height too. For this reason you not only group independent variables together, but in reality you are even talking about a completely different building.

This line of inquiry really has no benefit that I can see. In fact frankly I am still at a loss as to what exactly you are taking issue with about the 'official story'.

'I'm down to 3000 characters so I'll try and quickly go through the rest of your post.

when I stack my Jenga blocks on top of each other, I will make sure they are well-aligned. That makes them stable. When there is an offset, the area each block stands on - the contact face - becomes smaller, so less force is needed to dislocate the center of mass beyond the contact face to cause the block's toppling (and less height can be achieved). So, 5 blocks well-aligned are under less "inner tension" than 5 blocks alternately offset half an inch left and right - although both stacks have the same height, one is less stable and prone to collapse than the other.

Tension is certainly the wrong word to use here, it refers to a 'pulling' force trying to tear an object apart. In this case we are talking about a compression. It is better not to think of the jenga blocks as 'stable' but to ask what perturbation would cause their failure. This is fairly unintuitive, but the perspective this gives is quite obvious when stated:

There is always some small perturbation of the blocks, from heating to air currents to simply air molecules randomly hitting the sides. In fact we can even count faces being slightly angled as part of this. This provides us a testable criteria. If the structure can withstand displacement equal to these perturbations without failing, it is 'stable'. By moving the blocks so they are slightly offset relative to each other, you massively reduce the angle whereby the centre of gravity moves outside of the contact area of the block. This is the condition for collapse.

From this we can easily see that there is a simple relationship between the angle the block must rotate and the stability of the structure, therefore there is also a simple relationship between the stability of the structure and the forces imparted to it.

I doubt I am breaking any new ground here or telling you anything you do not know, but as I am so unsure of your particular complaint or issue with the 'official story' I feel I should tread a little lightly and lay the foundations of a discussion.

Would you give me a helping hand with the p-delta?

I'm really not sure what sort of hand you want here. The towers did sway by a foot or two at the top but they were over a thousand feet tall and so the additional moment is really not going to be great. It's also significantly resisted by the adjacent perimeter walls and the hat truss. This sort of thing would require an extremely detailed and large scale FEM. Years of work no doubt even with a decent team.

I would prefer a most simple model with a simple core, simple slabs and a simple perimeter wall
...
I believe
...
that the towers did not collapse accidentally

Let us start with the belief. How do you believe the towers were made to collapse?

Originally posted by exponent
This line of inquiry really has no benefit that I can see. In fact frankly I am still at a loss as to what exactly you are taking issue with about the 'official story'.

It's quite easy, and I will break it down as much as possible.

There is a tall, vertical structure. It is damaged at a point at 75% of its height. It descends straight down - implodes and explodes at the same time - and falls through and out of itself. This phenomenon is so unique it doesn't even have a name. It never happens in nature. And cannot be compared to anything that has been seen before or after.

Not even to conventional building demolitions, as these usually start the demolition sequence at the bottom to drop the building onto its own footprint (and to keep the debris field as small as possible).

Enter the "official story". Words in NIST report about collapse mechanism, after conditions for a collapse initiation have been met after some "tweaking" of the input variables of the computer model it never elaborates on: none, zero, nada, rien, nüscht. Merely a footnote referring to a paper by the world's leading material sciences professor who uses religious words like "doom". The same professor proposes his "inevitability theory" five years later (to dispel the myths of planted explosives) and compares the phenomenon with - building demolitions.

So, my issue with the official story should be clear. This thing was unique. It caused repercussions of all sorts. Its implications are huge. Pretending this is nothing unusual or worth looking into, brushing it off as "inevitable", common knowledge and self-explanatory is an insult to science. The call for an independent, thorough and transparent analysis is very justified, especially since the omissions of the"official" research up to now regarding the collapse sequence come across as silent a scream for explanations.

I doubt I am breaking any new ground here or telling you anything you do not know, but as I am so unsure of your particular complaint or issue with the 'official story' I feel I should tread a little lightly and lay the foundations of a discussion.

Let me build upon it by explaining where I'm heading.

I would like to show that there already is a physical description for instability, and that is pressure, or tension. My argument is that both are roughly, although not exactly, the same in the sense that both measure the amount of force exerted on a given area. I wish to explain this with the example of two stacks of five jenga blocks.each.

One stack has all contact faces perfectly aligned, the other one is built with a slight offset to each block so the sum of contact faces is smaller. Both stacks have the same weight and height, but effectively and technically, the first one is under less pressure/tension than the second one, because the ratio weight to (contact) area in the first is smaller than in the second. The greater the offset, the smaller the contact area, and the greater the tension/pressure. The greater this tension/pressure, the less stable the stack is, the less energy is needed to collapse the stack.

Even if I got technical terms wrong and strained physics lingo and have oversimplified things (please correct me and throw the right technical terms at me so we speak the same language) and you already suspect where I'm going with this - can we agree that what I called "tension/pressure" or "inner tension" in this example, the tendency to collapse, has a say in the stability of the stack?

(with "inner" tension I mean, of course, the same phenomenon like when you unroll a poster and it has a tendency to roll up again)

I'm really not sure what sort of hand you want here. The towers did sway by a foot or two at the top but they were over a thousand feet tall and so the additional moment is really not going to be great. It's also significantly resisted by the adjacent perimeter walls and the hat truss. This sort of thing would require an extremely detailed and large scale FEM. Years of work no doubt even with a decent team.

Yes, of course, FEM and such. It sure has its merit if you want a detailed description, I will not deny that. However, my approach is a layman's one for orders of magnitude, estimates, general assertions and rough comparison. It should not be that hard or complicated. I got this:

To a structural engineer, a skyscraper is modeled as a large cantilever vertical column. Each tower was 64 m square, standing 411 m above street level and 21 m below grade. This produces a height-to-width ratio of 6.8. The total weight of the structure was roughly 500,000 t, but wind load, rather than the gravity load, dominated the design. The building is a huge sail that must resist a 225 km/h hurricane. It was designed to resist a wind load of 2 kPa—a total of lateral load of 5,000 t.

reply to post by Akareyon


Hi Akareyon just a quick quote (busy just now) re the mass of the Towers the 500,000 tons is more likely the mass of both towers.

Best quoted fig I have seen for the steel is 96,000 tons per tower there would be about 100,000 tons for concrete the rest made up by plumbing,electrics,heating,glazing and cladding.

Weights for some others to compare.

Empire State Building, NYC = 365,000,000 kg
John Hancock Tower, Chicago = 174,500,000 kg

The Empire State Building has thicker floors a 3d grid of steelwork ( not open plan) brick work to the walls and an outer masonry skin.

reply to post by wmd_2008

I know and I agree, but thanks for the seperate figures for the elements anyway. I just used the 500,000t figure because it was the ealiest and also the most common one. If you find the time, feel free to quote a source, though. Shall we use the 500,000t figure anyway because that's the one we used for the last 20 pages or do we agree to use 250,000 tons from now on?

reply to post by Akareyon


Hi Akareyon
The figs I gave in the previous post were based on averages given on various sites but having another look at my files I found this link I had saved which gives I think the most accurrate figs for the mass of the towers.

Mass of Towers

The main reason I think for the supposed 500,000 ton fig per tower can about was the fact that some conspiracy sites got info on the amount of concrete used during the construction of the WTC site now some of the figure was actually used in the construction of The Bathtub and other buildings.

Looking at the bathtub alone.

152 framework segments, each measuring 22 feet across, to form a large box measuring four city blocks by two city blocks (about 500x1,000 feet / 152x304 m). This box, commonly referred to as a "bathtub," formed a water-tight perimeter wall for the two towers' foundation structure.

Taking the dimensions above and using the average mass of 1 m cube of standard structural concrete (2.4 tons) and average height of bathtub walls 22 m and approx 1 mtr thick we get 912 (perimeter) x 22 x 1 x 2.4 = 48,153 tons.

The other problem was of course that there were other buildings at the WTC not just the towers.Now you may see that is far to easy with this subject to jump to conclusions if you don't really understand construction/construction issues.

I will take on board another comment you made re "explosions" when tens of thousands of tons of material is falling many hundreds of feet it's going to make noise lots off it , when Mr Joe Public sees what looks like to him a CD that noise is no longer noise its an explosion.

reply to post by wmd_2008

Yeah, thanks for reading my post, I'm glad you understood and addressed the arguments I made

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