When you look at the crane illustration I did in post #86, can you really honestly say it's obvious cutting the cord would cause total collapse?
How high could the plane have hit, and still caused a total collapse?
- Thread starter Henkka
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Mendel
Senior Member.
and NIST FAQ #18
plus "pancaking" for the less detailed explanations, it's pretty much literally what I read in my news magazine back in the day
no, because it doesn't reflect the WTC situation in any way, as I have pointed out
if you're just ignoring what people post, why do you keep writing?
So this bit by NIST:
Does not seem very well reasoned to me... It seems to be only talking about the one floor immediately below the collapse initiation. Have you ever watched videos of martial artists breaking a stack of bricks with their hands? It's like saying if you manage to break the first brick, then you will inevitably also break the entire stack, which is obviously not true. Each brick takes energy to break, and that energy is subtracted from the kinetic energy of the hand, causing it to decelerate and come to a stop.
I thought you agreed it took a minimum of 5-6 floors to cause a total collapse, so the plane could have hit that much higher. Admittedly the crane thing is a bit silly and cartoonish, but by lifting up those 5-6 floors and dropping them they would have a whole lot more kinetic energy (and thus destructive potential) than they would if they just progressively succumbed to damage from heat. So I'm actually stacking the deck in favour of total collapse, not against it. If you think a plane hitting 6 floors below the roof could cause total collapse, obviously those same floors being dropped should also easily cause a total collapse.
econ41
Senior Member
The experts agree with NIST and Bazant.
And the "public" accepts it. So what?
How do you know that the "mainstream" explanation for Tacoma is correct? (It is, by the way.)
Most people accept it. I don't and several other members here don't. In varying degrees of both detail and self-assurance. I'm prepared to explain ANY claims I make. But let's not get too far off-topic. If you don't accept my explanations and supporting arguments - well it is a discussion forum...
It is rife with problems I can both identify and prove to anyone who is interested, honest and can follow the physics arguments.
Prove me wrong! Prove "us" wrong! Or go and find a forum with members who do not dare to challenge experts like Bazant when they make an occasional error.
Jeffrey Orling
Senior Member
I think there were a number of people / groups who try to offer explanations... mostly to appease the public who found a total building collapse "impossible".. Strong buildings, structures... don't collapse. Of course the public hasn't a clue about structure. (and many other technical explanations for "things" they "take for granted". A highly technical explanation would not "work" for the public. So out came the "dumbed down" versions... pancake collapse, for one. Public understand how explosions "destroy" things. And of course they know CDs use explosions to fatally weaken a structure.
ROOSD is really nothing more than the basic engineering concept that every element has a structural capacity and when the capacity is exceeded it fails... This includes... steel, concrete, connections - bolts and welds.. even composites and sub assemblies. WTC floors were designed to support X pounds square feet and if the load exceeded X the floor, or its components would fail - meaning collapse. When that floor mass impinged on the slab below, that slab's capacity was exceeded and it too collapsed. And the process repeated until all the floors below the first to fail had likewise failed. There was not feature of the design to arrest such a process. That would require a much stronger slab.
The acronym is just a handy way to describe a pancaking collapse. Pancaking was introduced early by the mainstream media... and seemingly easy to comprehend.
The column free open office design allowed the floor destruction to "spread" throughout the entire footprint. A more traditional grid/lattice frame would more likely confine a collapse to a single bay. The column free concept allowed tenants more flexibility in their office layouts including wall-less office landscape furniture arrangements. I believe that at the time no discussion took place about the vulnerability of a column free slab to experience floor wide collapse. So... the flexible (inexpensive and quick to build) column free slab was not blamed for the unstoppable collapse. Once the entire footprint was involved... the collapse would gut the interior. This happened to all three towers. Building integrity depended on not more than 3 or 4 floors collapsing.
In 7WTC the lateral structural elements were responsible for a local failure to spread throughout the footprint. But those very structures were what allowed the tower to be build over a sub station. I believe the building's engineer Cantor reflected that if the transfer trusses failed... the entire building would collapse as seen. Likewise, for the twin towers, the engineers did not expect a collapse of 3 or 4 continuous floors which they knew would be fatal to their design.
The initiation was the result of excessive heat which caused the steel to weaken, expand, and distort the frame to exceed the design specs and fail locally, which led to a floor collapse which went runaway. This sort of runaway failure process is an attribute of "complex systems".
The vulnerability of steel frames to heat/fire was completely understood. The mechanisms to suppress fire failed on 9/11. The towers did not stand a chance if fires burned too long. They did.
econ41
Senior Member
I outlined the possibilities in my post #90 which you seem to ignore.
Please read what is posted and answers your questions. I said:
Columns either "missed" or impaced in alignment., Those are the two options that caused confusion in historic debates.
Reality was columns MISSED, >>> ROOSD. Whether you choose to listen to the experts who are wrong OR recognise what actually happened which is clear from visual evidence.
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econ41
Senior Member
The NIST FAQ#18 frustrates me. It appears to be written for a layperson audience and is "good enough" for THAT audience. It is excessively conservative in two main factors which lead to the sort of issues you identify.
I'm prepared to explain the engineering more rigorously BUT it is too far off topic given @Landru's comment.
You are pushing the analogy too far without considering the reality of the WTC collapses. There was overwhelmingly more energy than was needed to ensure continuous progression to global collapse.
You will remain confused until you recognise the key factor - the dropping columns and debris MISSED landing on the top of the lower tower columns.
Not whilst you continue to ignore the key issue. Dropped onto WHAT??? Columns ends MISSING or columns ends IMPACTING.
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I can understand intuitively that the columns missing would cause a worse collapse than if they impacted in alignment as you put it. But a worse collapse doesn't necessarily mean a total collapse. It also seems very intuitively possible that in such a scenario, the top block and a number of floors below it would be reduced to rubble, but then much of the tower would remain standing... Especially if we're talking about the minimum scenario of a top block consisting of 5-6 floors. It is extremely difficult to intuitively understand how 5-6 floors falling could smash over 100 floors below to rubble.
econ41
Senior Member
@Henkka. The debris fell down the Open Office Space and sheared off the floors. Like this:
You should see from this diagram why:
a) The progression of the collapse was unstoppable once it started; AND
b) Why missing the columns is fundamentally different from impacting and crushing(buckling) the columns.
The only significant resistance to free fall acceleration were the forces shearing the joist to column connections labelled "It Fails Here" and the need to increase momentum as the debris of each successive floor adds to the falling mass. In fact, momentum accumulation was the major factor. Shearing and some friction forces were of little significance. The columns were simply not in line contributing their full strength to resisting collapse progression.
You should see from this diagram why:
a) The progression of the collapse was unstoppable once it started; AND
b) Why missing the columns is fundamentally different from impacting and crushing(buckling) the columns.
The only significant resistance to free fall acceleration were the forces shearing the joist to column connections labelled "It Fails Here" and the need to increase momentum as the debris of each successive floor adds to the falling mass. In fact, momentum accumulation was the major factor. Shearing and some friction forces were of little significance. The columns were simply not in line contributing their full strength to resisting collapse progression.
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FatPhil
Senior Member.
That's not a good analogue because the tiles can, and do, support themselves after they are broken, and the additional load caused by one more breaking above you is utterly negligible. It's not the tiles breaking the tiles, it's the human. Note that the pencils are placed between the tiles specifically such that it requires very little extra force from the human to regain the momentum and energy lost during the breaking of the previous tile. The trick only works if you can break them one at a time - it looks continuous to the human eye, but it's not - no tile is supported against the incoming downward force. So, as long as the human can keep up the downward force to keep his speed up, he will inevitably break the entire stack, he doesn't need all the energy in advance.
Mendel
Senior Member.
Yes. The NIST explanation does not involve kinetic energy at all. You pile up enough weight on a board, and it breaks. Don't do it carefully, then the unevenness of the "shock" makes it break more easily. The energy source is potential energy, but all you really need to break something is a force, which is gravity.
We're breaking metal bolts in the WTC towers, not bricks.
If you had just "floor rings" on your crane, it'd obviously work, but your diagram looks like 2 homogenous blocks, which is not at all like the core&tube structure of the WTC.
View attachment 52480
Gamolon
Active Member
Jeffrey Orling
Senior Member
they are not wood... look like plaster or similar.
Gamolon
Active Member
What
What's not wood?
Jeffrey Orling
Senior Member
the blocks in the video
Gamolon
Active Member
Who said they were wood?
Jeffrey Orling
Senior Member
no one said... but the material would make a difference in the chopping.
Gamolon
Active Member
It's not just "5-6 floors" smashing over 100 floors below to rubble. The 5-6 floors impacted the top concrete floor of the remaining structure and destroyed it. Beam/floor/structural connections were severed/sheared on that floor level. All the destroyed concrete and structural components that had their connections severed at that floor level add to the 5-6 floors which became 7 floor levels worth of descending debris that will now impact the next floor after the first impacted floor. Not to mention everything that was ON those floor levels.
Etc. etc.
There came a point were the core remained standing and the floors were stripped away. Then the reaming core components collapsed.
What is so hard to understand?
The towers weighed about half a million tons. They were built such that they could resist hurricane force winds (and earthquakes) swaying only a foot at the top (about half a percent out of its footprint).
It's hard to imagine that cutting off the top 6-10% of such a structure and dropping it a few feet onto the rest of it would be enough to raze it to the ground.
The way to get people to imagine it is to describe a simpler structure, with fewer columns and fewer floor-to-column connections that is very, very strong (for its weight), and then show how the movement of that top section down through the structure breaks it up into tiny pieces.
Also: it will be useful to show that (and why) less than 5% wouldn't do it. That is, a smaller top section would behave as the person who has a hard time understanding the collapses would expect.
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Uh, because he delivered enough force to break all of them? But conceivably if he was weaker, he would have broken the first few bricks, not all of them. That was my contention with the NIST FAQ question #18, it kind of sounds like they're saying six floors coming down is enough to cause the floor below them to fail, and then that is sufficient to destroy the entire building. But obviously you can conceive of the possibility that the top block would have decelerated and stopped, or been reduced to rubble, and much of the building would have remained standing. Like this:
Source: https://youtu.be/aTVXH9dJjhU?t=120
By the way, here is John Gross of NIST categorically saying they did not do any calculations on why the collapse wasn't arrested by the lower floors:
Source: https://www.youtube.com/watch?v=ZAuA19IPCso
Gamolon
Active Member
So there are different aspects (force, load, materials) for scenarios where someone could break all the blocks or NOT breaking all the blocks.
Again, 6 floors became 7 floors of debris that hit the next intact floor which was destroyed. Then it became 8 floors worth of debris impacting the next intact floor.
Different structure with different components and different design.
So you as of right now, you cannot conceive of any possibility/process of a "top block" descending on a "bottom block" and destroying the entire building? See video below.
I'm not understanding why you cannot conceive that the explanation of what happened at the twin towers is what happened.
https://youtu.be/aTVXH9dJjhU?t=120
The thing with these verinage demolitions is they're started near the middle, and the top block is used to destroy an equivalent number of floors as are in the top block. You will not find a video of a verinage demolition where the top 10% is used to destroy the lower 90%.
The video is instructive because everyone understands that the energy to destroy the tower comes from the body of the person impacting the top block.
Now we need a similar demonstration where the energy comes solely from dropping some part of it on the rest.
Gamolon
Active Member
So what?
What does this have to do with how much of a dropped load the first floor of the remaining lower section could resist before being destroyed?
Why to you insist on treating the upper and lower blocks as solid objects when they were constructed of different components which were bound together by varying connections?
See the video I posted above. Is that magic or something?
The video I linked in my post above shows exactly what you are describing!
Gamolon
Active Member
Near the middle? Top block is used to destroy an equivalent number of floors are in the top block? Did you watch the verinage video?
Three floors destroyed about 9-10 floors below!
What are you talking about?
Gamolon
Active Member
Again, watch the verinage video I posted.
I'm treating them as tightly bound, very strong structures.
One of the questions I have about this -- and I think it's been discussed on this forum (can't recall where) -- is how much a structure is pre-weakened before verinage (or any other demolition). One problem with using actual demolitions to disprove the truthers' claims is that they are claiming that the buildings were actually demolished.
I don't think the lower floors were entirely destroyed there, but it's hard to tell because of the dust cloud. It's also important to keep in mind you're looking at a manmade, controlled demolition. Often they remove a lot of the supports from the floors where they want the collapse to initiate and do the coup de gras using some machinery. Here it was just pulled, but they also use hydraulic machines to buckle the remaining columns simultaneously. So it's all done in a planned, synchronized fashion by professionals. The buckling is induced so the top block starts going at near free fall, causing it to gain immense kinetic energy it then uses to smash what's below. There were no hydraulic machines simultaneously buckling the columns in the WTC towers, just randomly scattered fires. And yet, the fall of the top of the North Tower is impeccably smooth and symmetrical.
Gamolon
Active Member
The lower section was strong enough to support the upper structure prior to being destroyed. Explain how the smaller, upper block crushed the larger, lower block. The lower block weighed much more that the upper block.
Gamolon
Active Member
Why are you shifting goalposts now? We aren't discussing the initiation of the collapse. We are currently talking about the smaller, upper section impacting the larger, smaller section and how a smaller section could have destroyed the larger one.
We can easily imagine cutting (or weakening) the columns so the structure is on the verge of collapse and just needs a little bump from the top to bring it down. It's not the weight of the lower section that matters but its strength.
Gamolon
Active Member
But how Henkka? I don't understand.
How did the smaller, upper block destroy the larger, lower block? The lower block supported the upper block previously? The lower section weighed MUCH more than the upper section?
Gamolon
Active Member
So it's all about overcoming the strength of a component or connection to make it fail eh?
Here's a question.
Do you think the first floor (concrete, trusses, connections, beams, etc.) of the lower remaining section that was impacted by the 6 floors of the upper section should have resisted and stayed intact? If so, explain your logic.
How the collapse initiates affects how much it can destroy what's below… The important part being how much speed it picks up before meeting significant resistance, ie impacting the lower sections. In these controlled demolitions, the initiation of the fall is deliberately set up so that the top block can freely accelerate for a time. This was not the case with the WTC towers. Again, this is a planned, manmade thing. Your position would have to be then that airplane impact and randomly scattered fires can result in an even smoother, more total collapse than these planned demolitions.
Gamolon
Active Member
Explain how you are using the "strength of the lower section" to come to the conclusion that the first floor of said lower section, impacted by the upper 6 floors dropping on it, show resist that impact and stay completely intact.
I think the general idea is that whatever would rip the lower section apart would also rip the upper section apart, turning it into rubble that behaves almost like a fluid. So you'd have some horrific damage for a few floors until the majority of the mass has been ejected to the sides or fallen through the elevator shafts, seeking the path of least resistance, and basically dissipating the downward momentum, eventually stopping (as in the video Henkka linked to).
But if the lower section is pre-weakened to point where the columns actually fail when jolted from above*, the path of least resistance really is down through the structure.
Like I say, truthers claim that the buildings would have had to be demolished to come down the way they did. So showing them videos of demolitions where they come down that way doesn't really help.
*Edit: by an upper section that presumably hasn't been weakened in the same way, precisely to ensure that it stays together as a "pile driver" while the weakened structure below is destoyed. As I understand it, demolition is a very developed science.
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Jeffrey Orling
Senior Member
What destroyed the towers was the runaway collapse (pancaking) of the floor slabs. Columns were "bypassed" and play no role at all in mitigating the collapse.
A typical floor could only support two or three floor masses placed on it... not dynamically dropped. When a floor's load bearing capacity is exceeded.... it collapses/fails. That's what happened and it repeated on every floor until they were all destroyed. This was predictable. But no one expected the load on a floor to be 2 -4 times its design capacity.
Mendel
Senior Member.
you are still thinking in terms of kinetic energy, this is wrong
think in terms of weight
if you put too much weight on a structure, it breaks
it doesn't matter much if you decelerate it first
the weight that breaks floor 72 also breaks floor 71
rather, it breaks the connections that attach it to the sturdy vertical columns
[it also breaks the floor, but all we are concerned about is that it comes loose)
FatPhil
Senior Member.
No. Noone understands that, because it's false, for the reasons I stated above.
FatPhil
Senior Member.
That's because it's the solution to a minimisation problem derivable from little more than Newton's laws of motion and gravity. It says nothing about the absolute probability of failure at any other ratio. And, for the N+1-th time, says nothing about completely different building types.
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