Mendel
Senior Member.
You already wrote this:
And now you disavow that when it suits you.
Truther tactic.
Start your model with the fire-damaged building, if you're honest.
Buckled Structural Steel in Building Fires
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Truther tactic.
Start your model with the fire-damaged building, if you're honest.
I honestly don't understand the contradiction you claim you've caught me in here. What are the two claims you think I've made that can't both be true?
You can get a bit lost in the weeds debating what a scale model of the WTC collapses should be like. And I have no idea what the specifications should be. But if you zoom out a bit, the big picture is that literally nobody has built one, in 22 years. So I kind of doubt that anyone will in the future, either.
Mendel
Senior Member.
1) You claim the effect of the fire does not puzzle you.
2) You claim you need to "reconcile" the facts that the building performed well for 3 decades and then collapsed.
But 2) is reconciled by the effect of the fire, so via 1) you should know that already.
What will looking at a model provide you with that looking at the actual collapse doesn't?
econ41
Senior Member
Correct. And the reasons why have been explained many times.
Hence my objection to the debating trick of the conflation of "understanding the collapses" with "by building a model".
The towers' collapses can be explained with any necessary degree of detail. It cannot be achieved by modeling for the reasons which have been given many times.
.
econ41
Senior Member
Exactly the point I have made many times. Including my personal preference to set NIST aside - it is redundant.
There is sufficient evidence in the video record of the actual collapses to support reasoned explanation. Backed by any necessary structural details which are in the public domain.
Unfortunately, I tend to agree. Doubts like these will persist in minds like mine.
The fires affected the towers locally. They collapsed globally. Like most truthers (with whom I therefore sympathize), I wouldn't understand the collapses even if the parts of the structure that were affected by the fire had been completely vaporized.
Mar 27, 2021
The paper is 30 cm x 41 cm and 90 g/m2.
The cardboard is basically what you get in a pizza box, cut into 7.5cm squares.
Those are AA Duracell batteries. You must put an equal amount of batteries on each floor and the floors must be equally spaced throughout the tower.
My (not rhetorical) question is: in order to use these sorts of materials to meet (something like) Hoffman's challenge, how big would you need the piece of paper & cardboard squares to be? (You can also specify the grade of paper if you like.)
Ideally, you will keep the floor heights and bases [spans] equal, so that the structure is a series of cubes, but I'm willing to hear why that's unreasonable.
If you want more or bigger (or smaller) batteries, just ask!
You can also cut holes in the paper as you choose to weaken the walls.
You can (and probably should) connect the tower to the base (the big piece of cardboard) using as much tape as you like. If you need a bigger or heavier base, that can also be arranged.
The structure must be strong enough in its initial state to let me poke a hole through the paper with the scissors at any point.
The structure must also be strong enough to let me shift the base about 10% of the width of the tower back and forth instantaneously (simulating an earthquake). I think that makes any "wind testing" moot.
The initiating failure must be brought about using the scissors in the ordinary way, cutting the paper, cardboard, or tape as much as you like somewhere in the top 20% of the structure.
This event must cause the whole structure to be destroyed. (It's not good enough that all the floors and batteries end up at the bottom of the tube, which remains stranding.)
The solution using the smallest piece of paper wins.
(Note: you are of course free to ignore this challenge, ridicule it, or meet it as you choose. Once it occurred to me I thought it was too good to keep to myself. I don't think I'm going to be able to meet it with the 30x40cm piece of paper I have. But I'm going to try.)
[The thing I like about this challenge is that it lets us scale up instead of down. That is, if we can't get the 41 cm tower to progressively collapse because gravity doesn't scale well, we can imagine it -- or even build it -- bigger until we reach floor heights where a gravity-driven collapse becomes possible.]
Mendel
Senior Member.
Yes.
But you would understand that dropping a 20-storey-skyscraper onto an 80-storey skyscraper is not normal operations? And the fact that this hadn't happened in the 3 decades prior meant that the "'performance' of those same sub systems" was uneventful during that time? And that they only failed because of the fire, or because of the skyscraper-onto-skyscraper event?
I'm not sure how many times I have to say this. No, I would not understand how a 20-storey building, dropped, through a distance of one storey, onto a perfectly good 80-storey building, could completely destroy both. It would be a huge disaster, yes, but I would need to know more about what happened, especially inside that bigger building, all the way down to the ground, before I would say I understand how it happened. It would puzzle me.
I know roughly how building demolitions work. So if you rule out that sort of mechanism, I don't know what to think.
Mendel
Senior Member.
The batteries are 15mm thick. The paper is 41cm high. 20% of that is 82mm.With 3 floors, the drop distance would only be 82-3×15=37mm, less than the thickness of the batteries. That's not a lot of kinetic energy.
The static cardboard carrying capacity, with a 7 cm edge length, is probably more than 3 layers of batteries, even if you cover the cardboard completely.
I don't think the original WTC towers would've survived a 6m (20 ft) displacement Earthquake event. Not even the big California Earthquakes had that much, and the East coast isn't that tectonically active.
Then, your chosen method of collapse initiation is demolition of vertical support; the actual collapse was initiated in a large part by loss of rigidity of horizontal elements.
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Mendel
Senior Member.
That's not what I asked.
I asked, do you understand that buildings are not designed for this; and that, if they're weak to this, it won't be visible during normal operations? Because normal operations don't induce loads that are in any way comparable?
econ41
Senior Member
For what it is worth this comment is based on a false premise::
You are correct when you say: "I would not understand how..." Neither can I. In fact I fully comprehend why it was impossible. We can leave that side track for now.
The source of that premise - the "dropped, through a distance of one storey" - is wrong. It was an assumption made - legitimately - to support the Bazant & Zhou 2001-2 "limit case" hypothesis. It did not happen. It could never have happened BUT became a foundation for both sides getting explanations wrong which dominated discussion from 2001 through to about 2009-10>> and remnants of the error still persist.
You are correct when you say: "I would not understand how..." Neither can I. In fact I fully comprehend why it was impossible. We can leave that side track for now.
Loads are quantifiable. They are therefore by definition comparable in many ways.
Mendel
Senior Member.
I am using "comparable" in the sense of "similar".
What will looking at a model provide you with that looking at the actual collapse doesn't?
These are the sorts of issues I'd love to address. I went with an earthquake displacement of "several meters" (per Wikipedia), but I'm sure a convincing model could make do with less (5%? 1%). Remember that most buildings are structurally capable of swaying in the wind much more than they actually do. They are "dampened" for comfort, not survival.
The main reason I'm proposing only 6 floors in that 41cm model is to have roughly 7cm drops. A bigger model could distribute the weight over more floors (or increase the drop). It would be interesting to talk about why that is necessary to get the desired result. After all, other parameters to adjust include the column strength and the floor connections.
A model simplifies the forces involved. It's like a working diagram of them.
This is interesting. What would you predict as the result of dropping 20 intact storeys on 80 intact stories?
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FatPhil
Senior Member.
So a model changes the forces involved?
The full complexity of forces involved in any event is incomprehensible by a human mind. So we imagine simpler combinations forces, disregarding those that cancel out--all those "equal and opposite" forces. This can be done quite precisely or it can be done in way that distorts the reality.
In this case, the important thing is to correctly represent the forces that destroyed the building along with the opposite forces that were trying to hold it up. The latter were, obviously, unequal to the task.
Oystein
Senior Member
A beverage can.
Can resist certain lateral wind loads. Can resist you stepping on it. Will collapse completely and rapidly when you stand on it and lightly tap the side.
Now of course you will immediately reply that certain conditions you haven't spelled up above are not met, or that the lateral strength is somehow not sufficient, or that the weight on top is disproportional. Or whatever.
In other words, you will demand something more complicated.
We may be able to find a mate for you at Scarborough Fair.
Oystein
Senior Member
Electro-magnetic fields instead of gravity.
Oystein
Senior Member
You need not one but several models.
One each for each aspect of the WTC events that you don't understand. Maybe perhaps:
- Plane piercing wall
- Plane arranging fuel (jet fuel as well as office contents)
- Fire spread
- Transfer of heat from fires to steel structure
- Truss sag
- Wall pull-in by floor trusses tugging laterally
- Creep of vertical columns
- Buckling of wall following creep and pull-in
- Response of structure above failure to buckling (Load re-distribution, tilt, ...)
- Response of structure below failure to tilting upper structure (floor slabs overloaded...)
- Accretion of debris layer and its effect on upper and lower structure
- Response of perimeter to floor slab removal
- Response of core
- Etc
- Etc
Or are there steps in that list that you think you could understand without the help of a model?
Oystein
Senior Member
So would a model satisfy you where we build an upper part hanging from a crane, dangling one or more floors above a lower structure, then let drop? And pretend that the gap is
?
Such a model would no longer present any external lateral load, nor in internal lateral load induced by the pull-in of sagging trusses. Earlier, I think you mentioned the term "lateral force" more than once as something to be considered by your desired model - are you throwing this requirement out now? If so, then please swear a holy oath to never again bring up any modelling requirement that involves lateral loads.
Or admit at once that the scenario you just described is NOT AT ALL what you wish to be modelled and just a red herring.
Let's begin here. (You are right that I'm going to demand something more complicated.) Consider:
A beverage can with you standing on it is (ridiculously) overloaded compared to the WTC. I think the steel constituted something like 40% of the total weight of the towers. Even putting a full can on top of an empty would be off the scale.
The WTC did not collapse when is was in fact "lightly tapped on the side".
I don't want separate models. If we begin with your can again. I would want you to be able to tap it hard enough to pierce it, then heat a small strip of it enough to weaken at most 5% of it, about 20% from the top. I don't think there's a load you can put on top it that would let you pierce the hole *and* crush the whole can after the heating. The model has to survive the "airplane impacts" but not the "fire".
Here's another way of explaining it.
I'm sure we can all agree that a single, unloaded column (either from the core or the perimeter), at a height that would let it stand alone by itself in a strong wind (nowhere near the self-buckling limit), would not be able to satisfy my conditions: to sustain "piercing" 20% from the top and collapse completely after a couple meters of it around the point where it was pierced is heated to the point failure. There's just not enough potential energy in the system. You could pierce it, and heat it, but the top length of the column wouldn't destroy the bottom length.
So my challenge is just to add the fewest possible components (more columns, spandrels, trusses, floors, loads, etc) to the model that would let us sever a few columns with lateral impact and, after heating the remaining columns and trusses at the level of the impact, watch the whole thing come down.
For me, the next step would be scale it down, using weaker and weaker materials. In each iteration, the model would need to be able to do those two things: sustain piercing, succumb completely to local heating. The goal, again, would be smallest, cheapest model of a total progressive WTC-like collapse.
econ41
Senior Member
bbbbb
As I said in the earlier post it was a scenario that led to lots of misunderstanding over at least 8 years. And Bazant himself fell for the same error in a 2007 paper with Verdure. https://web.archive.org/web/2007022...s/Papers/ProgressiveCollapseWTC-6-23-2006.pdf
It introduces the "Crush Down/Crush Up" hypothesis which an astonishing number of academics, professionals and online debunkers fell for and still haven't seen through.
There are four fatal assumptions in that paper three of them independently fatal. And most debunkers plus many academics don't recognise the simple reality.
You show that you are not serious - you ignore my helpful comments and query the sidetrack issue. Ah, well, Two can play the same game.
Let's not say I don't try to help. The scenario is not possible. It was utilised as a legitimate scene-setter premise for the Bazant & Zhou "limit case" hypothesis published 2001/2. It NEVER happened. It never could happen.
As I said in the earlier post it was a scenario that led to lots of misunderstanding over at least 8 years. And Bazant himself fell for the same error in a 2007 paper with Verdure. https://web.archive.org/web/2007022...s/Papers/ProgressiveCollapseWTC-6-23-2006.pdf
It introduces the "Crush Down/Crush Up" hypothesis which an astonishing number of academics, professionals and online debunkers fell for and still haven't seen through.
There are four fatal assumptions in that paper three of them independently fatal. And most debunkers plus many academics don't recognise the simple reality.
Mendel
Senior Member.
That's a scaling problem.
Assume the gravity load of a soda can can be supported by 4 steel straws.
For a can 10cm high you need 4×10cm of steel straws.
For 2 cans, you need 4×10cm and 4×20 cm of steel straws because the second can is higher up.
For 3 cans, it's 4×(10+20+30)=4×60cm of steel straws.
For 5 cans, it's 4×150.
For 10 cans, it's 4×550 cm.
The steel straws per can ratio is 40 for 1 can, 120 for 5 cans, and 220 for 10 cans.
As you scale up, the ratio changes.
You cannot expect the same ratio at all scales.
Asking for the same ratio is asking for the impossible.
(And that was not part of your challenge conditions, either. It's an objection you thought up because you didn't like the suggestion, creating the other kind of impossibility I referred to earlier.)
Gamolon
Active Member
Both the 20 storey and 80 storey structures you speak of were constructed using many components/assemblies that were connected together by various means correct?
Using a simplified scenario, what components/assemblies of the bottom of the 20 storey structure and the top of the 80 storey structure would first be subjected to the enormous forces of the 20 storey structure being dropped onto the 80 storey structure?
The floors correct?
Gamolon
Active Member
Are you going to address my post #44 regarding your belief that there was 425,000 cubic yards of concrete used in both towers?
Okay. Let's imagine structures with four colums, at the corners, and footprints of exactly the same size.
I assume you're suggesting that all four columns of the top building would miss the columns of the lower building. So the bottom floor of the top building would hit the top of at least one column of the
____
*Edited. Oops.
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Mendel
Senior Member.
If you push a book half off the table (at a corner, so the center is still supported), does the table then only support half the weight of the book?
It's interesting that you're suddenly willing to consider a simplified model. The point is that you want the floor connections to be decisive. But if the top section is outside the footprint of the bottom section then the floor connections of the top section are taking much of the hit. The columns of the bottom section are being impacted by the floors from above.
It's basically a question of considering Mick's bookshelfish model as being strong in three dimensions rather than two and then trying to get the top section to fall down through the bottom section in the same way. It doesn't work.
Mendel
Senior Member.
Yes, but I'm not trying to explain the WTC collapse. I'm hoping to get you to think more precisely.
That's a better answer to @Gamolon 's question than you gave before.
Which columns, and which floors?
Yes, that's the question. Why did it work for Mick, and why do you think it won't work in 3D?
It's obvious why it works in 2D. And, I think, obvious why it wouldn't in 3D. (You'd be stacking and dropping box-frames on top of each other rather than loose boards.)
Mendel
Senior Member.
That's the first time you mention frames. Could you answer Gamolon's question again and include them? Sketch a diagram, if you like.
Gamolon
Active Member
Ok.
See diagram below. It is a top down view. Black is the bottom floor of the upper 20 storey section and red is the top floor of the lower 80 storey section. The black 20 storey section drops onto the lower 80 storey section.
The first impact is the upper right corner column of the 20 storey section (black) impacting the top floor of the lower 80 storey section (red).
How big do you think the load/force is at that impact point between the column and the floor and what to you think would happen to the components involved?
I imagine the impact would be sufficient to damage the red floor, perhaps to detach it from the top right red column.
I'm not sure how you're imagining the column connections. But let's assume one connection at each column that is able to carry the load it needs to, plus some safety factor. Let's say each connection has the strength to carry the ful weight of the floor (so the four connections could carry four floors, and the top section is, say, at least 8 floors, so more than enough to overload the connections.
It seems to me that, if we imagine the floor as perfectly rigid, the top right red connection and the bottom left red connection would feel the most strain, and the other two would serve as the end points of an axis of rotation, a fulcrum for a kind of teeter totter. The top right connection would fail downwards, and the bottom left connection would fail upwards.
Shortly thereafter the black floor would impact the top of the bottom left red column. This would have similar impact, but acting upwards on the black floor, perhaps detatching it upwards from the lower right black column and downwards from the top right black column as it teeters.
One of the problems with these models (and also Mick's) is that we assume that the floors cannot be damaged or deformed in any other way than at the floor-column connections. (This is also implicit in the "floor pancaking" image of the process where each floor falls perfectly "flat" on the one below distributing the impact across the entire surface equally.)
Note (for later): on your model lateral forces between the column are transferred only through the floors, right? In the WTC, there were spandrels connecting the columns. These would come into play.
At the end of the day, a 3D model puts two out four floor-column connections outside of the immediate impact of the forces involved (they act more like hinges than brackets). So my prediction would be that, unless the columns have been assembled out of loosely stacked blocks (as Mick does), the collapse would be arrested (by the vertical strength of the columns) sometime before the black building has been completely destroyed.
It would be a mess. But not a total progressive collapse.
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