# Claim: Jim Hoffman's "9/11 progressive collapse challenge" can't be met

This is precisely why it seems like it would be easy to build a simple structure that demonstrates these principles with loads in the 1-3 pound range.
it is simple to demonstrate the principle that if a shelving unit you buy at home dept says each shelf can support 50lbs maximimum and you put 49lbs on it plus drop 10 , it will break. and that 59 lbs will drop on the next shelf (holding 10lbs) so now that shelf is experiencing 69lbs so it will break and land on the shelf below it. and so on and so on. the weight each consecutive lower shelf gets hit with is progressively larger ie. unstoppable.

maybe keiths real life numbers are too big. and smaller numbers will be easier to visualize.

The math problem:

each TT floor (cement slab+floor joists) weighs 10lbs.
each TT floor (cement slab+floor joists) currently holds 10lbs of weight. (carpet,furniture,drywall,paper, etc)
so each "story" (floor plus furnishings) weighs 20 lbs.

each TT floor (cement slab+floor joists) can hold 60lbs of weight.

how many "stories" can fall on floor 100 before it collapses down upon floor 99?

We are still chasing undefined and moving goalposts. This is the latest version of the core challenge as set by @Thomas B
This is precisely why it seems like it would be easy to build a simple structure that demonstrates these principles with loads in the 1-3 pound range.
So he looks for a "simple structure" - a physical model than demonstrates "these principles". AND - he once again specifies who it is to be demonstrated to:
The principles that need to be explained to laypeople are pretty simple.
So he wants to explain "principles" to "lay people" - AND - read the thread - he is targeting those lay persons who NEED simple models of the type that Mick constructed in the previous episode.

He is not looking for heavy grade academic or professional papers or modelling intended for sophisticated audiences. And for reasons I've already posted it would be better if the model visually resembled what it was modelling. Rather than a generic analogous demonstration of principles. Because the target lay person audience is not good at visualising real events from generic model examples. Which was much of the original reason for the challenge.

Then Thomas once again denies reality:
What puzzles me is that these simple mechanisms can't be demonstrated with simple machines in a laboratory or workshop setting.
They can. Some of them have. But the target - the goalposts - keep moving. And "we" keep forgetting who is the target audience.

Simple physical models to explain principles to lay persons who are not good at visualising and NEED physical models.

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We are still chasing undefined and moving goalposts.

I think you are all confusing him with your over complicated language. Maybe try responding to him as if he was your 10 year old nephew. or me
well i actually think he is just trolling us. but Mick says we're supposed to assume people are being sincere, so i'm faking it

If he hired me i would ask for sketches and interview him for details.
"what are you meaning when you say 'progressive collapse'?
"why specifically is dropping a brick not acceptable?"
"why are you saying "cubes""? are we ignoring tube in tube construction?

what is inside these 'cubes'? does the cube have a top and a bottom?
how are the walls of the cube constructed? do they mimic the spliced,staggered mesh pieces of the twins or are you using a solid wall of wood?
how is the 'floor' attached to the walls of this cube?
what specific principles are you wanting to see demonstrated with this model?

etc.

if the model visually resembled what it was modelling. Rather than a generic analogous demonstration of principles. Because the target lay person audience is not good at visualising real events from generic model examples
they have square shelves. i have some in my garage right now. and in a few closets.
But most lay people can grasp Mick models. I can. and i'm a barely high school graduate lay person. I'm not sure taking Thomas' word on what lay people can comprehend is the way to go.

lay people need steps. or 'chaining' is the teaching term. lay people can't compute everything at once.

i would first model why the floor progressively collapsed AFTER the initiating event.
then i would model why the exterior mesh got pulled in or popped out as they lost their lateral supports. (and possibly simultaneously the inner core)

youre on your own for the initiating event. youd have to build something with see through walls. and columsn that could sag. maybe a substance that would sag with water applied vs fire. kids molding clay does that if you wet it within a day or so of drying. ??? would take a while for the water to penetrate though. lots of spritz bottling!

Thanks Deidre for the effort you have expended in the explanation. It is tempting to review and respond to all your comments BUT that only compounds the problems. Let me just comment on a couple of points in what I think is the order of importance.

First your two comments under spoiler:
well i actually think he is just trolling us. >> He effectively is trolling - whether he intends it or not. He has all of us chasing him all over the place as he keeps moving the goalposts. I'm possibly abnormally aware of procedural tricks. The other members are totally ignoring that aspect. Bottom line - and I see it often - I should just abandon the chase. The other members are mostly avoiding the people aspects that the lay person target raises. Remeber they are all technical persons - people differences not main focus in their field of view. Has anyone other than me raised the question of the "target audience of lay persons?"

THEN " but Mick says we're supposed to assume people are being sincere, so i'm faking it << I comprehend and respect Mick's viewpoint BUT I no longer have the patience to treat as serious those persons who are deliberately playing debating trick games. I'm well experienced in resolution of conflicts by conciliation processes. BUT I differ from Mick in that I will deliberatly choose to cut my losses with those who are dishonest game players. They are not worth the effort. And I repeat I comprehend Mick's preference.
I think you are all confusing him with your over complicated language. Maybe try responding to him as if he was your 10 year old nephew. or me
Agreed. But my "complicated language" is directed at only two issues - (1) getting Thomas to decide what "Challenge" he wants to debate and (2) sticking to his "lay person target". And ALL the ohers are looking at a target other than the lay person. Thomas wants the topic to be what he thinks the challenge is but needs to debate it to get his thinking clear. That is why everytime he raises something and I respond he thinks of another problem to include. Notice that I'm the only one he has suggested should drop out of the debate. He even PMed me on the topic. The others are all willing to play along with and not explicitly comment on his shifting goalposts. So I think he comprehends that I have his measure.

Then Deidre - thanks for your suggestions and examples as to how to address a 10 year old. I can do it BUT we are stalled at this time by the ill definiton of both topic and target audience.
lay people need steps. or 'chaining' is the teaching term. lay people can't compute everything at once.

i would first model why the floor progressively collapsed AFTER the initiating event.
then i would model why the exterior mesh got pulled in or popped out as they lost their lateral supports. (and possibly simultaneously the inner core)
Understood. Maybe I"m too stubborn BUT he wont even agree that is his topic....

THEN:
youre on your own for the initiating event. youd have to build something with see through walls. and columsn that could sag. maybe a substance that would sag with water applied vs fire. kids molding clay does that if you wet it within a day or so of drying. ??? would take a while for the water to penetrate though. lots of spritz bottling!
I'm one of few engineering or science qualified debunkers who has actually tried to explain the initiation of collapse to lay persons. And copped lots of abuse from engineers who want to keep it complicated... I don't see this discusion even getting near that - we cannot even discuss the high school simple stage of "progression" without dragging in the wealth of complicated academic and professional papers. Which the target lay person CANNOT understand >> which is why Thomas suggested models in the first instance...

Thanks again for the suggestions. You are right and I appreciate your insights. But pragmatics says I should leave the form of debate to keep going round in circles as favoured by the majority.

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I think you don't appreciate just HOW damned weak and brittle such a human scale structure would have to be to mimic the behavior of the full scale originals under the same gravity.
It would be easier if we could build a 1:200 model and test it in a gravity field 200 times stronger than planet earth's (or 40,000 times stronger? I haven't thought that through). It is kind of important that, between consecutive floor slabs, gravity can pull the entire 11 feet 8 inches before falling slab runs into the next static slab.
I do appreciate this. I know that a model designer would have to insist on a minimum floor height, beyond which the problem doesn't scale because of the way gravity works. I'm not sure it's 11 feet 8 inches, though. As I understand it, even in the WTC cases there would have been enough energy in consecutive drops of 2 or 3 feet.

Now, since the process can be demonstrated with the collapse of little as three floor slabs (but more effectively with six or seven) a "human scale" (6 - 9 foot) model seems possible to me. Using lead slabs and (balsa?) wood beams, columns, and connections should make it possible.

Or using stronger materials with deliberately weak connections -- as long as they're strong enough to withstand whatever "wind" (shear) conditions we decide are reasonable at scale. These "weak connections" (relative to the strength of the column materials) are already part of Mick's model. Especially the magnet ones, which would be ideal for my "kit", actually. It would be pretty impressive with six 2-foot cubes actually. If it could be made to work.

As part of a popular talk debunking controlled demolition theories, I think it'd be a show-stopper, if you will.

as long as they're strong enough to withstand whatever "wind" (shear) conditions we decide are reasonable at scale.
I just had an idea for how these shear forces might be modeled. All you'd need is a weight, a string, and a pulley. Connect one end of the string to the side of the model and the other, via the pulley, to the weight. Now lift the weight off the ground with the pulley, holding the pulley to the height of the point you've connected it to the model. The force on the model at the connection point is (if I understand pulleys) 1/2 the weight at the other end of the string.

I don't know what a reasonable force would be to apply to (which and how many points on) the model. But the useful thing about this is that it would provide a good sense of the proportions because we can compare the shear loading to the dead weight of the slabs.

So what are the attributes of characteristics of the twin tower collapses which to the layman seem counter intuitive?

How does mass which was "locked up" in a standing structure... become the force (mass) which destroys it?

Basic understanding of how structures work is necessary. In the broadest sense the forces (mass) are "aggregated" and "directed to more and more robust elements and eventually to bedrock/foundations.

So... a desk has 4 legs which transfer the weight of the desk and contents to the floor slab it is on... That floor slabs has many "objects/weights" bearing on it - super imposed loads. The slab is designed to support itself and the anticipated super imposed "live" and "dead" loads... like wall and mechanical equipment. Each slab is supported by beams and girders. The slabs inside the core were supported by beams and the beams by girders. The slabs outside the core... (column free open office space) was supported by the perimeter columns and a belt girder surrounding the core. The belt girder was supported by the 24 perimeter columns of the core.
The system repeated for each floor. The columns at each level at to support all the loads from all the levels above it... and so were progressively strong from top to bottom.

What happened to the twin towers is that a single floor slab was over loaded when material from above fell onto it. The slabs itself could break up or the joint between the slab and the perimeter columns and or the belt girder would fail. If these connections fail... the slab breaks free and drops.

Each element... slabs, beams, bar trusses, bearing angles, bolts, welds etc. were designed to support the live and dead loads. The twin tower slabs were designed to support 29,000,000 pounds divided into the number of connections that the slab had to the belt girder and the perimeter column panels. Note that the double bar trusses were supported on spandrels at the perimeter because they were spaced at 80" on center... and the columns were at 40" centers... spandrels "moved" floor slab loads to the other columns... to "share" the loads.

Core side (double trusses shown in purple, Belt girder in red, transfer girder in yellow)

What failed when the floors were over loaded were the connections. The slabs broke free, dropped to the slab below and the process rapidlyy repeated until all slabs were destroyed/separated from the axial support structure. It's likely that after a number of slabs broke free and were falling the force of the impact on the lower slab was sufficient to shatter it. By the time this growing mass of broken slabs and contents tore through 10 or 15 slabs (we don't / can't know the number) the impact was shattering the slabs and contents to dust and gravel sized rubble.

The tower was 95% air by volume... and so if the perimeter at the base of the tower had maintained its integrity.... the entire tower inside the perimeter would be a compact mass of about 4-5 stories high. But a significant amount of the concrete was turned to dust and carried aloft on winds.

The perimeter at the lower reaches of the towers did not maintain their integrity and debris spread away from the tower... like a pile of gravel or sand would.

The above concepts are fairly simple to understand even for the layman. But to do so they need to know what the structural system was... and the most basic even intuitive understanding of structure. A table is not supported on dowel sticks because dowel sticks cannot support the weight (load) of the table top and the dishes etc.

Designers will use the least amount of material to do the job because it costs less to use less. However designs have a margin of error built into their designs. A table may support 50 pounds or 100 pounds but collapse when 200 pounds is placed on it. People understand this. It is not rocket science.

Laypersons would likely have difficulty understanding Euler self buckling (from their own weight)... of columns which are too slender.

Early on the term pancake collapse was used and this was taken too literally. But it was a runaway progressive collapse because the building's slabs were not designed to arrest such a collapse... slabs were designed for "normal office floor loads".

With a little understanding of the structure and intuition the mystery of the collapse becomes trivial.

No need to demonstrate what essentially everyone understands.

Having said that these building collapses and the the designs of these towers have been instructive to designers of high rise towers.

A table may support 50 pounds or 100 pounds but collapse when 200 pounds is placed on it. People understand this. [...]

No need to demonstrate what essentially everyone understands.
This is essentially a model. The layperson thinking about the WTC is entitled to ask, "How would this work with a stack of 10 tables each loaded with 100 pounds?" The lower tables would, of course, have to be significantly stronger than the upper tables.

Deirdre's bookcase is also a model. And the layperson is entitled to say, "I get how the shelves would collapse, but what about the frame of the bookcase? How would it be destroyed?"

At this point you can say that models aren't useful and go to an explanation of "what actually happened", or you can use the layperson's mental model (a stack of tables, a bookcase, or whatever) and suggest some modifications that, while no longer making for good furniture, would represent the strengths and vulnerabilities of tall buildings.

I think our unwillingness to take the second route is part of why CTs can continue to thrive. In other words, to return to the OP, it seems to be a fact that the challenge of "reproducing" a WTC-style progressive collapse hasn't been met.

No... my example is not an analogy to the WTC floor collapses.
The weights on the table top do not separate the table top from the legs.
Table failure could be the top breaks... say a piece of glass... or if wood... the legs may buckle... least likely is the joint between the top and the legs fail.

A glass table with 4 holes in the corners which receive a bolt down to secure the glass top to steel tubing legs which fit over the bolt heads on the table below.
Perhaps maybe somewhat similar to the lower levels would be a glass top table with glasses and dishes which someone drops a stack of plates on the top table... its top shatters all the dishes and the shattered glass drop to the table below.. they shatter the glass... the legs lose support and they too fall over.

Deirdre's bookcase is also a model. And the layperson is entitled to say, "I get how the shelves would collapse, but what about the frame of the bookcase? How would it be destroyed?"

oh yay!!! so you do understand the first model i suggested.

as far as the frame (and core) that is why Mick used small jenga block type pieces to build his walls vs using a 6inch x 8 foot plank of wood.
This is how TT were built...

This is essentially a model. The layperson thinking about the WTC is entitled to ask, "How would this work with a stack of 10 tables each loaded with 100 pounds?" The lower tables would, of course, have to be significantly stronger than the upper tables.
This might be a common misconception among laypeople regarding how the towers were actually constructed. Most tables have their slabs (tops) resting on TOP of their columns (legs) whereas the Twin Towers had their slabs (tops) resting on many trusses which connected to the INSIDES of the columns (legs) via truss seats which is why the "bookcase" model is a closer match.

I think our unwillingness to take the second route is part of why CTs can continue to thrive. In other words, to return to the OP, it seems to be a fact that the challenge of "reproducing" a WTC-style progressive collapse hasn't been met.
I tend to disagree.

No matter what model you come up with to show it's possible for progressive collapse or how it may have happened, it's not going to be 100% accurate and because it's not 100% accurate there will always be those people who will use those shortcomings to continue the CTs and cast doubt.

To a CTer, no model is going to invalidate the building blocks that they used to construct their conclusion that the Twin Towers were demolished. Building blocks such as:

1. It looked like a controlled demolition
2. Witnesses said they heard "boom boom boom" as the towers collapsed
3. No tower has ever collapsed from fire alone
4. They see "squibs" emanating from the tower during the collapses
5. There was an analysis done that showed the towers would withstand a plane impact
6. Large pieces of steel assemblies being ejected sideways at high velocity due to explosions from demolition
7. Supposed evidence of thermite

Just to name a few...

This might be a common misconception among laypeople regarding how the towers were actually constructed. Most tables have their slabs (tops) resting on TOP of their columns (legs) whereas the Twin Towers had their slabs (tops) resting on many trusses which connected to the INSIDES of the columns (legs) via truss seats which is why the "bookcase" model is a closer match.

So suppose you produced 16- 1/4" thick plaster squares 17 1/4" x 17 1/4" and made a 4 sided frame from 36 pieces of lattice 1/2" x 2" x 24" long and "tack" / spot glued them edge to edge to make 4 - 4 sided square tubes. Begin by assembling each side - 18"w x 24"w x 1/2" thk..

Next drill small 1/16" holes then tack glue the 4 sides into a tube of 18 1/2" x 18 1/2" x 24"h... with inside dim of 17 1/2" x 17 1/2"

Next insert the toothpicks into the 1/16" Ø holes at the lowest level of each tube and carefully place one of the plaster squares inside resting on the toothpicks... Repeat for the next row of holes and so on.
Do this for each of the 4 wood tubes and then place one tube on top of the other.. tacking a few spots of glue to attach one tube to the other.

You now have your 8' tall model 18 1/2" x 18 1/2" with 16 plaster floors at every 6".

Turn on a fan and see if the tower topples

Next set up your video cameras and get a pail full of sand and gravel and metal ball bearings etc... set up a ladder climb to the top and start pouring the contents onto the top plaster floor.

At some point the top plaster floor will fail... perhaps the toothpicks break.. or the plaster cracks... and then the same will happen to each successive floor.
The tubes may break apart and fall down as well.

Publish the video on the WWW and tell the truther movement to shut up.

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Mick used small jenga block type pieces to build his walls vs using a 6inch x 8 foot plank
This is fine so long as the blocks are connected in ways that let them deal with shear forces. Your cheap bookcase would use the 8-foot plank solution, right?

The layperson would want to know why it's not silly to build a skyscraper out blocks that are held together only by friction. (Of course they weren't held together only by friction in the WTC. But they were in Mick's model.)

The solution is to build (or at least imagine) a model that can survive a demonstrable amount of shear (in all directions) and totally collapse when the floor connections are overwhelmed from above.

I tend to disagree.

No matter what model you come up with to show it's possible for progressive collapse or how it may have happened, it's not going to be 100% accurate and because it's not 100% accurate there will always be those people who will use those shortcomings to continue the CTs and cast doubt.

To a CTer, no model is going to invalidate the building blocks that they used to construct their conclusion that the Twin Towers were demolished. Building blocks such as:

1. It looked like a controlled demolition
2. Witnesses said they heard "boom boom boom" as the towers collapsed
3. No tower has ever collapsed from fire alone
4. They see "squibs" emanating from the tower during the collapses
5. There was an analysis done that showed the towers would withstand a plane impact
6. Large pieces of steel assemblies being ejected sideways at high velocity due to explosions from demolition
7. Supposed evidence of thermite

Just to name a few...
Of course ANY model lacks the details and can't BE the actual collapse "in miniature"... this gives the truthers an escape... so there is no way to PROVE to their satisfaction that physics drove the collapse not demotion devices.

there will always be those people who will use those shortcomings to continue the CTs and cast doubt.
This is of course true. But it's really a general argument against any debunking effort. (You could say the same thing every time Mick debunks a UFO photograph.) You're right that the claim that progressive collapse can't be reproduced experimentally is just one claim among many. But every little bit helps.

The layperson would want to know why it's not silly to build a skyscraper out blocks that are held together only by friction. (Of course they weren't held together only by friction in the WTC. But they were in Mick's model.)
This is exactly the point I made above.

No model is going to be 100% accurate and people who want to push/believe CTs will forever say it's not accurate and thus proves/shows nothing.

But every little bit helps.
How? If a model can never be 100% accurate in explaining the towers collapsing and providing a loophole for CT pushers to invalidate it, how does that model help in making CTs less believable and in turn making them less popular?

I do not understand at all why the "100 mph wind" target has been chosen as one of the few arbitrary target points.

The collapse is mostly a matter of vertical forces, motions and load bearing capacity.
If you want to consider lateral load bearing capacity, I think you should, at least preliminarily, tackle this separately and explain in what ways a tall building is vulnerable to wind, where it would typically get stressed and be liable to fail, and what ideas engineers implement to address and mitigate those vulnerabilities. Or how, specifically, that problem was taken care of by the structural design of the twin towers.

Then, when you look at the actual failure modes that the actual twin towers underwent, I think you will see that the design elements that address wind shear simply don't play a role: Since the floor trusses failed first, the main means of transferring lateral loads went down the tube (literally!), but the lateral resistance scheme did nothing to protect the floor trusses from failing the way they did - there was no need for it.

Then, construct a model that demonstrates the pancaking failure mode. Mick's model went some way towards this. Deirdre's flimsy bookshelf does, too. Perhaps you could have a model where you fix the vertical elements so you see the pancaking slabs only.

Once your target audience has understood pancaking, the next step would be to build a model that demonstrates the importance of lateral bracing: Make the vertical supports wobbly, but supported by enough lateral elements to be stable, then slowly remove one lateral element after then other.

One aspect at least of the early stage of accelerated collapse progression is how the walls acted like sharp blades to cut floor trusses from columns - either as a falling axe (top part wall descending on inside of stading lower wall), or vice versa standing wall chopping off slabs as top wall slides outside of bottom wall.

Another model to show how air and dust get pushed out as slab falls on slab, reducing volume rapidly.

Etc.

Building blocks such as:

1. It looked like a controlled demolition
But it did not look like a controlled demolition. No CD video I’ve ever seen looked like the WTC collapse. I would even argue that the CTers might have just as much difficulty making a scalable model that convincingly recreates a collapse like what was seen using explosives.

Your cheap bookcase would use the 8-foot plank solution, right?
well no, in my mind i cut the sides into pieces and just stack them. but i didnt want to confuse you before we established you understood the floor/shelf collapse bit.

(Of course they weren't held together only by friction in the WTC. But they were in Mick's model.)
that's where you get into scale issues. if you scale the actual tt mesh connections down to Micks size then friction might be actually too strong.

a demonstrable amount of shear
again scale. 100mph against the Twins with their humongous footprint and humongous weight, is like almost nothing once scaled down to Mick's size.

Next insert the toothpicks
you would need to test that the toothpicks can hold 6 floors. (well the scale of the weight the twins held...assuming i am adding hte furniture weight into the floor for simplicity.) I dont have a pot scale to measure small weight amounts. AND i dont want to do the math, to make my own model.

but yes, you make sure the toothpicks can hold 6 floors then i'd weigh the loose sand before i started pouring it in from the top because you want the loose sand to only equal the 6 floors.

did i say that right?
If the toothpicks can hold 6 floors then the buildign is "sturdy enough". I think thats why they keep bringing up wind... they just want proof the model is sturdy enough to be realistic. well...realistic to their minds in that teeny tiny scale.

Okay, I'm no engineer, but every time I hear someone talk
about building a scaled-down version, I think of Godzilla.

Besides the "scaling gravity" issue, every Gojira fan knows
how easy it is to be taken out of a scene when the big guy
steps on a "hotel" miniature, and it collapses completely
unrealistically...laughably, into big, stiff chunks that more
resemble a cheap toy, than, well, an actual hotel. Obviously Toho was tying to save money wherever they could...but I've often wondered "What if the set designers were given unlimited time and money?" Sure their fancy "hotels" would look more realistic, when stepped on by Katsumi Tezuka's big, rubbery foot...but could they ever look totally real? My point is (though I agree with those who have said that no matter how perfectly a model replicated the real thing, CTs would still find some reason to disqualify it) that it seems to me that trying to really, accurately represent the the necessary strengths, weaknesses, connections, etc. in a miniature, would be incredibly expensive...if possible at all.
(Who, realistically, would be motivated to try?) And the slightest imperfection would still get it waved off.

p.s. The new Godzilla (vs. Kong) comes out this week...presumably a 160-200 million dollar budget means
that "hotels" will get crushed with the latest, finest CGI available.

I do not understand at all why the "100 mph wind" target has been chosen as one of the few arbitrary target points.

The collapse is mostly a matter of vertical forces, motions and load bearing capacity.
If you want to consider lateral load bearing capacity, I think you should, at least preliminarily, tackle this separately and explain in what ways a tall building is vulnerable to wind, where it would typically get stressed and be liable to fail, and what ideas engineers implement to address and mitigate those vulnerabilities. Or how, specifically, that problem was taken care of by the structural design of the twin towers.

Then, when you look at the actual failure modes that the actual twin towers underwent, I think you will see that the design elements that address wind shear simply don't play a role: Since the floor trusses failed first, the main means of transferring lateral loads went down the tube (literally!), but the lateral resistance scheme did nothing to protect the floor trusses from failing the way they did - there was no need for it.

Then, construct a model that demonstrates the pancaking failure mode. Mick's model went some way towards this. Deirdre's flimsy bookshelf does, too. Perhaps you could have a model where you fix the vertical elements so you see the pancaking slabs only.

Once your target audience has understood pancaking, the next step would be to build a model that demonstrates the importance of lateral bracing: Make the vertical supports wobbly, but supported by enough lateral elements to be stable, then slowly remove one lateral element after then other.

One aspect at least of the early stage of accelerated collapse progression is how the walls acted like sharp blades to cut floor trusses from columns - either as a falling axe (top part wall descending on inside of stading lower wall), or vice versa standing wall chopping off slabs as top wall slides outside of bottom wall.
View attachment 43581

Another model to show how air and dust get pushed out as slab falls on slab, reducing volume rapidly.

Etc.
The diagram does not explain how the ROOSD mass aggregated... ie broke free from the structure. That would be the so called "initiation" which took place almost completely hidden by the facade of the building.... and it "appeared" after the building burned for about an hr. One would have to hypothesize something driven by fire/heat..... and mechanical damage that heat would cause. I have described a process I called sink hole top drop. SHTP proposed that the fire heated beams and girders inside the core... which pushed and or pulled columns out alignment reducing the bearing area leading to buckling in the center of the core. This led to the hat truss structure losing support and that led to the antenna to drop and plunger down onto the floors below... breaking much of the apart, destroying the belt girders and that lead to tens of thousands of tons pouring down on the in tact floors below.... VOILA ROOSD mass.

The diagram does not explain how the ROOSD mass aggregated...
That's correct.
My diagram has one, and only one purpose: To visualize how floors would be chopped off the columns either by the descending wall (acting as a blade) passing inside the standing wall, or by the standing wall while the descending wall passes outside.
The idea is to demonstrate one concept at a time.
What Deirdre upthread called "chain-linking" (? or similar) .
My drawing is just a very early draft for an actual, simple physical model.

That's correct.
My diagram has one, and only one purpose: To visualize how floors would be chopped off the columns either by the descending wall (acting as a blade) passing inside the standing wall, or by the standing wall while the descending wall passes outside.
The idea is to demonstrate one concept at a time.
What Deirdre upthread called "chain-linking" (? or similar) .
My drawing is just a very early draft for an actual, simple physical model.
Yes... however I would think that the overloading was the more likely cause of the perimeter truss joints failing. I recall photos of bent over angles on the core and the perimeter and the likely cause was over loading and less likely having columns come down... certainly didn't happen at core side.

The "problem" is more a matter of sequence. Why would the top block translate in both north-south and east-west to allow this sort of guillotine effect?

What made the top blocks drop? Was it loss of axial capacity of the columns at the floors of the plane strike zone? Seems like a good possibility. But would capacity loss be uniform enough to cause the entire block to break free? 2wtc seems to suggest the SE capacity was much lower and it the top block tilted to the SE... as the NW side columns buckled. (path of plane)

1wtc seems to have been a more symmetrical loss of capacity... through the center (path of plane)... plus antenna drop is a tell.... capacity in the center of core was lost before the perimeter. Perimeter of top acted a bit like a rigid box translated by the forces from the interior collapse perhaps.

The top drop is the transition phase resulting from the unseen initiation process. Initiation... what happened from plane strike until top's drop is mystery for which there is no consensus.... except that heat was the driving force... collapse was driven by gravity... transition is driven gravity and lateral forces.

Maybe

One aspect at least of the early stage of accelerated collapse progression is how the walls acted like sharp blades to cut floor trusses from columns - either as a falling axe (top part wall descending on inside of stading lower wall), or vice versa standing wall chopping off slabs as top wall slides outside of bottom wall.

That overlapping of columns with perimeter "sheets" acting as knife edges is near certainly what actually happened. The horizontal displacement varied between the four pairs of sides - two pairs per tower. This single frame clip from a motion *.gif by truther researcher "Achimspok" shows it. Admittedly for the most extreme tilt affected situation. BUT the "columns missing columns" AKA "bypassing" was the key feature of collapse as set up by the initiation stage. AND BTW "tilt" per se is not relevant - the columns missed because of the inevitable way the columns failed. They missed BEFORE there was any tilt. "Tilt" resulted from columns failing and missing NOT the other way round. The YELLOW arrows are my addition and are effectively the same points - left to right reversed - as those labelled "Cuts here...and here" by Oystein.

And that same graphic effectively answers Jeffrey Orlings comment:
The diagram does not explain how the ROOSD mass aggregated... ie broke free from the structure.
... Jeffrey's comment reflects an old point of confusion. The presumption that runaway progression was started by accumulation of mass and the question of "Where did the mass come from?" The model proposed by Oystein and confirmed as what actually happend by the annotated graphic frame clip shows clearly:

1) There was application of a concentrated linear knife edge load to the floors; AND
2) That carried much weight from the still at that time intact "Top Block"; THEREFORE
3) Whether or not there was also some accumulation of debris the perimeter knife edge loading was more than sufficient to start the floor shearing leading to runaway progression often described as ROOSD.

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If a model can never be 100% accurate in explaining the towers collapsing and providing a loophole for CT pushers to invalidate it, how does that model help in making CTs less believable and in turn making them less popular?
You seem to believe that CTists are of one mind and that no one ever escapes from the rabbit hole. It's actually a pretty varied and dynamic bunch, a population of people with different reasons to doubt (different questions about) the received view, changing their minds all the time on big and small matters. You can't help all of them with a single argument or demonstration, but you may be able to help some of them. That's where my optimism comes from, anyway.

To say that only a 100% accurate model would have any effect is like saying it's pointless to teach sixth graders engineering principles using plastic straws because no bridges in real life are built out of plastic straws and tape. But it does seem like an interesting activity and certainly gets them thinking about how different shapes have different strengths. They learn what trusses are in a very practical way. And they learn how a very light-weight structure can carry a pretty serious load.

To return to the OP, the claim is that top-down progressive collapse can't be reproduced in the same way that pretty much all other engineering principles can be modeled in practical ways in the classroom. We can debunk this either by reproducing such a collapse, or by persuasively arguing that, unlike truss design for bridges, the mechanics of progressive collapse in steel and concrete tall buildings "don't scale" to smaller structures and other materials.

I'm still hoping the first option is possible. But I'm listening to the arguments that are being offered here for the second. At this point, I think a convincing model is more within reach than a "100% persuasive argument" (if you will) for the irrelevance of such a model.

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That overlapping of columns with perimeter "sheets" acting as knife edges is near certainly what actually happened. The horizontal displacement varied between the four pairs of sides - two pairs per tower. This single frame clip from a motion *.gif by truther researcher "Achimspok" shows it. Admittedly for the most extreme tilt affected situation. BUT the "columns missing columns" AKA "bypassing" was the key feature of collapse as set up by the initiation stage. AND BTW "tilt" per se is not relevant - the columns missed because of the inevitable way the columns failed. They missed BEFORE there was any tilt. "Tilt" resulted from columns failing and missing NOT the other way round. The YELLOW arrows are my addition and are effectively the same points - left to right reversed - as those labelled "Cuts here...and here" by Oystein.
View attachment 43583
And that same graphic effectively answers Jeffrey Orlings comment:

... Jeffrey's comment reflects an old point of confusion. The presumption that runaway progression was started by accumulation of mass and the question of "Where did the mass come from?" The model proposed by Oystein and confirmed as what actually happend by the annotated graphic frame clip shows clearly:

1) There was application of a concentrated linear knife edge load to the floors; AND
2) That carried much weight from the still at that time intact "Top Block"; THEREFORE
3) Whether or not there was also some accumulation of debris the perimeter knife edge loading was more than sufficient to start the floor shearing leading to runaway progression often described as ROOSD.
In the photo at best two of the four facade could "slice" through the floor.
So no this was not THE mechanism which created the ROOSD mass... or what cause entire floors to break free.

Photos of the truss joists connections post collapse tell a different story...

Jury is out on this...

It's actually a pretty varied and dynamic bunch, a population of people with different reasons to doubt (different questions about) the received view, changing their minds all the time on big and small matters. You can't help all of them with a single argument or demonstration, but you may be able to help some of them. That's where my optimism comes from, anyway.
FULLY AGREED. THAT is why the "who is the target audience?" question is the key issue. It is certainly not "one size suits all" and the technical aspects are secondary.

"If [the WTC's] mode of structural failure is so likely to happen," [asked Hoffman], "why is it so difficult to reproduce?"
I think it's important to keep this question in mind. The OP claim is that Hoffman's challenge can't be met, but there have been many good criticisms on this thread of his particular formulation of the challenge. So, at the risk of moving the goal posts a little, it might be better to restate the claim in a way that captures the spirit, not the letter, of Hoffman's challenge. It's what the truthers who read his challenge take away from it.

Claim: a WTC-style top-down progressive collapse can't be reproduced.

I think this is widely believed in the truther community. They don't think any actual tall buildings are likely to undergo a similar collapse and they don't think anyone will ever build a model that does so. They literally think it can't be done and, therefore, that it didn't happen.

What they do think happened is that the buildings were demolished. And the interesting thing is that demolition can be reproduced, both in real buildings and in models. There is no problem "scaling" controlled demolition and showing how it works in classroom-sized models, or at least yard-sized (with safety goggles on, class!).

Interestingly, I'm pretty sure this goes even for verinage, which we often use to illustrate how a building can destroy itself. (Or am I wrong about this? Is it also not possible to scale-down the mechanics of verinage to demonstrate it to students? Are "simple models" of verinage hopelessly inaccurate and therefore not instructive?)

This is why I think this "argument" (this challenge) is so important. I think it has a lot of persuasive power simply to say, "No one has reproduced the mechanics implicit in the received view of the collapses. If they understand it so well, surely they can demonstrate it? We can point to all kinds of practical and theoretical reproductions of demolition. Why are there no models of top-down progressive collapse?"

We can answer the why-question or we can show them the model. I hope the second is possible.

To say that only a 100% accurate model would have any effect is like saying it's pointless to teach sixth graders engineering principles using plastic straws because no bridges in real life are built out of plastic straws and tape. But it does seem like an interesting activity and certainly gets them thinking about how different shapes have different strengths. They learn what trusses are in a very practical way. And they learn how a very light-weight structure can carry a pretty serious load.

Then Mick's model is more than acceptable for your needs. So what is the point of this discussion?

...
To return to the OP, the claim is that top-down progressive collapse can't be reproduced in the same way that pretty much all other engineering principles can be modeled in practical ways in the classroom. ...
This is patently and obviously false: Mick's garage model with wooden elements joined by magnets does reproduce that top-down collapse.

It just doesn't also, at the same time (or Mick has not demonstrated that it does), satisfy a couple or three other entirely arbitrary demands to reach certain data points.
The challenge, as has been pointed out variously, is arbitrarily set up with a view to making a scale model difficult, while not having a view to making it transparent and educational.

The "100 mph wind resistance" is a nonsense requirement because it is vague and undefined
The "80% of mass ends up outside footprint" is nonsense i.) because it's probably not even true that 80% of the WTC's mass ended outside their footprints and b) this is highly dependent on how materials react to impact (elastic/inelastic, brittle/ductile), and also the ground.

Drop those nonsense requirement, call them out for what they are (dishonest), and accept that top-down collapse has been reproduced - easily and trivially.

...
Claim: a WTC-style top-down progressive collapse can't be reproduced.
...
Dishonest nonsense claim: By including the vague, undefined qualfiers "WTC-style", you retain the threat of retroactively dismissing any model because you subjectively can feel that it is not quite "WTC-style".

Here is a better claim:
Claim: Top-down, accelerating progressive collapse has been reproduced already.

...you retain the threat of retroactively dismissing any model because you subjectively can feel that it is not quite "WTC-style".
I think this is a correct assessment, but I'm a little more charitable about it. That "threat" is just the ever-present possibility of critique in all science. There are always more questions.

But you raise an important point. I think that, with a little effort, we can make models that can help more people "subjectively feel" both the strength and weaknesses of the WTC design than Mick's model does in its current form. It is too easily dismissed by too many people. Making it 3D, removing obvious weak points along the column lengths, subjecting it to (reasonably scaled) shear forces before initiating the collapse, would all help.

(I've said this somewhere before, but the problem with Mick's model is that everyone knows in advance how it will react to dropping the top section; just by describing the design, we know what's going to happen. The demonstration doesn't hit us in the gut with a new insight. There's no subjective feeling involved.)

With some further development, I think at some point almost everyone will "feel" the point of the model correctly. That is, we will have adjusted people's (gut) intuitions about the structure of tall buildings. That's partly what physics demonstrations are for.

At that point, I think the only way to dismiss the model would be, as you rightly say, through dishonesty (that is, someone who does feel the force of the demonstration refuses to admit it). I'm not willing to say that as our models stand now.

It is too easily dismissed by too many people.
who?

it is adequate for most, including grade school students.
i have problems envisioning spatial concepts, and even i have no problem envisioning his model as a square. YOU are free to reproduce it and make it square if you need such a model, but that is more work then you likely realize... and for no real purpose.

Complaining that others aren't willing to do the work for you, is a lot different then saying the model cannot be reproduced square (or tweaked mathematically to your personal satisfaction.)

I think that, with a little effort, we can make models that can help more people "subjectively feel" both the strength and weaknesses of the WTC design than Mick's model does in its current form.
Let's take a characteristic of Mick's model and discuss how we can make it more like the WTC design. How about the "floor" to "column" connections? What, in your opinion, would make those connections in Mick's model more like the WTC connections to a point that people would be satisfied that they are physically modeled correctly?

How about the "floor" to "column" connections?
I think those are fine, actually. Most people understand that the floor connections only had to hold the weight of their own floor (+safety factor).

I think those are fine, actually. Most people understand that the floor connections only had to hold the weight of their own floor (+safety factor).
Ok, what about the column to column connections?