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

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).
Would 4 corner columns be a sufficient representation of the strength/weaknesses of the WTC design?

Ok, what about the column to column connections?
Laterally there aren't any (except via the floors). This is where a 3D square tower would be more accurate, joining the faces at the corners. (Also maybe a square core, that can stand up by itself, rather than a single column.) Something more like "tube in tube" rather than three separate columns joined only by the floors.

Laterally there aren't any (except via the floors). This is where a 3D square tower would be more accurate, joining the faces at the corners. (Also maybe a square core, that can stand up by itself, rather than a single column.) Something more like "tube in tube" rather than three separate columns joined only by the floors.
Yes, columns refer to the vertical components.

Would four corner columns be a sufficient representation of the WTC perimeter? If so, how would you suggest we connect the ends of those columns together?

Yes, columns refer to the vertical components.

Would four corner columns be a sufficient representation of the WTC perimeter? If so, how would you suggest we connect the ends of those columns together?
I meant there aren't any lateral column-to-column connections in Mick's model. In the WTC columns were not just connected to the floors, but also to each other by "spandrel plates" along the faces; and the faces were joined at the corners. The core was a pretty conventional highrise in its own right, with beams between the columns. (It's not completely unreasonable to represent the core as a single column, so long as it doesn't need lateral support not to buckle under its own weight and is connected to the outer walls via some sort of "hat truss" to keep it from falling over when all the floors are removed.)

I do think that a four-column model (corners only) would be enough so long as they are joined by "spandrels" (or just beams) not just via their connection to the floors. But that would be very different from Mick's model and more like my stack of cubes.

[3] The structure must remain standing if a second slab is placed on top of each of the six slabs. (Essentially, the load on each "floor" can be doubled without causing the structure to fail.) [EDIT: That is, the structure must be able to remain standing when loaded with 12 slabs.]
Is double the the floor weight the upper limit of floor connection failures? Meaning that if I put 2.1x the floor load on the floor connections they will fail? You don't want to create floor connections that can support 10x the floor weight. So how do we determine what materials to use when trying to replicate the WTC connections in regards as to when they would fail?

Is double the the floor weight the upper limit of floor connection failures?
To meet the challenge (having it collapse) the designer would probably want to make the connections as weak as possible. So 2 slabs is the lower limit (I'm guessing most models would buckle with 3).

If the designer wanted to make them stronger they'd be free to, however. If, for example, they decided to meet the challenge by having a progressive Bazant-style column failure collapse it wouldn't matter that the floor connections were much stronger because the cubes would be destroyed along the length of the columns.

For me, after building and experiencing my model first hand the issue of being able to build a "better" model was simple. one of time (and, to an extent, money.) I feel there are no real issues in terms of physics, or even really practicality. It's a super-simple engineering issue. Just lots of fiddly parts and connections with an assembly time of hours, for a couple of seconds. I built mine once, and have not felt the need to do it again.

I built mine once, and have not felt the need to do it again.
With all due respect to the energy you put into it, I have to admit I don't quite understand why you felt the need to built it even once. It behaved exactly as I would have predicted and as a much smaller, cheaper and easier-to-build model would behave. What curiosity did it satisfy for you?

With all due respect to the energy you put into it, I have to admit I don't quite understand why you felt the need to built it even once. It behaved exactly as I would have predicted and as a much smaller, cheaper and easier-to-build model would behave. What curiosity did it satisfy for you?

The intent was to create something that could be re-used to demonstrate the general principle of the progressive top-down collapse of a stable tall structure via floor stripping. The need came from people not understanding how the WTC Twin Towers collapsed.

The intent was to create something that could be re-used to demonstrate the general principle of the progressive top-down collapse of a stable tall structure via floor stripping.
But is floor striping the whole story?
Two columns not three. Fewer floors (and lower ceilings). Shorter floor spans. Plastic columns, metal floors. Maybe velcro floor connections?

Two columns not three. Fewer floors (and lower ceilings). Shorter floor spans. Plastic columns, metal floors. Maybe velcro floor connections?
if that is cheaper and easier, why dont you build it?

But is floor striping the whole story?
Pretty much. Floor stripping leads to column instability. Falling masses lead to collisions and hence forces in a variety of directions.

Two columns not three. Fewer floors (and lower ceilings). Shorter floor spans. Plastic columns, metal floors. Maybe velcro floor connections?
Go on then. Build a three-floor model to demonstrate the basics.

Go on then.
why dont you build it?
I apologize. I think we're talking past each other. I really was curious about what you learned from building the model so big. I'm not saying anything could be demonstrated by building it smaller.

Floor stripping leads to column instability.
In the case of the WTC the two "tubes in tubes" were independently stable. That is, if all the floors had been carefully removed, you'd have the outer walls and the inner core standing comfortably even in a pretty strong wind. So there's something more going on than just the stripping away of the floors. The action actually broke the columns (I imagine by pulling and pushing). It didn't just remove their lateral support.

In the case of the WTC the two "tubes in tubes" were independently stable. That is, if all the floors had been carefully removed, you'd have the outer walls and the inner core standing comfortably even in a pretty strong wind. So there's something more going on than just the stripping away of the floors. The action actually broke the columns (I imagine by pulling and pushing). It didn't just remove their lateral support.
The core might have remained standing if you really carefully removed everything else, and there was very little wind. The outer skin was absolutely not stable by itself and would have folded in a stiff breeze - or probably just from natural variations. They were very specifically designed to work together.

The core might have remained standing if you really carefully removed everything else, and there was very little wind. The outer skin was absolutely not stable by itself and would have folded in a stiff breeze
By "everything else" we're talking about only the floor pans and their trusses right? If this is true I have completely misunderstood the structural design of the towers. Looks like I have to go back to the sources.

I meant there aren't any lateral column-to-column connections in Mick's model. In the WTC columns were not just connected to the floors, but also to each other by "spandrel plates" along the faces; and the faces were joined at the corners. The core was a pretty conventional highrise in its own right, with beams between the columns. (It's not completely unreasonable to represent the core as a single column, so long as it doesn't need lateral support not to buckle under its own weight and is connected to the outer walls via some sort of "hat truss" to keep it from falling over when all the floors are removed.)

I do think that a four-column model (corners only) would be enough so long as they are joined by "spandrels" (or just beams) not just via their connection to the floors. But that would be very different from Mick's model and more like my stack of cubes.
My model with tacks of glue between columns is a bit closer to the twin towers... again it's a matter of scaling.

In the case of the WTC the two "tubes in tubes" were independently stable. That is, if all the floors had been carefully removed, you'd have the outer walls and the inner core standing comfortably even in a pretty strong wind. So there's something more going on than just the stripping away of the floors. The action actually broke the columns (I imagine by pulling and pushing). It didn't just remove their lateral support.
not true for the perimeter... it would likely require lateral bracing.

The core might have remained standing if you really carefully removed everything else, and there was very little wind. The outer skin was absolutely not stable by itself and would have folded in a stiff breeze
the perimeter... it would likely require lateral bracing.
If this is true I have completely misunderstood the structural design of the towers. Looks like I have to go back to the sources.

So here's what Wikipedia says about tube structures:
In the simplest incarnation of the tube, the perimeter of the exterior consists of closely spaced columns that are tied together with deep spandrel beams through moment connections. This assembly of columns and beams forms a rigid frame that amounts to a dense and strong structural wall along the exterior of the building.

This exterior framing is designed sufficiently strong to resist all lateral loads on the building, thereby allowing the interior of the building to be simply framed for gravity loads.
I'm pretty sure that the WTC used this design principle.

You guys seriously predicate your understanding of the WTC collapses on the idea that the floor trusses were an essential part of the stability of the structural system? You think the shell and core would collapse on their own if the floors had been carefully removed one by one?

I think this may explain a lot about the frustrations on both sides of the truther-debunker debate. The truthers definitely hold the other view: the floors were suspended between two independently stable structures. The floors' contribution to the "stability" of the structures had to do with reducing sway and may have been important under extreme conditions (e.g., hurricane, earthquake).

I don't feel qualified to adjudicate this. But it explains a lot about why you can't persuade each other.

So here's what Wikipedia says about tube structures:

I'm pretty sure that the WTC used this design principle.

You guys seriously predicate your understanding of the WTC collapses on the idea that the floor trusses were an essential part of the stability of the structural system? You think the shell and core would collapse on their own if the floors had been carefully removed one by one?

I think this may explain a lot about the frustrations on both sides of the truther-debunker debate. The truthers definitely hold the other view: the floors were suspended between two independently stable structures. The floors' contribution to the "stability" of the structures had to do with reducing sway and may have been important under extreme conditions (e.g., hurricane, earthquake).

I don't feel qualified to adjudicate this. But it explains a lot about why you can't persuade each other.
The core was self supporting
The perimeter likely was not and did require lateral bracing from the floor plates

The core was self supporting
The perimeter likely was not
What makes you think not?

You guys seriously predicate your understanding of the WTC collapses on the idea that the floor trusses were an essential part of the stability of the structural system? You think the shell and core would collapse on their own if the floors had been carefully removed one by one?
Of course they were essential. This is just straightforward slender column buckling stuff. The outer walls were essentially just four 200 ft x 1400 ft meshes of steel and glass joined at the corners. Without bracing they would collapse.

This is just straightforward slender column buckling stuff. The outer were essentially just four 200 ft x 1400 ft meshes of steel and glass joined at the corners. Without bracing they would collapse.
Like I say, I don't know what to say. My understanding is that the outer shells could withstand the wind loads on their own (and that's what Wikipedia seems to say too), but maybe they're assuming the bracing of the floors. I don't know. I'll take a step back before I say any more about it.

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https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=910105

"The third major structural subsystem was the floors in the tenant spaces between the exterior walls and the core. These floors supported gravity loads, provided lateral stability to the exterior walls, and distributed wind loads among the exterior walls."
not quite correct
all "gravity loads" are transferred to the columns.

"These floors supported gravity loads, provided lateral stability to the exterior walls, and distributed wind loads among the exterior walls."
Yes, that's the sentence that made a penny a drop for me about why some of you might be thinking this. Again, I'm not qualified to say for sure, but my sense from Wikipedia article on tube structures is that the contribution of the floors to lateral stability would not have been essential in the sense that they'd simply buckly if the floors weren't there. Of course, they did "tighten" the whole thing up and made it more comfortable to work in the buildings. Like I say, I'm not claiming to know ... I'm just saying this is definitely what you're disagreeing with the truthers about at a fundamental level.

It may also mean that you're right that all this modeling stuff is pretty moot. So I'm that much smarter already. Thank you.

(and that's what Wikipedia seems to say too),
The note in Wikipedia leads to this:
Article:
They incorporated an innovative framed-tube concept for the structural system. The columns supporting the building were located both along the external faces and within the core. The core also contained the elevators, stairwells, and utility shafts. The dense array of columns along the building perimeter was to resist the lateral load due to hurricane-force winds, while also sharing the gravity loads about equally with the core columns. The floor system was to provide stiffness and stability to the framed-tube system in addition to supporting the floor loads.

Remove the floors, lose stiffness and stability.

not quite correct
all "gravity loads" are transferred to the columns.
You'll have to take that up with NIST. I just quoted their document for the "provided lateral stability to the exterior walls" part.

Remove the floors, lose stiffness and stability.
I get it.

I see where you're coming from. And I also see where the truthers are coming from.

The floors would definitely add stiffness and stability. But the design concept (as I've so far understood) is that the outer tube could stand (i.e., would not collapse) on its own. It would just sway a lot more in the wind.

I'll see if I can find a way to make up my own mind before I suggest what you should think.

It's just very clear why there's this deep clash about what models might be useful for.

Mick:
I'm gonna make sure I understand like, when you're talking about lateral loads, like, like, say, if you were to remove it, delicately remove the exterior and the floors. And you're just left with the core. Would that core column, be able to, the core of the building, be self-supporting?

Donald Friedman:
It would be self-supporting for gravity, but it would basically be a house of cards, we have very little lateral rigidity. So a wind load much less than the code required wind load in 1968, or now would be enough to destroy that. So yeah, it's what's called a tube structure. It was invented in the 60s, where you have closely spaced columns and beams at the exterior of the building. And that serves as your lateral bracing. John Hancock and Chicago's tube structure. There are any number of them here in New York. So it's a, it was for a while, sort of the in-vogue method of structural design for tall buildings.
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In other words, the later bracing is both the core and the out walls. They need to be connected. I didn't even ask him if the outer walls would be self-supporting because it never really occurred to me that people might think that.

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=910105
In the framed-tube concept, the exterior frame system resists the force of the wind. The exterior columns carry a portion of the building gravity loads, and in the absence of wind, are all in compression. Under the effect of wind alone (not including gravity loads), columns on the windward side are in tension and the columns on the leeward side are in compression. The overturning moments of lateral wind loads are primarily resisted by tube action, i.e., axial shortening (compression) and elongation (tension) of the columns on all sides of the tube. The columns on the walls parallel to the wind direction are in tension on the windward side and in compression on the leeward side. The shear force from the wind loads is primarily resisted by frame action (in-plane bending of columns and spandrels) along the two faces parallel to the
direction of the wind. In a framed tube system, the floor diaphragms play a key role since they carry lateral forces to the side walls of the building, thereby allowing tube action to take place. In addition, floor diaphragms provide lateral support for the stability of the columns.
Content from External Source

I didn't even ask him if the outer walls would be self-supporting because it never really occurred to me that people might think that.
But did it occur to you Wikipedia might say that?
This exterior framing is designed sufficiently strong to resist all lateral loads on the building, thereby allowing the interior of the building to be simply framed for gravity loads.

You'll have to take that up with NIST. I just quoted their document for the "provided lateral stability to the exterior walls" part.
I think you are not understanding my comment correctly.
Floor plates were part of the lateral stability... to resist wind forces.

However there was 3 classes of LOADS
superimposed dead loads - plumbing, mechanical, electrical, glass, interior walls (gwb, studs) HVAC equipment, elevator equipment
superimposed LIVE LOADS - people, movable furniture, office contents
The FLOORS transmitted the superimposed live & dead loads to the columns

In a framed tube system, the floor diaphragms play a key role since they carry lateral forces to the side walls of the building, thereby allowing tube action to take place.
This is a solid hit. The idea that the "tube action" only works when the floors are in place makes (some) sense. Given the other things that are said about the tube system, however, I'm imagining this "action" is mainly relevant to high wind conditions. I just don't see the (empty) shell collapsing on an ordinary day.

This is a solid hit. The idea that the "tube action" only works when the floors are in place makes (some) sense. Given the other things that are said about the tube system, however, I'm imagining this "action" is mainly relevant to high wind conditions. I just don't see the (empty) shell collapsing on an ordinary day.
You have no idea of the aggregate wind load of 25 knots on the side of the twin towers... and the demands it would make on the joints.

Here's Guy Nordenson talking about the design of the outer shell of the WTC in some detail. The whole clip is 5 minutes (from where I've cued it up). At 17:56 he says:
In a tube frame the idea is that the entire perimeter of the structure is mobilized so that the front and back faces of the building become like flanges of a box beam.

Here's Guy Nordenson talking about the design of the outer shell of the WTC in some detail. The whole clip is 5 minutes (from where I've cued it up). At 17:56 he says:

The corner connections are likely not rigid enough to make this into self supporting tube.

I didn't even ask him if the outer walls would be self-supporting because it never really occurred to me that people might think that.
"Most of the gravity and lateral loads are normally taken by the outer tube because of its greater strength." (Wikipedia)

I'm not arguing one way or another, I'm just trying to make clear why some people think the outer shell was doing a lot of structural work without need of the floors for lateral bracing.

At the risk of being a bore, this is why I really would like a good, detailed book on the subject.

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.
It is far from true that all other engineering principles can be modeled in practical ways in the classroom. Consider trying to model a fusion reaction, for example. This should be easy, since it occurs naturally in many places in the Universe. But we've so far been unable to reproduce one sustainably in the lab. Or consider the case of a satellite orbiting a planet; how would you reproduce this in the classroom, just using two masses and the gravitational force between them?

You might want to read up on scaling and the non-dimensional numbers. Understanding the Froude number would show you how to scale a 100 mph wind for a test on a model one-hundredth the size of the original structure, for example.

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