Use of Scale Model or Full Sized models for investigating 9/11 collapses

Can someone explain why free fall is insignificant for a layman such as myself?
It may not be insignificant - it depends on the specific features of the collapse in question.

The underlying problem is that free fall has become the subject of a truther "meme" which in effect says "free fall means CD must have been used because free fall does not happen with 'natural collapse'" (Where "natural collapse" is the 9/11 discussion euphemism for "fire damage")

So the usual context in which free fall is raised is a truther side claim that either explicitly or implicitly relies on "there was free fall THEREFORE it must be CD"

That is not true. Here is why:
Free fall acceleration may occur in a building collapse and it is a feature of the actual mechanism of that specific collapse.

CD is nothing more than one way of initiating a collapse by cutting selected members. It starts the collapse BUT does not cause part of the mechanism to exhibit "free fall". If some other agency caused failure of those same members the exact same collapse mechanism would ensue - without CD. Fire and motor vehicle impact are examples of plausible "other agencies".

So bottom line is free fall does not "prove" CD. Nor does it disprove. The proof rests on other factors than free fall.

Claims that "free fall == CD" are often associated with a number of other confusions about free fall. More detail if you want them. Not easy to explain in abstract - it could be best addressed by reference to a specific example post.
 
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I agree somewhat, but I think the puzzling thing about free fall has to do more with the fact that in order for an entire bldg to fall at free fall, all of the core and exterior columns, and flooring would have to fail at the exact same time. If one area of the bldg weakened before the other, the bldg probably would've leaned over as it fell. I believe things happened exactly as NIST explained, but the whole free fall thing has always rubbed me the wrong way, and that's probably due to my lack of engineering knowledge. And for something to fall for 2.25 seconds at free fall, when it took 8 or 9 seconds for it to collapse completely, that means the bldg fell a 1/4 of the the way without any resistance from beneath. Thats the part I have a hard time understanding or getting around it.
The example you are referring to was WTC7. The free fall was subject of some contention between D Chandler and NIST. It referred to a single "location" (which was not well defined) on one part of the building. So it was not the whole building. It was part of the external wall - after much of the internal structure has already collapsed. It has been measured by methods much more precise that either NIST or Chandler used. Those measurements strongly support that part of the free fall period was actually at accelerations in excess of G. Many people have refused to accept that "over G" is possible in that event. The free body physics to support that truth has been beyond many truthers and a lot of debunkers.

So it wasn't "the building" that fell without "any resistance from underneath" - it was only the perimeter façade wall. There are numerous other aspects which have caused confusion. Ask if you need more explanation...no guarantee but I should be able to assist or point you in a suitable direction.
 
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You could just let the model run. There's just no point.
So, to clarify: you think that NIST developed an expensive model that for you explains how the "facade collapse" could occur with free-fall acceleration -- but having done so they didn't bother to let the model run to the stage where it would show the facade collapsing at free-fall acceleration because there's "no point" in showing that the model could actually demonstrate how free-fall acceleration might occur.

And the only people who want to see the model demonstrate what you claim it could demonstrate (but doesn't because, hey, there's just "no point") are "truthers" and others who think a model of this unique event in the history of structural failures should actually demonstrate what it claims to explain.

Oh, and as I was saying:
If you're going to support the NIST WTC7 model I think you should explain why anyone should accept a computer model for which the input parameters are not available for independent verification, and if you're going to suggest an organisation like AE911 could produce a model I'd ask you what your response would be if they did so but refused to supply the input parameters for verification for the same reasons NIST has given.
 
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Econ41, I appreciate the effort you have made in your most recent post. I am not particularly interested in focusing on the free-fall acceleration issue to the exclusion of a discussion of models, as it is discussed elsewhere. I am interested in the question of modelling the Towers' collapse mechanism, whether in virtual space or physical reality (preferably the latter, of course). I agree that (iii) progression is the most interesting aspect of the collapse of the Towers, from my perspective at least. In other words, I am interested in how "Runaway Open Office Space Destruction" can be demonstrated, not discussed.
 
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So, to clarify: you think that NIST developed an expensive model that for you explains how the "facade collapse" could occur with free-fall acceleration -- but having done so they didn't bother to let the model run to the stage where it would show the facade collapsing at free-fall acceleration because there's "no point" in showing that the model could actually demonstrate how free-fall acceleration might occur.
Correct.

And the only people who want to see the model demonstrate what you claim it could demonstrate (but doesn't because, hey, there's just "no point") are "truthers" and others who think a model of this unique event in the history of structural failures should actually demonstrate what it claims to explain.
Correct.

Remember running the model wasn't free. Running the simulation for three seconds took a week (NCSTAR 1-9 page 544: "25 s of simulated time, which generally took up to 8 weeks "). Why would they tie up their computers for a week to simulate one possible outcome once it was obvious the building would collapse.

And I'm not being dismissive to Truthers, just pointing out that they are the only ones interest in such esoterica. And yet they ask other people to answer their questions.
 
On the more general topic of models (and esoterica), there are some interesting attempts at a 2D model of the skin of WTC7 here by @OneWhiteEye using a mass-spring system. This is something I'd considered doing in JavaScript.

http://the911forum.freeforums.org/wtc-7-trace-data-t353-75.html


OWE notes it "wrinkles up like a crushed beer can instead of dropping like a stone". Now this is obviously an incredibly simplistic model, but it might be interesting to do something like this with more attention paid to correcting for scale, and demonstrate the change in resistance after buckling, and hence downwards acceleration. 3D would be preferable though.
 
Correct.


Correct.

Remember running the model wasn't free. Running the simulation for three seconds took a week (NCSTAR 1-9 page 544: "25 s of simulated time, which generally took up to 8 weeks "). Why would they tie up their computers for a week to simulate one possible outcome once it was obvious the building would collapse.

And I'm not being dismissive to Truthers, just pointing out that they are the only ones interest in such esoterica. And yet they ask other people to answer their questions.
If only NIST would release the input parameters like a credible scientific research organisation, eh? Then its work could be examined by independent experts and those questions could be answered. After all, what took NIST's computers weeks won't take that long in future.

AS I WAS SAYING:

If you're going to support the NIST WTC7 model I think you should explain why anyone should accept a computer model for which the input parameters are not available for independent verification, and if you're going to suggest an organisation like AE911 could produce a model I'd ask you what your response would be if they did so but refused to supply the input parameters for verification for the same reasons NIST has given.
 
If only NIST would release the input parameters like a credible scientific research organisation, eh? Then its work could be examined by independent experts and those questions could be answered. After all, what took NIST's computers weeks won't take that long in future.

AS I WAS SAYING:

If you're going to support the NIST WTC7 model I think you should explain why anyone should accept a computer model for which the input parameters are not available for independent verification, and if you're going to suggest an organisation like AE911 could produce a model I'd ask you what your response would be if they did so but refused to supply the input parameters for verification for the same reasons NIST has given.

It would be great to have that data, but we don't. I know it makes you and others very suspicious, but that's it. This has been discussed a thousand times.
 
It would be great to have that data, but we don't. I know it makes you and others very suspicious, but that's it. This has been discussed a thousand times.
Just out of curiosity, what was NIST's reasoning to not release the data? Is the data sealed, and will it ever be released in the future under the freedom of information act?
 
I agree somewhat, but I think the puzzling thing about free fall has to do more with the fact that in order for an entire bldg to fall at free fall, all of the core and exterior columns, and flooring would have to fail at the exact same time. If one area of the bldg weakened before the other, the bldg probably would've leaned over as it fell. I believe things happened exactly as NIST explained, but the whole free fall thing has always rubbed me the wrong way, and that's probably due to my lack of engineering knowledge. And for something to fall for 2.25 seconds at free fall, when it took 8 or 9 seconds for it to collapse completely, that means the bldg fell a 1/4 of the the way without any resistance from beneath. Thats the part I have a hard time understanding or getting around it.
Mick shows a buckled column and its inability to support any load. At some point in this buckling it may well be incapable of supporting even its own mass and would continue to buckle even without a load. This is due in part to the heating due solely to the act of being buckled quickly under a massive overload. The parts that bend heat up. Buckle a column like this and it may well snap as well, at which point the upper portion is no longer lined up with lower portion and thus there is literally nothing supporting the load.

In addition, note that in Mick's example the model has been simplified to show a column that is still constrained from lateral movement at it upper and lower ends, an infinitely strong constraint. Thus the model takes into account only axial overloads. In a real life situation the connections between column sections would snap.
Imagine the column connections at the eighth floor, as they attach to and support, the cantilever trusses that are the foundation of the upper forty storeys. As the trusses push north and tilt down on their south end due to the internal collapse, these columns are tilted to the north, the column connection to both the trusses and/or the ground level foundation, will shear away. These columns are now free to move laterally at the top and/or bottom and are thus completely incapable of supporting any vertical load at all.

Think of a 2X4 , 8 feet long. Stand it on end and brace it to a foundation, it will stand on its own. It is also capable of supporting an axial load, if you brace it laterally to other columns.
However, remove the bracing to a foundation and it can support much less, remove only the connection to its upper lateral bracing and it can support much less. TILT it even a few degrees off vertical with either , or both, upper and lower lateral constraint(connections) absent, and it will be utterly useless in supporting any load at all.

The WTC 7 fea shows exactly this occurring at the juncture of the preexisting Con. Edison building, and the newer upper forty storeys. Those trusses move to the north.
So, we have massive vertical overload due to loss of the core support, and the lower 7-8 storey columns tilted to the north, probable shearing of column connections at foundation and/or the trusses that support the newer upper structure. We also have buckled columns most likely those nearer to core,, some of which may have snapoed
Is an eight storey free fall suspicious? No!

Additionally, the collapse of the core structure would see the floor beams, and girders being pulled down on their south ends while on the north end , the perimeter kept that end up. This is a force pulling the upper structure down and to the south. When the collapse sees the lower perimeter fail this force will add to the gravitational force on the northern side of the building due to its( the northern part of the upper structure) mass.
This would suggest the possibility of exceeding an acceleration of greater than 'g'. In fact one analysis of the last seconds , including the 2.25 seconds of 'freefall' does indicate a ramping up of acceleration to, and slightly beyond 'g'.

So, why not model this? Twofold, first its an incredibly complex period and the true nature of individual structural members as connections cannot be known.
As reference, note that in the fire spread computer simulation, since it cannot be known what doors were open and which were closed, the Sim was run with the parameter of all doors open. Do you model all 8th floor connections sheared, in the collapse Sim?
Second, its not necessary. At the point when the exterior begins to move, the building is completely doomed to global collapse. NIST was tasked with design reccomendations to mitigate global collapse. That would require that they make reccomendations for design, that will arrest the progression of collapse such that the structure never reaches this point, so why bother with the last three of seconds of a 16-20 second collapse? Its the previous 13-17 seconds that are important wrt progressive collapse.
 
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But the crucial part of the collapse, the initiation, is visible, and is what interests investigators. The complete collapse all the way to the ground is less so and doesn't really justify massive investigation.
Actually, even that is mostly hidden. The exterior walls mean we have to make educated guesses as to what occurred in the time period leading up to initiation. This includes not only the towers but #7 as well. We do have, for #7, the very obvious in falling of the EPH, followed by the more western structures, and the 'kink' and then final global collapse. For the towers we have the inward perimeter buckling.

Cube Radio says that if an aircraft computer design failed we wouldn't say its due to unknowns. True, because in an aircraft design and its behavior in a virtual world, the designer controls every initial state of the aircraft.
In a forensic application , computer simulation of a building collapse must make educated guesses as to the conditions on the initial structure. For the aircraft impact simulation this can be pretty close to reality. But! The aircraft is another story. Thus the aircraft speed and attitude were modeled several times using values incorporating the margin of error for these parameters which were derived by observation of the video(s). The results bracketing these margins were compared to observation of the structure's exterior after impact. Whatever matched best was considered more accurate and thus its predictions of the interior damage were also considered as accurate as possible.
Fire spread and heating simulation has less solid data to go by for initial conditions. We do not know exactly where the jet fuel went, we can't know that. We don't, and can't, know the exact amount or distribution of flammable contents in the structure after the impacts. We make educated guesses for these and compare to what is observed from the outside, and compare to forensic examination of steel samples from the fire/impact floors that can be positively identified. From that we make a determination that the simulation was as accurate as possible.
For structural response to fire we have the results of the aircraft impact simulation to go by as an initial condition of the building, and we have the results of the structural heating from the fire simulation, both of which have a certain amount of accuracy and inaccuracy. Thus we begin this computer analysis knowing full well that the details are in the ballpark of the real situation, but not perfect.
By now everyone should see how the difficulties are getting greater, in precisely matching reality.
Suggesting that this is directly comparable to the virtual flight test of an new aircraft design is rather odd.
So, the NIST engineers go as far as collapse initiation. At this point its obvious that the building is doomed, their task is to recommend how to not have a building get to this point. What happens after this point is irrelevant to NIST and to any engineer trying to design better buildings.

Similar issues arise wrt #7.
 
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Econ41, I appreciate the effort you have made in your most recent post. I am not particularly interested in focusing on the free-fall acceleration issue to the exclusion of a discussion of models, as it is discussed elsewhere. I am interested in the question of modelling the Towers' collapse mechanism, whether in virtual space or physical reality (preferably the latter, of course). I agree that (iii) progression is the most interesting aspect of the collapse of the Towers, from my perspective at least. In other words, I am interested in how "Runaway Open Office Space Destruction" can be demonstrated, not discussed.
Mick already did that with his models.

As far as letting the NIST model continue, they would need to include computer memory capable of approx ten times as many connections as the impact and above portion of the buildings, the run of the simulations that were done took weeks(months?) to complete , so we could expect approx ten times as long and thus ten times to computer time expense.
To what end, what is the value in it besides partially satisfying the curiosity of a few internet denizens?
 
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NIST recognises a 2.25 second period of free-fall but as we agree econ41 the model does not proceed to this point in the collapse sequence. For now I will suggest that it concerns me because freefall acceleration is without doubt a feature of the collapse that has raised questions and is a frequent focus of objections to NIST's conclusions; it cannot be disputed that this aspect of the collapse is not explained by the model. The "realistic advantage" as you put it would be to demonstrate with a model (to the "layperson", if you will) that this controversial and unique aspect of the collapse has indeed been fully understood, as is claimed.

However, I would prefer to focus on the collapse sequence of the Towers, as there are no successful models of this of any kind, to my knowledge.

The questioning and objection you refer to here comes almost exclusively from the so-called , truther, groups.
It does not come from any established engineering organization.

You bristle whenever anyone brings up trutherism but you are addressing truther issues whether you are one or not and thus those issues will be treated that way. Its not personal.
 
The questioning and objection you refer to here comes almost exclusively from the so-called , truther, groups.
It does not come from any established engineering organization.

You bristle whenever anyone brings up trutherism but you are addressing truther issues whether you are one or not and thus those issues will be treated that way. Its not personal.
Is it true that the design if the wtc 1,2, and 7 were unique and were built like a bldg within a bldg and that there are no other bldgs standing like them. Some have argued the towers were flawed and had design issues and that they chose cheaper costs over safety.
 
Econ41, I appreciate the effort you have made in your most recent post. I am not particularly interested in focusing on the free-fall acceleration issue to the exclusion of a discussion of models, as it is discussed elsewhere. I am interested in the question of modelling the Towers' collapse mechanism, whether in virtual space or physical reality (preferably the latter, of course). I agree that (iii) progression is the most interesting aspect of the collapse of the Towers, from my perspective at least. In other words, I am interested in how "Runaway Open Office Space Destruction" can be demonstrated, not discussed.
(My emphasis.)
Understood - and that is the specific narrow focus I am taking - despite the temptations to join in the several other discussions which are in progress.
The two topics we are joining together are:
1) Understanding the "ROOSD" process; BY
2) Demonstrating using physical models.

The gap between us is that:
a) You see models as both necessary and better than other methods; WHILST
b) I do not see any need for modelling of ROOSD (And don't see further modelling, esp physical modelling, as practical/possible for other main stages of the collapses but those not our current focus.)

I will need to press you further as to why you see modelling as either necessary or superior. And, continuing my intention to keep tight focus, here are what should be two points of final focussing:

First a minor one. "ROOSD" is one of the three mechanisms of the Twin Towers global progression collapse. The three are (1) "ROOSD" - runaway down the open office space WHICH (2) Left the perimeter unbraced causing/allowing "Perimeter Peel Off" WHILST (3) A similar strip down mechanism removed the bracing in the core leading to core columns falling. Discussion of these issues often is ambiguous as to whether ROOSD includes the total three mechanisms. My focus on the single mechanism.

Second as to the types of physical models. We have a "two by two" selection.
x) Do we model part or all; AND
y) Do we want it to "look like" the real thing OR do we want to demonstrate the engineering/physics. The two are not mutually exclusive BUT some engineering models focused on a specific detail may not look like the real event.

I'm taking your need as being for something that "looks like" the real thing to suit a lay audience AND deals with the full mechanism.

However lets first look at some building blocks. The key reason why ROOSD happened was identified in this post by deirdre:
i think this pic says it all. the little clips holding Everything together are really all that matters, not sure how to model that.

For ROOSD to happen all that was needed was for a big enough impact load to land on that section of floor and it would easily shear the clips. Do you understand that and do you agree?

Now that is only one element and for each floor there were many such elements - but the same truth applies to each. I see no difficulty in concluding that - if one would shear THEN under the same circumstances - all the others would also shear. Do you agree?

Now let's take that part of the mechanism and put it into a physical model. This is one of the best efforts I have seen:
The core was not brought down by forces from above, although that played a part. It was brought down by a combination of lack of lateral support, and then being pushed and pulled from the side by a violent wave of thousands of tons of steel and concrete.
Look at the center column here:

Now try to imagine what the collapse (of this model) would look like if it were 30x as high. Even if I tape up the center column quite firmly, it's still going to collapse by itself at a certain height, and the vast accumulation of debris in the falling wave is going to make it collapse (in sections) much quicker.
IMO it looks very similar to the real thing. Near enough for me to see that it is relevant and a sound modelling of the qualitative aspects of that single "vertically aligned stack of joists" part of the total mechanism. (Note "qualitative" - we may need to discuss "quantitative' at a future stage.)

We are now at a decision point:
Are you comfortable that you comprehend the mechanism of single floor joist element failure as the building block of "ROOSD"? AND
Do you agree that those single building blocks add up to the complete ROOSD mechanism?

No problem if you don't - it simply means I have more explaining to do. If so say which bits.

If you are in agreement so far we have all the foundation work completed to address the real questions including":
A) Would a model do it better?
B) For whom?
C) Why? / Why not?
...and some more that will no doubt emerge. :)


And I may wander off to join in a couple of the other mini-discussions - in case some old friends think I have totally lost the plot and some potential new friends doubt that I ever knew where the plot was. :rolleyes:

EDIT PS Thanks to Mick West for help with the graphics.
 
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Is it true that the design if the wtc 1,2, and 7 were unique and were built like a bldg within a bldg and that there are no other bldgs standing like them. Some have argued the towers were flawed and had design issues and that they chose cheaper costs over safety.
Yes, and no. The towers were a new design when built. The relatively lightweight floor pans alowed building higher. The outer "tube", inner "tube" were also connected and mutually braced each other, by way of the floor truss connections. This resulted in wide open floor space. No columns or walls to contend with over a huge area on each level.
Further reducing mass was the lack of diagonal interlevel bracing. Packing columns close in the core and connecting them to each other with heavy beams was strong enough.
This design has been copied , albeit modified, in other structures.

WTC7 was built on a lot that already had the Consolidated Edison station. To avoid the expense and disruption of rebuilding that elsewhere, the new structure was built around and over it. To accomplish this basically, an elevated foundation for s forty storey building was constructed above the existing building while being tied into the rest of the new structure going up over the expanded lot.
 
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Is it true that the design if the wtc 1,2, and 7 were unique and were built like a bldg within a bldg and that there are no other bldgs standing like them. Some have argued the towers were flawed and had design issues and that they chose cheaper costs over safety.

Fazlur Khan's tube design in now almost universal in skyscrapers. The BOK building in Tulsa Oklahoma is a "little brother" of the Twin Towers, with a "tube-in-tube" design. (http://en.wikipedia.org/wiki/BOK_Tower) The Willis Tower in Chicago (formerly the Sears Tower) is a bundled-tube design.

All engineering involves trade-offs. It's not a "flaw" that tube designs use the floor system for lateral stability, it's a feature. The weakness in the design is that in the unlikely event of a series of floor collapses, the entire structure will "unzip" itself. This was recognized early-on. Part two of the Twin Towers' collapse involved a series of floor collapses, but at that point the buildings were a total loss, anyway. The improvements in tube-in-tube designs since 9/11 have involved mostly improved means of evacuation rather than trying to staunch a progressive collapse.

There have been progressive floor collapses in other high-rises (see the Parker Building) which went from roof to ground floor in places, although the entire structure did not collapse, since it was built in the classic manner of a skeleton of massive columns and girders. Nonetheless, the Parker Building was still a total loss, and firefighters were killed by the floor collapses.
 
Yes, and no. The towers were a new design when built. The relatively lightweight floor pans
EDIT PS Ooops - Redwood types faster them me. still - i'll leave the post "as is" - they are closely related themes.

There were two big goals - get the open office space and get the height. The height challenge required design to be light as possible. The OOS design also helped.

The comments about design issues need to be kept in context. All three towers were pushed way outside their design envelopes. They are probably among few buildings ever to be so severely traumatised. They failed.

Any building pushed outside its limits will fail. The only variables being how far outside before it fails and how it ultimately fails. Most professionals would regard the structural success of all three as outstanding. The failures were not in structural design They were in lack of safe egress for occupants and redundancy of fire fighting systems. And, again, both of those weaknesses only resulted after the building was pushed way out of the envelope. How the events should impact on the commercial and risk management aspects of future designs is a much broader question...
...not one I can pretend is within the scope of the "modelling" topic. :rolleyes:
 
Cube Radio says that if an aircraft computer design failed we wouldn't say its due to unknowns. True, because in an aircraft design and its behavior in a virtual world, the designer controls every initial state of the aircraft.

True. As I've mentioned in passing, a failure in an aircraft design wouldn't be from the calculations "being too difficult", it would be from the designers not taking something into account in their calculations.

The first commercial jetliner, the DeHavilland Comet, suffered a series of catastrophic mid-air breakups. This was not from problems in calculations; it was that the designers failed to consider that its square passenger windows induced structural stresses that eventually resulted in catastrophic failure.

The first failures were blamed on bad weather or possible sabotage; it was only after a series of failures that the designers realized that something was seriously wrong with the design. The square passenger windows were replaced with ovals, and after that, the Comet had an excellent safety record, though its sales never really recovered. A pity, as it was really an excellent design, except for that initial flaw.

Subsequent designs by other firms of first-generation jetliners learned from the initial failures of the Comet. As I said, in engineering you learn more from failure than from success.
 
... As I said, in engineering you learn more from failure than from success.
Doctors bury their mistakes - engineers tend to make them in full public view.
Tacoma Narrows Bridge and Westgate Bridge as examples.

And Tacoma is "on topic" - they did physical modelling in the post collapse studies.

AND model testing was better then "explaining" in that instance. :)
 
On the more general topic of models (and esoterica), there are some interesting attempts at a 2D model of the skin of WTC7 here by @OneWhiteEye using a mass-spring system. This is something I'd considered doing in JavaScript.
That one wasn't too serious, just a throwaway. Another before it in the same thread was a bit more serious in that it attempted to address a very narrow point: how much elastic restoring force (relative to mass density) does it take to have a snap-down effect of the right magnitude in a structure that size? What I'd call "the crossbow" for lack of a better term.




(Details here)

Supported at the corners until the structure has sagged, then corner support removed. Elastic rebound then causes the corners to descend quicker than the middle, as shown in the graph where red is freefall, green is corner and blue is midpoint horizontally. Obviously the corners snap down at greater than g, while the middle exhibits less than g; the structure as a whole (motion of center of mass) descends at freefall.

This is one of the proposed mechanisms for over-g, maybe the most commonly encountered, with minor variations. The idea is, part of the structure is already in motion and, by the time the perimeter support is lost, the part in motion "drags down" the perimeter. In this case via elastic strain potential as opposed to entrainment/impacts. In other words, the interior applies downward force which is added to the existing gravitational force acting on the exterior. The 2D model can be extended to 3D by way of symmetry to a block (supported on opposing edges) or a cylinder (supported on the circumference), the latter being perhaps mechanistically more closely analogous to what's been proposed for WTC7, though not much so.

Looking at displacement...



...then comparing to mid/outer traces of the real thing...



It appears there's something similar going on. How exactly that could be the case, I don't know. Conceptually, a piece of cake. All is well until you consider how stiff the structure has to be to have something like this happen to any appreciable degree; startingly so. Naturally, when rolling in a disintegrating structure, it becomes a bit more troublesome. Fractured members are notorious for applying no force.

My opinion is maybe looks are deceiving, and this wasn't the (primary) mechanism.
 
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Regarding the last graphic, taken from video. If the NW corner was freefall, or even over-g a bit, the middle part of the building is well under freefall. This is still just considering the perimeter, not the interior.
 
OWE notes it "wrinkles up like a crushed beer can instead of dropping like a stone".
I think the only real value of this particular exercise was in seeing one similarity between this and the curtain distortion in the NIST physics sim (as well as a number of quite involved FEA works I've seen since then): excessive ductility. There's no way a real structure would deform like that.
 
Just to give an idea of how prevalent excessive ductility is in modeling, there's an example on the previous page. Mick posted this graphic:
No, it just means there was very little resistance from beneath.

The key to understand this is "buckling".

Columns are incredibly strong, but only as long at they are straight and vertical. Once a column bends a bit, or deviates from vertical, then the amount of weight it can support is greatly diminished, the bending or deviation increases, and BAM, the column collapses. It gives very little resistance, just as if it had been blown away.
This supports Mick's point to a 'T' but will overestimate resistance afforded because fracture is not incorporated, making the point even more strongly.
 
This is one of the proposed mechanisms for over-g, maybe the most commonly encountered, with minor variations. The idea is, part of the structure is already in motion and, by the time the perimeter support is lost, the part in motion "drags down" the perimeter. In this case via elastic strain potential as opposed to entrainment/impacts...
Or simply a form of rotation due to different fall err..."dynamics" in the relevant bits of the collapsing interior. If some tied together bits of the collapsing interior have the bit nearer the external face going "faster" - there is a plausible basis for additional pull down adding to gravity on the external façade. That was how I explained it on that other forum...took a while to get acceptance.

And there is the ever present need when discussing freebody accelerations esp "over G" to not lose the plot and start thinking velocities. (Hence the cop outs "dynamics" and "faster" in what I just posted - to save many words of pedantry.)
 
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...because fracture is not incorporated...

That's brilliant! In all these years of 'arguing' with so-called "truthers" I had completely forgotten the aspect of simple steel fracturing (besides the attachment points, which can be thought of as a sort of 'Achilles' Heel' of any building).
 
...This supports Mick's point to a 'T' but will overestimate resistance afforded because fracture is not incorporated, making the point even more strongly.
Fortunately for most of the column fail bits of WTC collapse explanations the difference between "as designed" and "failed" is orders of magnitude - which is what Mick was saying with me using different words. The various stages of real event WTC collapses can all, in my experience, be explained by ball park approximate maths simply because the failure states result in overwhelming loads for what is left after failure. Whether columns bend/buckle/break/whatever. Same goes for most of the other failure mechanisms i.e. floors shearing in ROOSD and core beams stripping down in shear a process analogous to ROOSD but little discussed.
 
Or simply a form of rotation due to different fall err..."dynamics" in the relevant bits of the collapsing interior. If some tied together bits of the collapsing interior have the bit nearer the external face going "faster" - there is a plausible basis for additional pull down adding to gravity on the external façade. That was how I explained it on that other forum...took a while to get acceptance.
Yes, there is that, too. The devil in the details for me tends to be the bit that involves being "tied together". None of this stuff stretches - at all - as far we're concerned at the macroscopic level. It complicates the mechanism somewhat. So, the members are not rigid bodies and they can bend and break but won't rotate much or elongate before they break.
 
That's brilliant! In all these years of 'arguing' with so-called "truthers" I had completely forgotten the aspect of simple steel fracturing (besides the attachment points, which can be thought of as a sort of 'Achilles' Heel' of any building).
Glad you liked it! You can dismiss about 90% of all idle chit chat with this simple fact.
 
Yes, there is that, too. The devil in the details for me tends to be the bit that involves being "tied together". None of this stuff stretches - at all - as far we're concerned at the macroscopic level. It complicates the mechanism somewhat. So, the members are not rigid bodies and they can bend and break but won't rotate much or elongate before they break.
Agreed BUT it only takes one strand of still joined together bits of steel to impart the downwards pull. Not rigorous argument but reverse the logic - what chance in such a collapsing mess that there was not one or more bits tied together and tied to the façade?
 
Glad you liked it! You can dismiss about 90% of all idle chit chat with this simple fact.
Provided the audience accepts the simple fact. Won't post my two favourite examples of "bleedingly obvious fatal errors" which no one but me seems to see as obvious.
 
Agreed BUT it only takes one strand of still joined together bits of steel to impart the downwards pull. Not rigorous argument but reverse the logic - what chance in such a collapsing mess that there was not one or more bits tied together and tied to the façade?
Good point. But run through the forces associated with each member of an ideal representation in your mind; think about what it takes for something to pull down on something else for a sustained interval, deformation allowed but neither elongation nor large connection rotations allowed. It's starts to get a little gnarly in the details. Something can provide a brief impulse "yanking" on another part but, without differential motion over the interval, it's gone in a blip. Sustained acceleration requires a series of blips that act over time, or elongation to allow a part to continue exerting force until the relative velocities are the same. Rotation is more forgiving, maybe.

Is it time for diagrams?
 
Provided the audience accepts the simple fact. Won't post my two favourite examples of "bleedingly obvious fatal errors" which no one but me seems to see as obvious.
Well, heh heh, of course. It was in this forum not too long ago where it was rejected by T Sz, who brought out a photo of a severely bent box column to illustrate how much rotation could be accomplished without fracture... except his example WAS fractured, you couldn't see it from the view he chose.
 
Good point. But run through the forces associated with each member of an ideal representation in your mind; think about what it takes for something to pull down on something else for a sustained interval, deformation allowed but neither elongation nor large connection rotations allowed. It's starts to get a little gnarly in the details. Something can provide a brief impulse "yanking" on another part but, without differential motion over the interval, it's gone in a blip. Sustained acceleration requires a series of blips that act over time, or elongation to allow a part to continue exerting force until the relative velocities are the same. Rotation is more forgiving, maybe..
All agreed - its not a point worth much effort in the context of this thread.
Is it time for diagrams?
Nah. Drifting too far off topic.
 
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Well, heh heh, of course. It was in this forum not too long ago where it was rejected by T Sz, who brought out a photo of a severely bent box column to illustrate how much rotation could be accomplished without fracture... except his example WAS fractured, you couldn't see it from the view he chose.
Don't tempt me. The "bleedingly obvious" examples include some where people (from both 'sides') dispute axial contact occurring between falling top bit of broken columns and static bottom bit.

And overlook that the starting assumption with the established state of top part falling means that the time for the impact (or "jolt" :rolleyes: ) is already past history.
 
Good point. But run through the forces associated with each member of an ideal representation in your mind; think about what it takes for something to pull down on something else for a sustained interval, deformation allowed but neither elongation nor large connection rotations allowed. It's starts to get a little gnarly in the details. Something can provide a brief impulse "yanking" on another part but, without differential motion over the interval, it's gone in a blip. Sustained acceleration requires a series of blips that act over time, or elongation to allow a part to continue exerting force until the relative velocities are the same. Rotation is more forgiving, maybe.

Is it time for diagrams?
Are you expecting that as the core failed, that the interior floors to the north of the core reached an equilibrium before onset of global collapse?
I envisage an increasing tilt of those floors, between north facade and their southern extent. These floors would largely be cantilevered from the north perimeter, with some support from the east and west perimeter. After all we know that they were all already tilted towards the north wall's kink. There is no reason not to suspect a similar tilt towards the core.
Concentrating on the western two thirds of the building, that constructed over the Con-Ed substation, this would cause a force on the north perimeter acting to pull it down and south. It would represent a mass with an initial velocity ( in rotation) , at the onset of global collapse(the approx last five seconds). If these floors continue to rotate the downward component of their instantaneous acceleration over and above that now being contributed by the fact that the north perimeter is also falling, will increase, thus also will extra force in the downward direction. This floor pan rotation would also ensure toppling to the south.

As for the eastern third of the structure, it did not rely on the same core structure that the western two thirds did. Nor was the eastern part of the south face badly damaged, unlike the more western part of the south face. There is less impetus for this part of the structure to fall to the south. Its quite probable that global collapse began first on the western south side. This would pull the southern part of that eastern section while the northern side is still standing imparting a twist to that portion. I recall , though can't confirm, that this eastern portion did twist as it fell and thus ended up smacking the Fitterman building to the NE of WTC7.

But that's just my opinion, I could be wrong.
 
There was a LOT of weight above the impact points (referring to WTC 1 & 2). The simple FACT that WTC 2 collapsed BEFORE WTC 1 is evidence.

I do NOT understand why the "truthers" ignore this fact. It is so bleeding OBVIOUS!!!
 
There was a LOT of weight above the impact points (referring to WTC 1 & 2). The simple FACT that WTC 2 collapsed BEFORE WTC 1 is evidence.

I do NOT understand why the "truthers" ignore this fact. It is so bleeding OBVIOUS!!!
I agree WW, but I honestly think it has less to do with ignoring the "obvious" and more to do with their perception of what initiated the collapse. Which plane hit first is irrelevant to truthers, as far as I can tell...
 
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