Anologies for WTC 1 and 2 collapses....

[jaydeehess]

It struck me today that if the towers had been hit much lower down they may well have had the top portion topple over.
Say it was hit at the 55th floor. Then the top section would have its CoM approx 5X further from the hinge than the upper section of the towers had on Sept 11.
That would mean that a lesser tilt angle would put the CoM of that upper section at or beyond the side of the lower structure. At that point , if the hinge gave way , most of the mass of the upper section would be outside the footprint of the lower section and not impact it.

 

[Illuminist14]

It struck me today that if the towers had been hit much lower down they may well have had the top portion topple over.
Say it was hit at the 55th floor. Then the top section would have its CoM approx 5X further from the hinge than the upper section of the towers had on Sept 11.
That would mean that a lesser tilt angle would put the CoM of that upper section at or beyond the side of the lower structure. At that point , if the hinge gave way , most of the mass of the upper section would be outside the footprint of the lower section and not impact it.

I queried this not so long ago, and its true what you say the structures above the impact areas, were not tall enough to significantly activate an inverted pendulum effect. If it was as you say 55 floors up or lower we would surely see a toppling take place.

 

[tfk]

Illuminist,

Here's a great demo of the "illusion" that the angular velocity stops.

http://www.hotwebclips.com/Home/Show/NBbz2eIoVDQ

In this video, a tall tower of wood blocks is toppled from the bottom. The column starts to rotate, but then collapses at the bottom.

The angular velocity of the top does NOT stop, but the linear velocity increases constantly giving the ILLUSION that the angular velocity decreases.


tom

 

[tfk]

I queried this not so long ago, and its true what you say the structures above the impact areas, were not tall enough to significantly activate an inverted pendulum effect. If it was as you say 55 floors up or lower we would surely see a toppling take place.

I do not believe that this is true. Not certain, I just don't believe it.

Specifically, I don't believe that there is anything that you could do to get a tower, built with a lattice array of columns like the WTC, to topple to the side, even if this was your goal.

Obviously the ideal task would involve explosives & take out a "wedge" of supports on one side. The crucial task is to leave the remaining supports as strong as possible (to support the tilting upper segment) while taking out just enough to cause the collapse.

This ain't that far from the situation that WTC2 was in, just before collapse. And yet, it came straight down. I don't believe that lowering the impact point would have changed anything.

This relates directly to Bazant's argument in one of his early papers as to why the building did not (& could not) topple to the side. By the time the building got to about 8° of tilt (about what WTC2 did), the bending stresses on the remaining column would exceed the columns' ability to support the upper block, they buckle & the building comes approximately straight down. Keeping whatever angular momentum it had generated in its tilt, which would be "not very much".
___

Here's a pretty good demonstration of some radical removal of supports, down low, from a tower, and yet the tower comes straight down. Granted they intentionally removed the supports in a way to keep the load symmetrically balanced. So perhaps not a directly relevant video. But pretty fun, nonetheless.

This is a video capture of the start point.

There are over 5000 blocks in the tower, which stands 35' tall. There are 20 vertical blocks on the bottom (& as far as I can tell, every) story.

The guy in the white shirt is going to take out individual blocks from the bottom layer, one at a time. You should guess how many you think that he can remove before the tower collapses.

Don't read the blurb, it's got a spoiler in it. Just watch.

You'll be surprised at the result.

http://www.hotwebclips.com/Home/Show/DtxfXcV7Nn4

Tom

 

[Animal]

I queried this not so long ago, and its true what you say the structures above the impact areas, were not tall enough to significantly activate an inverted pendulum effect. If it was as you say 55 floors up or lower we would surely see a toppling take place.

No, gravity loads would cause structural failure long before any real toppling could occur

 

[Illuminist14]

I do not believe that this is true. Not certain, I just don't believe it.

Tom

Hi Tom,

I will defo watch the videos when I can, I get what you say now about the structures columns failing before the top section can actually get much further than 8 degrees.

No, gravity loads would cause structural failure long before any real toppling could occur

Hi animal,

So obviously as floor 55 is much further down the structure, there is more loading on the columns than the ones on the upper floors, therefore you're saying there would not be enough strength from the columns to allow for a tilt equal or greater than what we observe on 911?

 

[Sabretooth]

This was a quick little gif I threw together to kind of show what was happening during the collapse.

*Not to scale

 

[Illuminist14]

I doubt it did, as I cannot see any remnents of it in the rubble. But some claim the hat trusses survived damaged, but intact. Does anyone have any info on this? Surely they would have disintergrated or the connections would have sheared?

 

[tfk]

I doubt it did, as I cannot see any remnents of it in the rubble. But some claim the hat trusses survived damaged, but intact. Does anyone have any info on this? Surely they would have disintergrated or the connections would have sheared?

"Some say" lots o' things.

A relatively small subset turn out to be true.

I seriously doubt that the hat truss remained "intact". For WTC2, the upper block landed virtually on its side. A lucky photo shot shows it about to land on the Mariott.

There is little expectation that the truss could survive this "intact".

I don't think anyone knows how WTC1 roof landed, but it is most probable at some severe angle like this.

There is little doubt that pieces of each would remain attached to each other. Perhaps recognizable to someone who knew what to look for.

But "intact"?? Doubt it.

Why does anyone think that this matters?

 

[Loss Leader]

My problem with modeling - scale or full sized - is that it won't work. The collapse of the towers were pretty chaotic events. The total number of factors and the relationships between those factors is just unimaginably large. You could build a tower, let it stand for thirty years and then hit it with a plane; it will not come down exactly the same way as on 9/11.

The collapse will be similar, but it won't be the same.

So, no matter what the simulation shows, truthers will always be able to argue that the demonstration has not proven anything about 9/11. And, to an extent, they'd be right.

 

[Illuminist14]

"Some say" lots o' things.

A relatively small subset turn out to be true.

I seriously doubt that the hat truss remained "intact". For WTC2, the upper block landed virtually on its side. A lucky photo shot shows it about to land on the Mariott.

There is little expectation that the truss could survive this "intact".

I don't think anyone knows how WTC1 roof landed, but it is most probable at some severe angle like this.

There is little doubt that pieces of each would remain attached to each other. Perhaps recognizable to someone who knew what to look for.

But "intact"?? Doubt it.

Why does anyone think that this matters?

Pretty much my thoughts too.

Absolutely no idea! (Re why it should matter)

 

[jaydeehess]

I queried this not so long ago, and its true what you say the structures above the impact areas, were not tall enough to significantly activate an inverted pendulum effect. If it was as you say 55 floors up or lower we would surely see a toppling take place.

I did not mean to say that it would be a certainty that if hit a 55 floors it would topple, too may variables.
However the chances increase. One problem is that with the increased height of the upper portion you now increase the torsion stresses on the tilted structure. It could fail somewhere above the original impact level first.
Also you have increased the load on the pivot, both axial and lateral, as the upper portion tilts and it may simply fail there with a lesser tilt angle.

As for the hat truss, it may well have somehow survived all the way down BUT it would then be coming to a stop after attaining a velocity of 60-70 MPH. I don't believe it would have much chance of remaining in anything close to an intact structure during that.

,,,, and no, it is irrellevent to any discussion about any technical matter, whether or not it did survive. Interest in such a matter would be merely a curiousity along the lines of how one house is swept away by a tornado while the house 25 feet away is untouched.

Sabretooth, kudos on that gif. Simple but very illustrative.

 

[ozeco41]

I queried this not so long ago, and its true what you say the structures above the impact areas, were not tall enough to significantly activate an inverted pendulum effect. If it was as you say 55 floors up or lower we would surely see a toppling take place.

I do not believe that this is true. Not certain, I just don't believe it....

Curse you for that tfk. You caused me to engage "serious thinking mode". And that is not without risk at my age...

First issue is that I think you could be right in casting doubt.

Second issue is that I an also "not certain" - either way; AND

Third issue is that I may have made some "it will topple" claims in the past which could now come home to haunt me. If anyone finds any of my previous posts which could be embarrassing please use one of these but don't rub my nose in it.

Now getting more serious - here is my "thinking as I type" - warts and all with a couple of dead end paths left here for all to see:

The key factors in "topple or not" if damage started lower down are, I think:
1) There was/were no rigid block/blocks to give an easily defined "point" (or "line") of pivot about which the top bit would topple. However if the topside of lower tower and underside of top portion of tower did allow a "topple" there would be a "virtual pivot line". That line could be no further out than the perimeter columns on the toppling side. And by hopefully trivial logic or engineering gut reaction it would be way back - probably within the core area. The step forward for that thinking becomes "there was no need for the CofM of the Top Bit to be outside the lower tower perimeter". (Several hairy bits there but leave them till I pursue some more basics)

2) In order to create the overturning moment we need an upwards force exerted by the lower tower on the base of the top section. Thinking initially of this as a single "net" force it has to be out of vertical alignment with the Centre of Mass of the falling top bit to create the toppling moment. (The "net" simplification to initially at least avoid (a) the "virtual pivot" definition as stated above and (b)the details of what bits of lower tower reacted with what bits of "upper tower" - the "two halves of a wire basket" problem. So let me use "net" at least for a starter.)

3) Why the halt of tilting a 8^o? Having rewritten "1)" and "2)" I think the 8^o is irrelevant. Or rather it is consequential of something rather than causal. Let's see as I continue this "typing as I think"

4) Now having got this far I can see that I have gone down a dead end street. The two primary issues for "will it topple or fall straight down?" are
A) "where does the overturning moment come from"; AND
B) "what forms the pivot for the toppling?"
...and both of them relate to how the forces are distributed across the "wire baskets" of the interface between top portion and lower tower. So trying to treat it as simplified "net" forces gets me nowhere.

(I'm confident we can ignore "already turned to debris" material in between those two - at least in a first order bit of guestimation. )

At this stage let me keep the focus on the actual form of collapse initiation which occurred with WTC1 and WTC2 - i.e. aircraft impact starting fires which were not fought - but adding the hypothetical aspect of the postulated lower impact. Reason I'm not confident that a "cut out a wedge" initiation by demolition techniques creates the same scenario. Neither would a "grid of columns" traditional structural arrangement. We can return to the broader scope of tfk's "intuitive claim" later if I get anywhere with this bit of brain farting.

5) The tilt of the top part - most extreme in WTC2 - meant that the columns on the low side had failed - observable fact I suggest. The columns of the core were at best failing leaving the "opposite side" perimeter columns in tension. (And the other two sides a mixture which I will relegate by guess to "second order" importance)

(And let me state an obvious one - If the initial failure did not start the tilting then "straight down" is what would happen without doubt for a WTC Twin Towers "tube-in-tube" structural configuration. So we are not considering that scenario as a possible one for "top bit toppling over the side".)

6) Back to our scenario with "top bit' tilted as per WTC2. Given the tilt the "low side" was already transitioning into ROOSD and the core already transitioning into "core strip down mode".

Fine details aside that means that:
a) the low side resistance available to form part of the net upwards force and one side of the toppling moment is already down to the minimal strength of the floor joist connectors. No significant perimeter column strength is still in play;
b) the core is partly destroyed and reducing to the ultimate weakest situation of beam shear strip down forces So the resistance from the core is reducing. (AND that is a critical point for the next stage comment/assessment - see "####")
c) As the low side perimeter enters ROOSD and the core enters "strip down stage" - both of which must be happening to allow the 8^o - the perimeter columns on the high side are reverting to compression (Or are still in near full strength compression - it matters not if they never went into tension) AND those high side columns are, at that stage, the only significant column strength still available for upward resistance.

SO there is the overturning moment - strong force from high side perimeter, low force from low side perimeter and the "virtual pivot" somewhere between core and high side perimeter. And core contribution to overturning moment not very significant because it is closer to the pivot line of the centroid of the virtual "section modulus".

THEREFORE the top bit will continue to topple unless there is another factor.

#### What is the "Other Factor"

I think the clue lies in this comment by tfk:

...The crucial task is to leave the remaining supports as strong as possible (to support the tilting upper segment) while taking out just enough to cause the collapse...

(Not forgetting that tfk is contemplating an ab initio deliberate CD whilst I am at this stage still staying with a hypothetical WTC with lower impact.)

There are two conflicting factors in that statement - I agree with them BTW. They are - in my words:
A) Leave enough to act as pivot for toppling; WHILST
B) taking out enough to cause downwards movement.

Stated differently: For anything to happen - toppling or straight down - enough weakening has to take place. THEN that weakening has to leave behind enough resistance to induce toppling OR it will go "straight down".

Now we know that for WTC2 tilting started but did not transition to toppling.

What would happen if the initial impact and damage zone was lower?

The mechanism as I have explained it and as demonstrated by WTC2 on 9/11 saw the top portion retain geometry - the same mechanism applied lower down and with a "longer" top portion tilting would take the CofM of the top portion further outside the pivot area. With the pivot still intact on the high side and the resistance on the low side reduced to insignificant ROOSD levels.

So as first conclusion I "guesstimate" that a lower down initiation would lead to toppling.

So there is my first lot of thinking laid out warts and all.

Go for it anyone interested. Engineers first might be best.

I can revert to consideration of tfk's additional points and broader scope if I get out of this one without sever injuries.

 

[ozeco41]

This was a quick little gif I threw together to kind of show what was happening during the collapse.

*Not to scale

[qimg]https://lh3.googleusercontent.com/-HlXtXk2x5c0/T_XZsDvBgAI/AAAAAAAABJE/eel1VFkqGvI/s800/wtcani.gif[/qimg]

That is a great "gif" - perfect level of simplicity to get the points across.

A couple of suggestions based on what I would regard as current state of understanding the mechanisms. Not criticism of your effort - nor suggestions for more work:
1) The "top block" fell with some perimeters inside - as you show - however some were outside. So on a two dimensions drawing one inside and one outside. Plus maybe a bit of tilt.
2) The initial separation of lower tower floors was possibly caused by the falling "knife edge" of the upper perimeter where that perimeter fell inside. "Possibly" because falling floor debris may have got there first except where did that debris come from for the first few floors?
3)That mechanism in reverse where the top perimeter fell outside the lower would see the "rising" lower perimeter knife edge cut off the floors of the upper portion for those faces . No competition from "falling" floor debris here - it was heading the other direction. (And please forgive the "rising" when I mean "relatively rising" )
4) Given the above it is near certain that some of the top block was stripping floors concurrent with the lower tower floor stripping ROOSD.

 

[ozeco41]

Second Attempt

Could toppling have occurred at WTC with a lower point of aircraft impact?

OK so I have given some more thoughts to this question "Could toppling have occurred" - still from my limited perspective of the hypothetical scenario "Would/could lower down aircraft impact at a WTC Tower cause toppling?" (I'm not evading tfk's broader suggestion - merely trying to keep it manageable for a 71YO engineer's brain as a first step.)

So running into difficulties with my earlier attempt I thought "Can I approach this challenge from the big picture down and see if we (I?? ) can reach any valid conclusions?"

Here goes - still warts and all - wide open to challenge - and my doubts open on the table.


So what are the "knowns" we start with?

The first "known" must be that the impact and fires start to the scenario must be one which causes tilt of the top block. i.e. what actually happened with WTC2, didn't happen with WTC1 and which tfk is leading to in his more general question "Could you achieve it by deliberate CD?" The last one out of my scope at this stage. No tilt means no topple - I'll take that as a given unless we see a reasoned objection.

The second "known" is that the fire and impact zone underwent a cascade failure. The features of that failure being:
1) The weaker side failed first and must have meant failure of low side perimeter columns and core columns. The rotation>>tilt must have shortened the "gap" so shortened gap means columns failed. (for those with a sense of humour that is one of the points of failed understanding in "Missing Jolt' and a couple of other classics. )
2) The cascade failure then failed the "high side" perimeter columns (plus the "other two sides" columns failed somewhere through that process. Minor detail at this stage.)
3) and that sequence is what caused the tilt - i.e. one side gave way first. And that is a pre-requisite for toppling. (At least is must be in the scenario we are considering.)

The third "known" is that there is a massive difference between the strength of the columns as designed to hold up the top bit and the residual "hold up" strength once ROOSD/core strip down global collapse is under way. Possibly an order of magnitude - I or "we" may need to check the numbers later.

The fourth "known" is the (near) truism that tfk posted and I agreed - i.e. "The crucial task is to leave the remaining supports as strong as possible (to support the tilting upper segment) while taking out just enough to cause the collapse.?" Make that "requirement" rather than "task" for the hypothetical WTC2 scenario.

Now we know that for "real 9/11 WTC2" the cascade failure of the impact zone progressed to failure of the high side perimeter columns. At which stage the top portion fell downwards. We do not ("We" may not?) (I do not??? ) know about residual rotational momentum of the tilting motion but it is a factor we need to accommodate one way or another.

At that stage, where possibly the fall could transition from "tilting into toppling", the high side perimeter columns were the only ones with anything near original strength still in play. So the only parts of real on 9/11 WTC2 which could give the impetus towards toppling with "first order" forces. And they didn't. And if I'm wrong on that the remaining strength was less - so I am on the safe side of the unknown possibilities.

And we know that the high side failed. And that means that in real WTC2 9/11 there was not sufficient moment/(forces X lever arms) to cause toppling.

So for a hypothetical WTC2 lower down impact toppling would require one of two available mechanisms:
1) First scenario mechanism is that topple occurs or is started whilst the "high side" perimeter intact columns are still not failed which also means before gross downwards descent of the top portion commences. So ROOSD/core strip down has started on the tilt affected "low side" and the high side columns retain strength long enough to initiate the toppling. Toppling didn't happen in real WTC2 9/11 so we need to answer "Would it happen with a lower down impact point?". The (or one of the) key question(s) then follows - "What mechanism would stop the cascade failure of the impact zone for long enough that toppling could start?" And "Could it last long enough in the dynamics of a rapid conversion to downwards translation a la "ROOSD"?"

2) The second scenario is that the residual strength in the lower tower after ROOSD etc is initiated is sufficient to cause tilting whilst the top portion is falling. Said tilting by definition imposed on top of a progression collapse at a combination of very high velocities/accelerations. (leave the differences aside for now) And to qualify as a "toppling collapse" it would have to win the race with "straight down" collapse. Simply stated it would have to topple before the downwards movement reached ground level.

My gut feeling at this stage of thinking is:

For scenario "1)" there isn't any mechanism to stop progression of the cascade THEREFORE the downwards collapse starts at such a velocity/acceleration (we can sort out the differences if anyone wants to go there) that there is no way the toppling would overtake the downwards translation.

For Scenario "2)" I don't believe that the lower order forces available would overcome the dynamics of the main event - i.e. downwards movement at a large proportion of "G" - so I will leave that one out of consideration for now.

Anyone care to comment?

I've caught the bug and will still keep thinking. There are several potential holes in the logic which may need plugging.

BUT the preliminary hypothesis is "for WTC type towers toppling would not occur even with lower down aircraft impact." And that leads on to "tfk is right in his gut feeling - at least for WTC"


And before anyone says it:
Anyone from the school of "try the math(s) BEFORE you work out what you are trying to 'mathyfy' " will already know that I come from the school of "think first" try math(s) when needed.

 

[ozeco41]

Reading back through the thread - this bit of truth:

...give the false impression that the rate of rotation is slowing, when in fact it's the rate of descent that's increasing....

..which is the "race" condition I alluded to in my post. And 'descent' wins therefore not a 'toppling' collapse.

Plus the "top-portion" doesn't survive intact long enough to win the race....

...it doesn't even come in second. "Failed to finish"

 

[Sabretooth]

That is a great "gif" - perfect level of simplicity to get the points across.

And that's all I intended. Simplicity. I'm not much of a digital artist, so I just wanted to give the basic mechanic of the collapse. Of course, I can't recreate rotation, tilting, and the like, but I'm glad to see that I was able to make a point.

I'm curious to see if this could be created on a physical model. Based on what I know of the WTC construction, the outer walls were held by not much more than bolts at the floor joists...which were bolted at the core on the other side. I imagine a simplified model could be constructed using easily sheered pins to show the mechanics of the collapse? Hell, you could make the thing out of Tinker Toys and Construx if you wanted I would think...

 

[ozeco41]

And that's all I intended. Simplicity. I'm not much of a digital artist, so I just wanted to give the basic mechanic of the collapse....

I'm probably the worst digital artist. This was one of my early efforts - dated 26 Nov 2007 and about two weeks after I first posted on the Internet - 13 Nov 2007. As you can see I was starting to explain "global collapse" and what Major_Tom later called "ROOSD":

...rough but it got the point across. Try it today and it would probably get "nit picked" - times have changed.

... I'm glad to see that I was able to make a point.

..that is the primary objective. Not winning an art competition.

...I'm curious to see if this could be created on a physical model. Based on what I know of the WTC construction, the outer walls were held by not much more than bolts at the floor joists...which were bolted at the core on the other side. I imagine a simplified model could be constructed using easily sheered pins to show the mechanics of the collapse? Hell, you could make the thing out of Tinker Toys and Construx if you wanted I would think...

I lost the record of my posts when the Richard Dawkins Forum closed in 2010. But I once suggested a conceptual model - described in words but never built - it also got the message across.

Imagine two rectangular tubes of square cracker biscuits arranged tube inside tube style with toothpicks spanning from the inner tube to the outer tube. So a "tube in tube" tower with Open Office space "floors" represented by toothpicks. The crackers stuck together with 'blue tack' or similar. Then you have a sort of rectangular doughnut which you drop down the space between the tubes. Crude and never built but it explained one aspect of the global collapse - what M_T came to label "ROOSD". Back in those days people were still getting confused with pre-Heiwa "solid block" models, and Bazantian originated misunderstandings about crushed columns. It took a long time before the obvious reality of no (or very few) crushed columns became accepted. And we still see the big divide between those who explain what really happened and those who rely on abstract models which some of that group then take too far.

I think modelling the three aspects of WTC1/2 global collapse would be near impossible - for either of the two legitimate purposes I can think of:
1) testing that it would really happen the way it did. i.e. the legitimate engineering modelling. Too difficulty to do and no real benefit - reality too obvious for a model to help for engineering reasons; OR
2) demonstrating for non-engineers or non-physicists how the three parts of the actual collapse occurred. If they cannot visualise it without the model the model ain't gunna help much.

 

[Bell]

What does ROOSD stand for?

 

[ozeco41]

What does ROOSD stand for?

"Runaway Open Office Space Destruction"

Simply stated it means the process in the Twin Towers collapses where a mass of debris fell down the "Open Office Space" causing the floors in that space to become disconnected -- then the next floor -- and the next --in a rapid sequence - "Runaway Open Office Space Destruction". See my crude graphic a couple of posts back.

That left the outer perimeter columns standing with no bracing and they simply fell over. Meanwhile the core was also being destroyed. Hence my references to "three parallel mechanisms" of ROOSD, Core strip down and perimeter peel off.

The acronym "ROOSD" was chosen by Major_Tom over on the 911Forum where he has researched the details of the collapse progression (or "Global Collapse) stage of the twin towers collapses.

The explanations of WTC collapses tend to be in two classes:
1) Academic abstraction modelling which simplifies or ignores what really happened but allows quantified (mathematical) analysis; AND
2) Explanations derived from interpreting the actual collapse mechanisms. Gets closer to explaining real events but not as accessible to mathematics.

The number of adherents of class "1)" vastly outnumbering those in Class "2)". And some of us bridge the "middle ground' without getting confused too often.

I am one of those who prefer to work from understanding the "real mechanics" of what actually happened. Hence the content of recent posts.

Major_Tom has taken the detailed work much further than I did. Plus he attached the label "ROOSD" which was a good "marketing ploy". You will find opposition to M_T's work around here because he is reputed to be a truther therefore what he says must be wrong. I don't share that viewpoint. And I find the "ROOSD" label is very useful as shorthand.


...provided the readers are familiar with it. And they usually are these days. Add yourself to the numbers.