# How high could the plane have hit, and still caused a total collapse?

#### Henkka

##### Member
Hello everyone, I'm new here. I've had an interest in the various theories surrounding 9/11 for about a year now. Coincidentally, it got started when I randomly watched Mick interview a truther on his Youtube channel. Basically I've wanted to understand the various sides to the extent that I'm capable of as a layman. Those sides would be the truthers, the official explanations by NIST and others, and then skeptics/debunkers. Obviously skeptics mostly side with NIST, but it's interesting to see what skeptics argue on various issues where NIST didn't elaborate fully, like the total collapse of the towers. For this, Metabunk has been my #1 source.

I was recently writing up my thoughts on the WTC collapses, and thought of a question I wanted to ask to more technical people. That would be the thread title: How high could the plane have hit, and still caused a total collapse? The question came to me as I was looking at the rubble piles of the two towers, seen here:

I think it's fair to say that the rubble piles of WTC 1 and 2 are pretty similar. That is to say, both towers totally collapsed, with no parts left standing. It is not the case that because WTC 2 was hit lower, it collapsed totally while parts of WTC 1 survived because it was hit higher. So both buildings had a sufficient number of floors above the impact zone to cause a total collapse. From this we can reason two possibilities. Either:

1) Both towers had "more than enough" floors above the impact zone to cause a total collapse. In this case, the plane could have hit WTC 1 even higher than the 96th floor and still caused a total collapse. But how high?

or

2) WTC 2 had "more than enough" floors above the impact zone to cause a total collapse, while WTC 1 had "just the right amount" of floors to cause a total collapse. In other words, the 96th floor was kind of the "Goldilocks zone" of total collapse. If the plane had hit any higher, the building would not have totally collapsed. That seems a bit coincidental, though.

To illustrate my thoughts, I found a picture on Wikipedia that depicted where the planes hit, and edited it:

I created two clones of WTC 1 called WTC 1-2 and WTC 1-3, and wanted to see how it would look if the plane hit them higher. For WTC 1-2, I moved the plane up around 5-6 floors (Assuming the drawing is accurate), and then a couple more floors for WTC 1-3. Just trying to think about it intuitively, it's hard to say what would happen to WTC 1-2. If an impact on the 96th floor caused the real WTC 1 to totally collapse, I guess an impact 5-6 floors higher would have still caused major devastation. Maybe not a total collapse, though? But as for WTC 1-3, it's starting to stretch believability that such a thin slice of the building would cause a major collapse of the underlying structure at all. Maybe the roof would sort of cave in? And obviously if the plane went higher than that, it would be basically just scraping the roof... And knocking over the antenna. So when you think about it like this, it seems the building goes from "No collapse" to "Total collapse" very quickly just in the span of a few floors. If impacting near the roof would cause no collapse, and impacting the 96th floor causes total collapse, do all other possibilities fit between those? Would WTC 1-2 only have collapsed halfway, for example?

So that's my question, could the plane have impacted WTC 1 higher and still caused a total collapse? Or was it "just right", in your opinion? Sorry if this has been discussed before.

#### econ41

##### Senior Member
Greetings and welcome @Henkka. I'm a retired civil and military engineer and have contributed to the discussion of the WTC collapses since Nov 2007.

Therer are many issues you mention in your OP post. Let me start by responding to your primary question:
So that's my question, could the plane have impacted WTC 1 higher and still caused a total collapse? Or was it "just right", in your opinion? Sorry if this has been discussed before.
A "qualified" not "quantified" assessment AKA a gut feeling guess helped by a bit of thought.

I think the collapse would have been initiated by impacts higher than those on 9/11. For this post, I won't attempt to guess "How much higher?"

There are two critical factors - and comprehending them requires an understanding of the actual collapse mechanisms. What actually happened on 9/11.

The first critical factor is how much superimposed load is required to trigger the "initiation stage" and "drop" the "Top Block". That aspect is hard to "ballpark" quantify. For the real event the ~11 storeys Top Block of WTC1 was sufficient to provide the necessary weight in the right places. Let's leave that aspect for now - there are two ways forward viz qualified assessment with some ball park guesses OR more detailed quantification. And I doubt that the detailed quantification path is practical. With anything less than a major study analysis.

The second critical issue is - once the Top Block starts to fall will the collapse go into unstoppable progression. That one is easily answered "YES" - again provided you understand the progression mechanism of the "real event". Explanation later when required.

So, yes, in my opinion, impact at a higher level could have initiated a collapse. I cannot at this time guess how much higher. I'm sure that, once the "Top Blck" dropped it could not be arrested and global collapse would ensue.

So of the two factors I identified - triggering the initiation is the one that is key.

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#### econ41

##### Senior Member
Those sides would be the truthers, the official explanations by NIST and others, and then skeptics/debunkers.
Don't overlook "What actually happened" << That has to be the standard against which ALL the other player's claims are tested.
Obviously skeptics mostly side with NIST, but it's interesting to see what skeptics argue on various issues where NIST didn't elaborate fully, like the total collapse of the towers.
I "mostly side with NIST" but don't accept NIST as the unquestioned "authority".

Here's why: I saw the Twin Towers collapses reported on BBC News when in Wales on a vacation. (I'm English by birth but Australian since age 11 - 1952.) Saw "plane hits tower >> tower collapses" as two TV News "10-second grabs" - the default hypothesis wrote itself. I was unaware of the ~1hour delay and had no concept of the relative size of plane<>tower. Forgot about it and was unaware of the CT claims for CD until 2007 when a colleague in my then hometown asked my engineer's opinion of the CD of the Twin Towers. And for reasons of credibility in small-town politics I needed to give him a professional level explanation of the collapses. Eventually, I started posting 14 Nov 2007.

But within two weeks I decided that I would never accept NIST or Bazant (or any others) as "Authorities". The reason was tactical - the debate had not at that time polarised into the "truthers v debunkers" false dichotomy we see today. But all four sides of that 2007 debate kept switching goalposts between "What really happened" and "Did NIST (Bazant) explain it correctly". So I've maintained that strategic position. I have no substantial disagreement with NIST's conclusions tho I'm not a fan of some of their explanations. And one of Bazant's explanations caused more confusion even when he was right plus he has made at least one significant error.

#### Mendel

##### Senior Member.
From the answer to question 18 on the NIST FAQ:
Article:
This simplified and conservative analysis indicates that the floor connections could have carried only a maximum of about 11 additional floors if the load from these floors were applied statically. Even this number is (conservatively) high, since the load from above the collapsing floor is being applied suddenly. Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors. Since the number of floors above the level where the collapse initiated exceeded six for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated. In reality, the highest intact floor was about three (WTC 2) to six (WTC 1) floors below the level of collapse initiation. Thus, more than the 12 to 29 floors reported above actually loaded the intact floor suddenly.

So, as a rough estimation, a top block of 6 storeys would be sufficient to sustain runaway collapse once it fell.

And then consider that the aircraft did not hit the towers with wings level (some say the hijacker pilots banked deliberately to maximize damage), so an impact of fewer than 6 storeys from the top could've been sufficient since the damage extended below.

NIST didn't nail this down exactly because there was no need to.

So when you think about it like this, it seems the building goes from "No collapse" to "Total collapse" very quickly just in the span of a few floors.
A big reason for this is the runaway collapse. If the topmost intact floor (or even a damaged floor above) holds, there is no runaway collapse; if it fails, none of the lower floors can stop the collapse as the load only gets worse on the way down. So the breaking point for the critical floor is the threshold for collapse progression.

Regarding initiation, the conditions for the top block not to fail are even harder to pin down: if the weight is less, maybe the fires just need to burn longer and cause more/different damage? Again, that's a hypothetical scenario NIST didn't need to explore.

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#### Abdullah

##### Member
Building on the source @Mendel provided, which states that each floor can carry a maximum of 29 million pounds, and on the following

We get that any impact below the 105th floor would cause a total collapse

#### econ41

##### Senior Member
Building on the source @Mendel provided, which states that each floor can carry a maximum of 29 million pounds, and on the following

We get that any impact below the 105th floor would cause a total collapse
I doubt that your conclusion is reliable. Could you provide a link to where that *.pdf file and the quoted chart is located - I need to check the context. Remember that @Mendel's comment referred to collapse progression whilst I have stated my opinion that collapse initiation is the critical stage. Also your comment "any impact..." is a risky global comment.

#### Henkka

##### Member
Thanks for the replies! As it happens, in my illustration, WTC 1-3 is around 6 floors. So that would be the minimum according to NIST? Pretty interesting

#### deirdre

##### Senior Member.
which states that each floor can carry a maximum of 29 million pounds

that's a LOT of weight.

#### econ41

##### Senior Member
Thanks for the replies! As it happens, in my illustration, WTC 1-3 is around 6 floors. So that would be the minimum according to NIST?
I haven't a clue where you pulled that conclusion from. The NIST material quoted refers to the progression stage. As per my earlier comments progression stage is not, IMO, the critical stage. Initiation is the critical stage. Getting the collapse started. Once started it will continue.

Pretty interesting
Another interesting topic for speculation is: "Would the Top Block have toppled over if the plane had struck lower down the Tower"?

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#### econ41

##### Senior Member
that's a LOT of weight.
Yes, but it is not relevant to the topic question. I'll explain more fully if any of the participants want a discussion. (It applies to the wrong stage of collapse AND is part of an over-conservative throw-away comment but NIST when they didn't see the need to analyse the "progression" stage of collapse. Remember NIST said "global collapse was inevitable" or similar words.)

#### Mendel

##### Senior Member.
Thanks for the replies! As it happens, in my illustration, WTC 1-3 is around 6 floors. So that would be the minimum according to NIST? Pretty interesting
There is no "minimum according to NIST" because NIST did not study this question.

#### Mendel

##### Senior Member.
that's a LOT of weight.
The NIST FAQ states the size of a floor outside the core.
31000 sq. ft. ≈ 2900m², that's ~2.3 olympic swimming pools.
29,000,000 lbs ≈ 13200t ≈ 13200m³ of water
13200m³/2900m²≈4.55m

You could very carefully have filled up a floor with 15 ft. of water (if you had plugged all the holes) and not exceeded the total load of all vertical connections. Obviously, this needs to be done on an absolutely calm day, as wind loads would cause the connections to break; and some may break anyway (with the others following) as there's no guarantee this load would be distributed evenly according to connection strength, and that the floor joists could take it.

#### econ41

##### Senior Member
You could very carefully have filled up a floor with 15 ft. of water (if you had plugged all the holes) and not exceeded the total load of all vertical connections.
And compressed the water to whatever .... just under 12.5 feet gross and there is a floor slab taking some space.

#### Abdullah

##### Member
@econ41 sorry, I forgot to.mentiin that it was NCSTAR 1-6D

#### econ41

##### Senior Member
@econ41 sorry, I forgot to.mentiin that it was NCSTAR 1-6D
No problem. I'll see if I can find it - I don't rely on NIST and don't keep copies.

Got it. Will read it in the next hour or two.

#### Henkka

##### Member
I haven't a clue where you pulled that conclusion from. The NIST material quoted refers to the progression stage. As per my earlier comments progression stage is not, IMO, the critical stage. Initiation is the critical stage. Getting the collapse started. Once started it will continue.

Maybe I misunderstood then? Mendel quoted this from NIST:

Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors. Since the number of floors above the level where the collapse initiated exceeded six for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated.

I took that to mean it took a minimum of six floors to get the collapse initiated. And once it initiated, it could not be stopped.

#### Mendel

##### Senior Member.
I took that to mean it took a minimum of six floors to get the collapse initiated. And once it initiated, it could not be stopped.
No, it really says the opposite: if you drop 6 floors on it, it's definitely failing. That's because the question they were looking at was whether the weight of the top block was sufficient to trigger this progressive collapse. They didn't look for any minimum because the actual weight was already much more.

Conspiracy theorists often think like that, trying to use experts statements from one context to say something else that the experts never considered. That often means that stuff gets ignored that the experts would have considered if the other question had been posed to them.

I understood these floor strength numbers to say that if the impact was at Top-6 floors, the progressive collapse would have occurred, provided there was a mechanism to initiate it. So I estimate the maximum safe zone is the 6 top storeys, but it could actually be less if you examine it more closely.

For example, the columns get thinner and weaker as you go up the building. If the aircraft had hit higher up, it may have penetrated further into the buildung, and the initial impact may have caused more damage. It's hard to estimate how that affects the initiation mechanism.

It does look to me as if your 1-3 scenario should have resulted in total collapse as well, while the aircraft "landing" on the roof might not have.

#### deirdre

##### Senior Member.
I took that to mean it took a minimum of six floors to get the collapse initiated.
"collapse initiation" and "progressive collapse initiation" are 2 different things. (i'm pointing this out or you will never be able to follow a conversation with econ )

"collapse initiation" is what causes the 6 floors to fall.
"progressive collapse initiation" is what happens AFTER the 6 floors fall.

No, it really says the opposite: if you drop 6 floors on it, it's definitely failing. That's because the question they were looking at was whether the weight of the top block was sufficient to trigger this progressive collapse. They didn't look for any minimum because the actual weight was already much more.
I don't think this (last sentence) is correct.

Is this last sentence correct @econ41 ? and please dont give me some weird pool metaphor i won't understand and please start your [assumed] long confusing answer with a "yes" or a "no". Otherwise i won't know if your long answer is supposed to be saying "yes" or "no". thanks!

#### Mendel

##### Senior Member.
They didn't look for any minimum because the actual weight was already much more.
There's a logical distinction between the words "sufficient" and "necessary".

For example, to make a road trip, it's necessary to have fuel in the car, but it's not sufficient: you also need motor oil, engine coolant, and a driver.

To debunk a UFO sighting, it's sufficient to point out the exact flight on flightradar that the reporter mistook for aliens, but it's not necessary: many UFO reports can be debunked in other ways.

NIST FAQ #18 talks about the weight that is sufficient to trigger progressive collapse.
@Henkka asks for the weight that is necessary to trigger collapse.
(sufficient weight) ≥ (necessary weight), so there's a relation, but they're not the same.

#### Henkka

##### Member
NIST FAQ #18 talks about the weight that is sufficient to trigger progressive collapse.
@Henkka asks for the weight that is necessary to trigger collapse.
(sufficient weight) ≥ (necessary weight), so there's a relation, but they're not the same.

Yeah. Though I have a hard time understanding why a collapse of six floors could not be arrested. It is a very thin slice of the building, 6 floors VS. 104 floors...

#### Mendel

##### Senior Member.
Though I have a hard time understanding why a collapse of six floors could not be arrested.
It's because 6 floors are heavy, and the connections for a single floor aren't sturdy enough to stand up to 6 floors bumping into them. It's like the difference between a kid sleeping in their bed and a grown man jumping on it.

If it helps, look at the NIST FAQ numbers again.

#### deirdre

##### Senior Member.
Yeah. Though I have a hard time understanding why a collapse of six floors could not be arrested. It is a very thin slice of the building, 6 floors VS. 104 floors...

it's 6 [dynamic] floors vs 1 floor. then it becomes 7 dynamic floors vs 1 floor. then it becomes 8 dynamic floors vs 1 floor...and so on until you get to the ground (or below ground in this case).

#### econ41

##### Senior Member
Thanks, deirdre for this:
"collapse initiation" and "progressive collapse initiation" are 2 different things. (i'm pointing this out or you will never be able to follow a conversation with econ )
I'm still astonished at how many people don't recognise the difference. And disappointed at the number of debunkers who still confuse and conflate the two.

Thanks to deirdre for saving me the need to once again explain:
"collapse initiation" is what causes the 6 floors to fall.
"progressive collapse initiation" is what happens AFTER the 6 floors fall.
Those are the two that matter. "initiation" is very complex and still confuses many. "progression" is actually the easiest bit to explain if you simply describe what really happened. (And there is a long complicated story about who got things wrong for a long time - save that for another day!) This diagram should explain the key reason for the rapid progression for @Henkka 's benefit.

Once the "Top Block" started to "fall" it broke up and the debris missed the columns landing on the floors. The picture shows the progression stage well established - @Henkka if you need more explanation just ask. But with the debris missing the columns the only two big things slowing the collapse are:
(a) The force needed to shear the floor joists off the columns - marked "It fails here" in the graphic:
(b) The need to increase momentum as debris accumulated at each floor.

There was very little resistance from columns because the debris missed the columns.

#### deirdre

##### Senior Member.
back to the question at hand. If the plane had hit and entered at floor 105, would there have been a progressive collapse of the entire tower building?

yes or no?

edit: rephrase: If the plane had hit and entered at floor 105 and none of it's engines popped out of the building, would there have been a full progressive collapse in the tower without the antennae on top?

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#### econ41

##### Senior Member
back to the question at hand. If the plane had hit and entered at floor 105, would there have been a progressive collapse of the entire tower building?

yes or no?
I suspect "No!" but I've only given it a little bit of semi-serious thought. As per my previous comment - I'm in no doubt that IF collapse could be initiated it would then progress. But "initiation" is the critical stage. And I don't think I can even persuade myself so the rest of you should be safe.

#### Abdullah

##### Member
@deirdre if the collapse initiated, then yes, most definitely there would be a total collapse.m, even with just the static weight above

The only reason I stopped at fl105 is because I didn't have convenient access to data on the higher floors

#### econ41

##### Senior Member
edit: rephrase: If the plane had hit and entered at floor 105 and none of it's engines popped out of the building, would there have been a full progressive collapse in the tower without the antennae on top?
We crossed in posting - even with your edit I still say "No!" << BUT that is the engineer who is responding to "Will it collapse?" and I can not guarantee that it would.

BUT - reframe the question around the other way. You are on a high floor and the plane is coming in. You ask me: "Am I safe to stay?" and I would say "Get out of there - fast!"

#### econ41

##### Senior Member
@deirdre if the collapse initiated, then yes, most definitely there would be a total collapse.
That is evading the question. Would it "initiate"?

#### deirdre

##### Senior Member.
Would it "initiate"?
i dont really think Henkka was pondering the initiation event. i think he was asking if progressive collapse would happen if there was an initiation event.

i agree the initiation event is iffy on higher floors, but i'm not sure. ps. i wish you guys would stop saying "top block", it sounds too much like Gage's cardboard box analogy...imagining that is the only reason hennka would have said:
Though I have a hard time understanding why a collapse of six floors could not be arrested. It is a very thin slice of the building, 6 floors VS. 104 floors...

#### econ41

##### Senior Member
i dont really think Henkka was pondering the initiation event. i think he was asking if progressive collapse would happen if there was an initiation event.
Yes he probably was. But it is a "cannot get there" scenario.
i agree the initiation event is iffy on higher floors, but i'm not sure.
If we accept the NIST explanation that "column inward bowing" was the trigger for "initiation" THEN collapse from inward bowing needed to weight from above to push the misalignment past "p-delta" (Technical jargon). With less floors above there are fewer floors to push the inward bowing. The rest of "initiation" - the cascading failures driven by load re-distribution should still work BUT there is no trigger. << I think. i.e. engineers more or less gut feeling.

ps. i wish you guys would stop saying "top block",
I've been saying it for 15 years. Probably before I even knew about Gage - I can not remember. I know I started explaining WTC collapses long (6-9 months) before I got involved in the online discussion scene. Not sure I can either break the habit OR think of a better description. Then I'm stubborn. I still tend to put "meme" in scare quotes because it was a good word that was bastardised in common usage. And Richard Dawkins himself admitted defeat on that one.

#### Mendel

##### Senior Member.
We crossed in posting - even with your edit I still say "No!" << BUT that is the engineer who is responding to "Will it collapse?" and I can not guarantee that it would.

BUT - reframe the question around the other way. You are on a high floor and the plane is coming in. You ask me: "Am I safe to stay?" and I would say "Get out of there - fast!
The question is, what would you say to the people on the lower floors?

The construction of the WTC columns gets more flimsy as you go up, see e.g. this schedule of column 501:
Here is the column schedule. Not sure if this affects your calculations or not.
At floor 83, the column shape changes from a box beam to a simple I-beam ("wide flange"), dramatically decreasing in size up to floor 106, so what's to say the plane wouldn't have simply gone right through the core and initiated collapse then and there?

#### econ41

##### Senior Member
Some good points Mendel. I think I can answer them:
The question is, what would you say to the people on the lower floors?
Same thing. "Get out --- quickly!"

Because, for the Twin Towers design, progression was self-sustaining and unstoppable once started. And the lighter structure at the upper levels won't change "progression". "Initiation" is what is in doubt.
The construction of the WTC columns gets more flimsy as you go up, see e.g. this schedule of column 501:
Understood. Doesn't change either of my conclusions tho reduced weight changes the details of the two different explanations.
At floor 83, the column shape changes from a box beam to a simple I-beam ("wide flange"), dramatically decreasing in size up to floor 106, so what's to say the plane wouldn't have simply gone right through the core and initiated collapse then and there?
Doesn't that actually accept the point I'm doubting? "Will the collapse initiate?" AKA "Will the Top Block"*** start to "drop" down?

There is one factor that may change if your "plane right through core" scenario does occur. The progression stage as actually happened depended on falling "Top Block"*** columns missing their lower parts. Members could query whether that would still happen. I say "YES!" and can offer "proof" if we get the query.

*** With apologies to @deirdre.

#### Mendel

##### Senior Member.
Doesn't that actually accept the point I'm doubting?

You wrote "there is no trigger. << I think." I'm proposing that the collapse might have been initiated/triggered differently if the aircraft had impacted higher up, but I don't know, and doubt I could understand the drawings enough to answer this.

It kinda depends on how much the aircraft was retarded by the floor plates (and whether these were constructed the same at that level), how strong the facade and core columns were, and how the top 6 storeys were constructed (hat truss?). Inituitively, the top block "doesn't hold together as well" when it's smaller, but it's also lighter.

I don't have an "engineer's gut feeling", so I genuinely don't know.

There's also a fire aspect: if the fires can burn "through the roof", a draft/chimney may be created that really heats up the inferno. I feel that for this to happen quickly, a column or two would need to "drop down" locally, and that's probably not a realistic scenario.

#### econ41

##### Senior Member

You wrote "there is no trigger. << I think." I'm proposing that the collapse might have been initiated/triggered differently if the aircraft had impacted higher up.
Yes. I'm assuming outcomes from initiation which are similar to the real event. But I have given thought to how the outcomes might differ. Even if the inputs differ the net outcome should be the same.

Here's why: The debris will either fall on the office space floors - breaking up similar to the real event OR on the columns not yet broken up. If it lands on column ends, not broken up, it will halt there >> no initiation. If it breaks up it is similar to the real event but we need a trigger to start progression >> let's not go there ----YET! If it part breaks up it will continue to fully break up and progression happens. (Sorry about the bare assertions - more explanation if needed.),
It kinda depends on how much the aircraft was retarded by the floor plates (and whether these were constructed the same at that level), how strong the facade and core columns were, and how the top 6 storeys were constructed (hat truss?). Inituitively, the top block "doesn't hold together as well" when it's smaller, but it's also lighter.
Understood. This is why I recommend coming at the puzzle from a different perspective. It saves trying to resolve unresolvable complexities. By the way that is a valuable "trick" to thinking through a lot of WTC collapse complexities, And where a lot of "left-brained engineers" simply lock up >> yet another topic for discussion.
I don't have an "engineer's gut feeling", so I genuinely don't know.
I'm bluffing - I'm getting "rusty" with increasing age.
There's also a fire aspect: if the fires can burn "through the roof", a draft/chimney may be created that really heats up the inferno. I feel that for this to happen quickly, a column or two would need to "drop down" locally, and that's probably not a realistic scenario
Realistic or not I think the detail is absorbed by my reasoning approach coming from the three possible net outcomes.

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#### Henkka

##### Member
i dont really think Henkka was pondering the initiation event. i think he was asking if progressive collapse would happen if there was an initiation event.

i agree the initiation event is iffy on higher floors, but i'm not sure. ps. i wish you guys would stop saying "top block", it sounds too much like Gage's cardboard box analogy...imagining that is the only reason hennka would have said:

I think people on both sides of the issue have used similar language. Here is how the Wikipedia page on the collapses currently puts it, referring to Bažant's study:

From there collapse proceeded through two phases. During the crush-down phase, the upper block destroyed the structure below in a progressive series of floor failures roughly one story at a time. (...)

This continued until the upper block reached the ground and the crush-up phase began.

Another thing I wanted to ask you guys... In your opinion, how do the WTC collapses relate to a type of demolition done in France, called verinage? Such as this one:

Trying to post this in accordance with the "No-click policy" I recently learned about... It's a video of a verinage demolition where it looks like a top block of 6 floors is dropped on 8 floors. The top block quickly decelerates, breaks apart and falls off. Collapse starts about 2 minutes in. Though this might not be a particularly successful verinage demolition, as you can find videos on Youtube of such demolitions where the fall is much smoother and the destruction more total. But regardless, I'm under the impression that these demolitions are always started near the middle. So the top half is used to destroy the lower half. I've not seen a verinage demolition where the approximately top 10% is used to destroy the lower 90%, like what happened with WTC 1. Like if you think about it, weren't the WTC collapses basically verinage demolitions? Main difference being that the collapse was started with plane impact and fires, rather than hydraulic machines buckling the columns.

#### Mendel

##### Senior Member.
Main difference being that the collapse was started with plane impact and fires
the main difference is that the building in your video is a concrete construction while the WTC's structure was steel, with a central core group of columns, load-bearing columns on the facade forming a rectangular steel tube, and open-plan office floors suspended between that outer tube and the core.

The progressive collapse phase of the WTC towers did not crush the columns, it simply detached the floors from them, leaving the columns unbraced to buckle and fall.

#### econ41

##### Senior Member
@Henkka - you are clearly interested in understanding the WTC "Twin Towers" collapses. I am available to work through the explanation with you - it will take a few posts and a bit of discussion.

First let's clear up a couple of less important issues. This one first:
I think people on both sides of the issue have used similar language. Here is how the Wikipedia page on the collapses currently puts it, referring to Bažant's study:
Bazant is recognised as a leading expert in structural engineering and WTC collapses. But the "Crush Down/Crush Up" hypothesis which he published in the paper "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions" co-authored in 2007 with Mathieu Verdure is wrong. I recommend that you ignore it - it will only confuse our discussion of my explanation.

Then parts of this next comment:
Another thing I wanted to ask you guys... In your opinion, how do the WTC collapses relate to a type of demolition done in France, called verinage? Such as this one:
Most of the members here, including me, are familiar with Verinage. You are correct that the WTC Twin Towers collapses superficially resemble Verinage in two key aspects viz:
(a) They start partway up the building; and
(b) by releasing a "top block" to fall onto and destroy the lower tower whilst concurrently destroying the Top Block,

The Twin Towers collapses differ from Verinage in that most Verinage demolitions involve materials other than steel frames but that difference is not important to our understanding.

Then you identify a similarity of WTC collapses to Verinage with this:
I'm under the impression that these demolitions are always started near the middle. So the top half is used to destroy the lower half. I've not seen a verinage demolition where the approximately top 10% is used to destroy the lower 90%, like what happened with WTC 1. Like if you think about it, weren't the WTC collapses basically verinage demolitions? Main difference being that the collapse was started with plane impact and fires, rather than hydraulic machines buckling the columns.
You are correct that the start of the Verinage example you posted is analogous to the WTC Twin Towers. BUT the difference occurs at the point where the Verinage failed.

The first step in understanding the collapses is to recognise that they involved stages. Two main stages but four are needed to support full understanding. The Two main states are:
1) The "initiation stage" - from the damage of aircraft impact than a period of accumulating fire damage until the "Top Block" started to "drop" bodily; THEN
3) The "progression stage" of rapid global collapse to the debris heap at ground level.

For complete understanding we insert "2) The "transition stage " between those two. And split 3) Progression into two sub-stages. "3)(a) Early Progression" and "3)(b) Established Progression. Reasons for the four stages will become clear as the explanation and discussion proceeds.

Do you comprehend the need for stages? Do you want to continue the discussion? Do you have any preliminary questions?

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#### deirdre

##### Senior Member.
I think people on both sides of the issue have used similar language.
but how are YOU picturing the collapse? meaning do you think the top block fell through the building intact and only broke apart when it hit the ground?

add: ps. in the early days, even in online forums and certainly in written papers, it was engineers talking with/to other engineers about the collapse mechanisms. But Metabunk is primarily laymen readers oriented. How engineers understand certain phrases is different than how many laymen some phrases.

I've been saying it for 15 years. Probably before I even knew about Gage - I can not remember. I know I started explaining WTC collapses long (6-9 months) before I got involved in the online discussion scene. Not sure I can either break the habit OR think of a better description.
i know. how about APS? but you would have to write it as "APS (Above Plane Section)" each time...which would likely be annoying.

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#### Henkka

##### Member
the main difference is that the building in your video is a concrete construction while the WTC's structure was steel, with a central core group of columns, load-bearing columns on the facade forming a rectangular steel tube, and open-plan office floors suspended between that outer tube and the core.
Well, yeah, but it doesn't seem intuitively obvious why a steel building would be more susceptible to a top down, verinage-style collapse than a concrete building.
The progressive collapse phase of the WTC towers did not crush the columns, it simply detached the floors from them, leaving the columns unbraced to buckle and fall.
Weren't the core columns braced between themselves? Like I mean they had horizontal steel beams going across. This description reminds me a bit of the original Nova program, which showed the core columns as like long, swaying spaghettis jutting out from the earth, having to be braced by the floors. But in reality, the core was more like a traditional steel skyscraper, a building inside a building.
Bazant is recognised as a leading expert in structural engineering and WTC collapses. But the "Crush Down/Crush Up" hypothesis which he published in the paper "Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions" co-authored in 2007 with Mathieu Verdure is wrong. I recommend that you ignore it - it will only confuse our discussion of my explanation.
I'm just gonna respond to this part so the discussion doesn't become overwhelmingly convoluted yet. I'm not sure what your problems with Bazant's hypothesis are, but yeah I also have my doubts about it. I've glanced at the paper, but obviously I can't make heads or tails of his equations. But there are two problems that I think are quite fatal:

1) It is incredibly difficult to intuitively understand the idea of the upper block somehow staying intact while it crushed everything below at an incredible speed. Accelerating, even. While I'm admittedly just going off intuition, it's almost like cartoon physics or something.

2) The video evidence doesn't show the upper block staying intact. If you watch some of the close-up videos of the North Tower collapsing, it's pretty evident there is no block whatsoever riding on top of the billowing dust and debris. I know that Bazant submitted his study a mere two days after 9/11, so he had probably only seen the collapse a few times on TV, and from distant angles.

edit: 3) Actually a third problem I just remembered... Parts of the core remaining standing after the collapse, also known as the "spire". It's just another proof that the upper block could not have been intact all the way to the ground.

As such, it's incredibly strange to me that this is still the explanation presented on Wikipedia. Now you could say I shouldn't use Wikipedia as my source, but I think it's fine for understanding what is currently the mainstream, or consensus explanation among experts. How come in 20 years, researchers haven't supplanted Bazant's initial hypothesis, if it is so flawed?
but how are YOU picturing the collapse? meaning do you think the top block fell through the building intact and only broke apart when it hit the ground?
See above

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