How Buckling Led to "Free Fall" acceleration for part of WTC7's Collapse.

[Jeffrey Orling]

My view is that the interior structure failed first... in a runaway sequence... and that led to the entire floor system collapsing... and that led to the axial structures at the base up top floor 7 to be undermined and fail... and that led to the entire moment frame w/ curtain wall to collapse as a unit. Of course these "stages" were not distinct/discrete, but included transitions. Key words: runaway progressive.
Heat explains how beams and girders in the NE quadrant led to location floor collapsing which led to disruption / displacement of transfer structures above Con End and floors 5-7 which led to disruption/displacement of axial structures in the interior which led to the floors losing support and collapse which led to the perimeter axial structures at the base to be displaced/fail which led to the final act of the hollow moment frame and attached curtain wall to collapse as a unit including 100 feet of "fall" with no resistance (from the displaced columns at the base). My sense is the moment frame did not crinkle as shown in the NIST simulation, but rather came down as a largely intact hollow tube (as seen in the video). Note that the runaway floor collapses of the lower blocks' floors of the twin towers bypassed the columns including the perimeter as they collapsed. Same deal with 7WTC. Floor collapse undermined the columns which fell absent lateral support of the floor system.

 

[econ41]

I don't think the video record shows an empty shell.

It clearly shows the perimeter shell remaining intact/integral as it fell. THAT is the point you are denying. So stop shifting the goalposts.

At most, the eastern side of the interior has collapsed, but the center and western side are still there imo. This is shown by the fact that these remaining roof structures don't budge until a fraction of a second before the collapse of the perimeter:

So you agree "before"? THAT is the key issue. Sequence.

I think this is still on-topic if you guys are saying that a collapse of the entire interior is a necessary prerequisite for the two seconds of 9.8 m/s^2 acceleration to happen.

I agree that there was a period of acceleration averaging "g" and probably exceeding "g". That much is FACT no matter what is speculated about "necessary prerequisites". You are on dubious ground with your attempts to deny observed facts on the basis of unproven speculation. Even if we ignore the reversal of the burden of proof.

 

[Henkka]

It clearly shows the perimeter shell remaining intact/integral as it fell.

Yes I can see that, I just don't think that it is an empty shell, which is why it doesn't crumple like it does in the NIST simulations.

 

[Jeffrey Orling]

Yes I can see that, I just don't think that it is an empty shell, which is why it doesn't crumple like it does in the NIST simulations.

NISTs sim, I believe, shows what they think could have happened to the interior steel frame/structure of the tower.. which to me appears to show that the interior had NOT collapsed inside the "cage" of the perimeter but somehow crinkled up. What forces would cause the interior to crinkle as they imply?
Again... I lean toward these stages:

1. extensive fire caused beams and girders expand and to disengage from col 79 and perhaps 80​

2. floors supported on the disengaged beams and girders drop and fall down through the building​

3. collapsing floor mass (+/- 8 floors) disrupts the transfer structures on floors 5-7​

4. disrupted transfers lead to structural failures (beams& girders) propagating west and south involving the remaining interior​

5. Floors loose support throughout the footprint and drop down through the tower​

6. Massive floor debris pile cannot be contained by the perimeter at the base​

7. Columns at the base are damaged, displace, and they collapse (fail)​

8. Moment frame without support drops​

 

[Abdullah]

An engineer in Quora keeps insisting I bring another engineer. So I would appreciate if @econ41 or another engineer reviewed by argument, presented below:

1-The core and thrnnorth and south walls were braced against north-south displacements by the floor slabs.

2-the slabs were thems lives restrained against north south displacent by the east and west walls.

3-So the building is like a stack of east to west I beams lying on their sides, welded at their flanges. The slabs are the webs, and North and South walls are the flanges, and the beams are pinned to the east and west walls.

4-when the east lobe caved in, removing the "web" on that side, the "beams" utterly lost their rigidity and became free to rotate about the West wall

5-From their everything else follows.

 

[econ41]

An engineer in Quora keeps insisting I bring another engineer. So I would appreciate if @econ41 or another engineer reviewed by argument, presented below:

I cannot comment on your argument as it stands. It is too loosely defined. What are you trying to prove? Where does it fit in both the spatial context - what specific part of the collapse mechanism or the temporal context - when in the collapse process?

I suspect that you are clear in your own mind about these issues - but it is not clear to me from your writing.

Also can you point me to where the discussion is on "Quora" - I may be able to understand in the context of your discussion over there.

 

[Abdullah]

when in the collapse process?

I thought it was obvious. Sorry. I was trying to explain how the collapse of floors in the east lead to the buckling of all columns across multiple storeys.

Also can you point me to where the discussion is on "Quora"

https://www.quora.com/Do-you-agree-...n-Bell-24?comment_id=287382631&comment_type=2

 

[econ41]

I thought it was obvious. Sorry. I was trying to explain how the collapse of floors in the east lead to the buckling of all columns across multiple storeys.

It isn't as obvious as you think - and even I have difficulty fitting what you are saying into a realistic context.

https://www.quora.com/Do-you-agree-...n-Bell-24?comment_id=287382631&comment_type=2

Thanks for the link - I've joined the Forum and made a couple of preliminary comments to your opponent.

I understand the frustration for both of you. I doubt if I can help. If I was to advise you I would suggest a totally different approach to "argument". But it would not be your style. And, whilst ever you persist in presenting details, your opponent will persist in his two techniques - viz:
1) Telling you to get an engineer; and
2) persisting in his repeating of stock standard 2007-8-9 era truther counter claims.

He cannot see what is legitimate about your claims and it won't help you present your claims IF I weigh in as a legitimate engineer adviser.

 

[Abdullah]

I don't agree with what Abdullah is saying for a couple of reasons.

What are they?

 

[Henkka]

An engineer in Quora keeps insisting I bring another engineer. So I would appreciate if @econ41 or another engineer reviewed by argument, presented below:

1-The core and thrnnorth and south walls were braced against north-south displacements by the floor slabs.

2-the slabs were thems lives restrained against north south displacent by the east and west walls.

3-So the building is like a stack of east to west I beams lying on their sides, welded at their flanges. The slabs are the webs, and North and South walls are the flanges, and the beams are pinned to the east and west walls.

4-when the east lobe caved in, removing the "web" on that side, the "beams" utterly lost their rigidity and became free to rotate about the West wall

5-From their everything else follows.

I'm no engineer, so I can't really comment on the merits of this argument on the specifics... But as a layman, nothing about this really "clicks" with me in a way that makes the free fall acceleration make more sense to me.

This is a pretty silly thing, but it's one way I think about it... Look at David Chandler's measurement of the acceleration I posted on the previous page. Now ask yourself, what would that graph look like if a wizard waved a magic wand and made all the steel in the bottom eight floors instantly vanish into thin air? Well, it's sort of a trick question, because it would look the same. The building would go from being stationary to being in 9.8 m/s^2 acceleration in an instant, and free fall through those eight floors. Then it would start to slow down as it met more resistance. You're attempting to find some natural progression of fire-induced failures that can make the building move like that, and what me and Adrian on Quora are saying is that it's impossible.

 

[Abdullah]

it would look the same

No it wouldn't. There would be no periods of above freefall.

When confronted with this fact, Adrian acted as if I had just said the earth was flat.

 

[Henkka]

Well, I don't really know about that... NIST says in their FAQ that they measured 2.25 seconds of free fall. They make no mention of an acceleration above free fall. Is there reason to think it's not just some slight measurement error? Keep in mind both Chandler and NIST are measuring the fall from fuzzy videos taken from far away, and I think NIST even used the video that is looking up at the building. And I'm not sure how they measured it, but I'm sure their method is similar to that of Chandler, where he placed dots on the roofline using a program called Tracker. Even a slightly misplaced dot will introduce some fuzziness into the data. Another possible source of error is if the scale of the image was ever so slightly off, even by a little bit.

If there was some kind of "pull" on the roofline that would've helped NIST explain why the building came down so fast without explosives being needed, I'm sure they would have made note of it and included it in their explanation. Instead, they first claimed the fall was 40% slower than "free fall time", a rather nonsensical way of measuring it, and then they admitted the 2.25 seconds of free fall when pressed on it.

 

[Abdullah]

If there was some kind of "pull" on the roofline that would've helped NIST explain why the building came down so fast without explosives, I'm sure they would have made note of it and included it in their explanation.

I said elsewhere that the NIST WTC7 report seemed to have been written by bored, exhausted people.

They could not be bothered to explain something even more important, which was why column buckling spread across the entire cross section of the building, instead of being confined to the east.

 

[Henkka]

They could not be bothered to explain something even more important, which was why column buckling spread across the entire cross section of the building, instead of being confined to the east.

And my humble suggestion is they didn't explain it because they couldn't... Because it's impossible for fire to cause a failure like that. And they weren't just spitballing ideas, they had supercomputers running a highly advanced simulation of the building where they could test these things. But the final simulation doesn't really show the free fall.

Also, let's be real, two months isn't enough time to make any substantial changes to the report after they had worked on it for years. So the "40% slower than free fall time" thing in the August 2008 version was what they were going to go with in the final report. The only reason it was changed was because they were publicly asked about it. So the final version of the report in November doesn't really contain an explanation for all these things, because they had already committed in August to not explaining it.

 

[Abdullah]

it's impossible for fire to cause a failure like that

I just explained how fire caused a failure like that.

If those first columns had been cut by demo charges, the result would have been the same; global crippling of the building, followed by 8-stotry 3-hinge buckling of all columns.

 

[Henkka]

I just explained how fire caused a failure like that.

I would say you proposed an idea. But as for why NIST with their entire team of engineers, millions of dollars and state-of-the-art simulations didn't come to the same conclusion as you, you just said they were bored and exhausted. I don't think that's a satisfactory explanation.

I feel like you're doing engineering mental gymnastics to explain the collapse, when the obvious is staring you in the face... That the building collapsed in a manner resembling a demolition because it was a demolition. It neatly explains so many things about the collapse, like the symmetry, the free fall, the foreknowledge, everything. But instead, you're trying to rationalize how asymmetrical office fires caused the building to coincidentally collapse in a manner resembling a demolition. You have to ask yourself, what is the probability of that happening? One in a thousand? One in a million? I'd say it's even less than that.

 

[Abdullah]

I feel like you're doing engineering mental gymnastics to explain the collapse, when the obvious is staring you in the face... That the building collapsed in a manner resembling a demolition because it was a demolition.

Are you kidding me? If a building is on fire and it collapsed, the obvious conclusion is that it fell due to fire!

 

[Abdullah]

NIST with their entire team of engineers, millions of dollars and state-of-the-art simulations didn't come to the same conclusion as you

If they ever bothered to come to a concluaion at all, it most probably would be mine. Their own simulations were what made me come to this conclusion.

 

[econ41]

Well, I don't really know about that...

We understand that - hence the efforts to help you understand.

NIST says in their FAQ that they measured 2.25 seconds of free fall. They make no mention of an acceleration above free fall.

They said: "gravitational acceleration (free fall)" -like so many in these discusions not always as rigorous distinguishing "free Fall" from "F F Acceleration" as would be desirable. Both NIST and Chandler used crude methods of video measurement. The possibility of acceleration above free fall arose in extended discussion on another forum which identified about a dozen sources of error including the ones you identify:

Is there reason to think it's not just some slight measurement error? Keep in mind both Chandler and NIST are measuring the fall from fuzzy videos taken from far away, and I think NIST even used the video that is looking up at the building. And I'm not sure how they measured it, but I'm sure their method is similar to that of Chandler, where he placed dots on the roofline using a program called Tracker. Even a slightly misplaced dot will introduce some fuzziness into the data. Another possible source of error is if the scale of the image was ever so slightly off, even by a little bit.

The topic has been given some cursory examination here on Metabunk. BUT "above free fall" was NOT the subject of NIST's assessment or Chandler's questioning. The NIST WTC7 Report was "released for public comment" PRIOR to publication. Chandler asked for clarification of the NIST measurements. NIST responded by explaining in more detail what it had found. Both NIST and Chandler were discussing the broad picture of the findings - both accepting that their similar methods of measurement were adequate.

If there was some kind of "pull" on the roofline that would've helped NIST explain why the building came down so fast without explosives being needed,

Why would NIST need to explain what did not happen?

I'm sure they would have made note of it and included it in their explanation.

Your "sureness" has no post hoc interest for NIST even without the false presumption of CD

Instead, they first claimed the fall was 40% slower than "free fall time", a rather nonsensical way of measuring it,

True fact despite your assessment.

and then they admitted the 2.25 seconds of free fall when pressed on it.

And that "admitted" is the stock standard truther misrepresentation. NIST mentioned the FFA in the draft report. Chandler asked for more details. NIST gave more details. Even tho by that stage they would be aware of Chandler's mendacious intention. Truthers rely on a false meme - the presumption that free fall proves CD. It doesn't but TM leading figures like Chandler rely on it to mislead their followers. For the rational rest of the world free fall is simply a feature of a collapse mechanism.

 

[Landru]

Henkka is banned from this thread for off topic posts.

 

[Abdullah]

Why would NIST need to explain what did not happen?

You should clarify what it is that did not happen. Was it the whole building falling at freefall? Or the "pull"? Or the roofline exceeding G?

 

[investigating911]

They said: "gravitational acceleration (free fall)" -like so many in these discusions not always as rigorous distinguishing "free Fall" from "F F Acceleration" as would be desirable.

I have been having trouble understanding this part for a while now. What sort of physical situation would cause gravitational acceleration (free fall, which from I understand, is g=9.8 m/^2 for all falling bodies on Earth) to be different from free fall acceleration?

I also don't understand what the difference is between gravitational acceleration (free fall) and free fall acceleration. From what I have read, gravitational acceleration for all bodies on Earth is assumed a constant (g) of 9.8 m/^2. So how is free fall acceleration different? I would assume that free fall acceleration would be defined as, rather than a constant, a period of time where downwards acceleration of a body towards the Earth's surface is at 9.8 m/s^2 or greater. Or is it the stage at which terminal velocity is reached where there is zero net force because gravity and air resistance cancel each other out (I would think not, since in that case there would be more forces than gravity acting on the falling body)? Or something different entirely, such as the stage of free fall acceleration being where gravity is the only force acting on a body and the downwards acceleration of the object being constant at g=9.8 m/^2 (which I assume would be the stage for a falling body before the upwards force from air resistance/drag acted to reduce the magnitude of the downwards acceleration below 9.8 m/^2)?

Also, what would cause a falling body to fall faster than free fall (i.e. accelerating downwards to the Earth's surface at higher than 9.8 m/^2), such as during the 2.25 seconds where the outer curtain wall of WTC 7 could have accelerated downwards faster than 9.8 m/^2?

 

[investigating911]

This is offtopic, and it's becomubgvsimumporrant that I am going to create a new thread for it

Returning to the topic in this thread, from what I have understood:
as per @Jeffrey Orling,

1. collapse of lower floors leads to collapse of upper floors as well, with collapsing floors kicking out exterior columns at base of WTC 7 in all directions

2. upper hollowed out curtain wall of WTC 7 is temporarily in a state where the only force acting on the curtain wall is gravity, due to missing base exterior columns being kicked out by lower collapsing floors, and so the upper curtain wall of WTC 7 is temporarily in a state of free fall acceleration

3. the upper curtain wall of WTC 7 falls as a unit without significant crumpling, at or near free fall acceleration, only crumpling and falling in a roughly east-west configuration when it impacts the lower surviving portion of WTC 7.

 

[Abdullah]

1. collapse of lower floors leads to collapse of upper floors as well, with collapsing floors kicking out exterior columns at base of WTC 7 in all directions

He apparently thinks:

1. The east floor area slammed into the transfer floors at around level 5.

2. This somehow pushed the girder at the north edge of the core sudeways at a bottom floor (even though this girder was not a single monolithic object?)

3. Whuch took all the remaining north core columns with it.

4. Which caused the entire north floor area to cave in +but not the West penthouse?)

5. The debris crashed down and pushed the north h wall out

 

[investigating911]

He apparently thinks:

1. The east floor area slammed into the transfer floors at around level 5.

2. This somehow pushed the girder at the north edge of the core sudeways at a bottom floor (even though this girder was not a single monolithic object?)

3. Whuch took all the remaining north core columns with it.

4. Which caused the entire north floor area to cave in +but not the West penthouse?)

5. The debris crashed down and pushed the north h wall out

I'm not familiar with how exactly the progression of events at the base of WTC 7 took place, but the explanation by @Jeffrey Orling that the floor collapses kicked out the base exterior columns on all sides of WTC 7 makes sense to me.

I'd think also that with the absence of base columns and the relatively short vertical distance of the upper curtain wall to the ground, the upper hollowed out curtain wall falling as a unit at or near free fall acceleration before it impacted the lower portion of WTC 7 wouldn't be unreasonable.

If all that is generally true, I'm still confused as to how it would be possible for the upper curtain wall of WTC 7 to initially accelerate downwards at greater than g=9.8 m/s^2.

 

[Mendel]

I also don't understand what the difference is between gravitational acceleration (free fall) and free fall acceleration. From what I have read, gravitational acceleration for all bodies on Earth is assumed a constant (g) of 9.8 m/^2. So how is free fall acceleration different? I would assume that free fall acceleration would be defined as, rather than a constant, a period of time where downwards acceleration of a body towards the Earth's surface is at 9.8 m/s^2 or greater.

Take a broomstick. Rest one end on the table, hold the other end away from table, and let go.
• The broomstick undergoes "gravitational acceleration", because it falls down due to gravity.
• The broomstick is not in free fall, because one end is supported.
• Parts of the broomstick close to table accelerate at much less than g
• The free end of the broomstick accelerates at more than g due to leverage
• (g varies across the surface of the Earth, but is approximately 9.81m/s² everywhere.)

g is not like some "speed limit" on falling things, like c is for light. It only is that for free fall, but a collapsing building is generally not free falling. (Some loose debris that gets ejected might be.)

Free fall acceleration can not be greater than g; it is exactly g.

Also, what would cause a falling body to fall faster than free fall (i.e. accelerating downwards to the Earth's surface at higher than 9.8 m/^2), such as during the 2.25 seconds where the outer curtain wall of WTC 7 could have accelerated downwards faster than 9.8 m/^2?

The cause would be for that falling body to not be in free fall, like that broomstick end.


Note also that if you have a measurement with error, then measuring a true free falling body will have some data at less than g and some data at more than g, so the errors can cancel out and you get the true g acceleration. It does not mean that the acceleration actually exceeded g at any point.

 

[Mendel]

• (g varies across the surface of the Earth, but is approximately 9.81m/s² everywhere.)

Wolfram Alpha gives g=9.80213m/s² for Manhattan.

 

[investigating911]

Take a broomstick. Rest one end on the table, hold the other end away from table, and let go.
• The broomstick undergoes "gravitational acceleration", because it falls down due to gravity.
• The broomstick is not in free fall, because one end is supported.
• Parts of the broomstick close to table accelerate at much less than g
• The free end of the broomstick accelerates at more than g due to leverage
• (g varies across the surface of the Earth, but is approximately 9.81m/s² everywhere.)

g is not like some "speed limit" on falling things, like c is for light. It only is that for free fall, but a collapsing building is generally not free falling. (Some loose debris that gets ejected might be.)

Free fall acceleration can not be greater than g; it is exactly g.

Okay, so free fall acceleration is the state where the only force acting on an object is gravity and downwards acceleration is exactly g?

Assuming parts of the WTC 7 upper curtain wall temporarily did come down at faster than free fall, would that indicate that there was a surviving portion of lower columns still connected to that portion of the curtain wall (as the bottom end of the upper curtain wall would still be supported), which would provide the leverage for the free end of the curtain wall to accelerate downwards at more than g?

If that was the case, wouldn't that also suggest that the floor collapses didn't uniformly kick out all the base columns of WTC 7 and that some base columns remained standing and connected to the upper (but hollowed out) curtain wall of WTC 7?

Note also that if you have a measurement with error, then measuring a true free falling body will have some data at less than g and some data at more than g, so the errors can cancel out and you get the true g acceleration. It does not mean that the acceleration actually exceeded g at any point.

Right, I would think the faster than g downwards acceleration would be (as discussed before by others in this thread) mostly an artifact of grainy video footage pixels and errors in estimating the exact acceleration of the drop of portions of the building from this footage.

Assuming the acceleration didn't exceed g at any point, it would make sense if the floor collapses kicked out nearly all or all the exterior base columns, leaving the only force acting on the upper hollowed out curtain wall being gravity. Because of the short vertical drop from the upper curtain wall to the ground, before other forces like air resistance/drag would start becoming just as important as gravity, I think it would be reasonable to assume that the upper hollowed out curtain wall of WTC 7 would fall at or near free fall acceleration and as a unit, with very little crumpling, at least initially (which appears to be what was observed from footage)?

 

[econ41]

Okay, so free fall acceleration is the state where the only force acting on an object is gravity and downwards acceleration is exactly g?

When discussing free body motions and accelerations "over-G" I suggest it is best to be pedantically accurate.

"Free Fall" is a state of zero restraint - no external resistance and the body is free to fall under gravity. << No problem there.

"Free Fall ACCELERATION" ["FFA"] is a motion. YES it is the motion that results from the state of Free Fall. BUT - and this aspect is critical to "over-G" and other complexities of free body movements - FFA can result from other scenarios.

So it is best to NOT define free fall acceleration as "the state where the only force acting on an object is gravity" but rather FFA is "...the motion (NOT "state") where there is ZERO NET force acting other than gravity".

Because "over-G" and a few other complications can arise when other forces are in play. I can give a fuller - possibly clearer - explanation if it is needed.

So assessed over-G motion of that infamous point or zone on the WTC7 facade can come from THREE sources:
1) Errors of measurement. (The NIST and Chandler measurements were crude and subject to as many as 10 or 12 sources of error.)
2) Precise measurements which are accurate AND over-G actually happened OR
3) IIRC NIST and Chandler claimed "average of G" - and given the nature of "average" that makes it highly probable that it was both oveer and under G.

One likely cause of over G is "torquing" as some rotary motion imparts rotational vectors. Another is linear pulling from other partially still attached internal components which are moving ahead of the falling perimeter and "dragging it after them".

 

[investigating911]

When discussing free body motions and accelerations "over-G" I suggest it is best to be pedanticlly accurat.

All right, I think I understand, free fall acceleration of a body is the motion where there is zero net force acting on the body other than gravity, and the downwards acceleration of the body is exactly g. If accelerations are over g, there isn't free fall acceleration of a body, as it would indicate that there are other forces besides gravity acting on the body.

Would you say that this general sequence of events for the collapse of WTC 7 would be reasonable:

1. collapse of lower floors leads to collapse of upper floors as well, with collapsing floors kicking out exterior columns at base of WTC 7 in all directions


2. upper hollowed out curtain wall of WTC 7 is temporarily in a state where the only force acting on the curtain wall (at least initially) is gravity, due to missing base exterior columns being kicked out by lower collapsing floors, and so the upper curtain wall of WTC 7 is temporarily in a state of free fall acceleration


3. the upper hollowed out curtain wall of WTC 7 falls as a unit without significant crumpling, at or near free fall acceleration, only crumpling and falling in a roughly east-west configuration when it impacts the lower surviving portion of WTC 7.

 

[Abdullah]

If that was the case, wouldn't that also suggest that the floor collapses didn't uniformly kick out all the base columns of WTC 7 and that some base columns remained standing and connected to the upper (but hollowed out) curtain wall of WTC 7?

Thus is why I say that the floor collapses did not happen. At least beyond the east area. Instead, thus happened:

https://engineering.stackexchange.com/a/51820/27982

>Therefore, it can be said that after the caving of the east floor area, WTC7 behaved as a box column which had lost one of it's flanges. That this crippling was the buckling mode is supported by the NIST simulations, which show the tower twisting south by "over 1m".

 

[econ41]

Sorry @investigating911 - I posted #469 prematurely - part finished draft. Now completed.

 

[investigating911]

Thus is why I say that the floor collapses did not happen. At least beyond the east area. Instead, thus happened:

https://engineering.stackexchange.com/a/51820/27982

>Therefore, it can be said that after the caving of the east floor area, WTC7 behaved as a box column which had lost one of it's flanges. That this crippling was the buckling mode is supported by the NIST simulations, which show the tower twisting south by "over 1m".

But the NIST simulations differed from what was actually seen from some angles, which was with the curtain wall more or less falling straight down, initially at or near free fall acceleration.

I agree that it seems the collapses initiated in the east area and complete floor collapse at the base of WTC 7 might not have occurred, but what besides uniform floor collapse would result in the curtain wall of WTC 7 appearing to fall straight down? If there was deflection and twisting to the south, it wasn't visible on for example the BBC footage of WTC 7 collapsing. It does appear to have been visible on some other footage


Source: https://www.youtube.com/watch?v=7ZiMG84hws0


From 0:00 to 0:03 in the above video, WTC 7 appears to tilt to the west (maybe even some movement to the southwest also visible) as it falls.

From 0:09 to 0:14 in the above video, WTC 7 appears to tilt to the south as it falls.

Overall, it seems from close up footage that the curtain wall of WTC 7 when viewed from just a few blocks away, fell to the south and west, but from farther away like in the BBC and CNN footage (CNN footage seen here
Source: https://www.youtube.com/watch?v=mY7F4xMcE4s
from 3:05 to 3:20) or from the angle used by AE 9/11 Truth (which is viewing WTC 7 from the northwest and looking roughly east-southeast) appears to show WTC 7 falling straight down.

It's possible that when viewed from directly north or close up and just northwest of WTC 7, the tilt of WTC 7 to the south and west as it fell was not that apparent, and so it appeared that WTC 7 came straight vertically down.

 

[econ41]

All right, I think I understand, free fall acceleration of a body is the state where the only force acting on the body is gravity, and the downwards acceleration of the body is exactly g. If accelerations are over g, there isn't free fall acceleration of a body, as it would indicate that there are other forces besides gravity acting on the body.

Yes.

Would you say that this general sequence of events for the collapse of WTC 7 would be reasonable:

1. collapse of lower floors leads to collapse of upper floors as well, with collapsing floors kicking out exterior columns at base of WTC 7 in all directions

There are two other plausible mechanisms. And the overall situation could be a mix of all three.

2. upper hollowed out curtain wall of WTC 7 is temporarily in a state where the only force acting on the curtain wall (at least initially) is gravity, due to missing base exterior columns being kicked out by lower collapsing floors, and so the upper curtain wall of WTC 7 is temporarily in a state of free fall acceleration

I have doubts about:
1) All three of those assertions; AND
2) whether we are getting lost in details we can never be sure of.

3. the upper hollowed out curtain wall of WTC 7 falls as a unit without significant crumpling, at or near free fall acceleration, only crumpling and falling in a roughly east-west configuration when it impacts the lower surviving portion of WTC 7.

I'm not convinced of the final assertion.

 

[investigating911]

There are two other plausible mechanisms. And the overall situation could be a mix of all three.

What are the two other plausible mechanisms besides lower floor collapse?

I have doubts about:
1) All three of those assertions; AND
2) whether we are getting lost in details we can never be sure of.

From looking at different angles of the collapse, it appears to me now that it can't exactly be concluded that WTC 7 fell completely straight down. It appears to have fallen to the south and west.

I'm not convinced of the final assertion.

Would it at least be reasonable to say (which was the subject of earlier debate in this thread) that the curtain wall of WTC 7 at least initially came down as a unit at near free fall acceleration without crumpling, unlike what was seen in NIST simulations of the collapse of WTC 7 (where the modeled curtain wall displayed inwards and downwards crumpling as it fell)?

 

[econ41]

What are the two other plausible mechanisms besides lower floor collapse?

The topic is how buckling led to free fall. The first two (accepted) premises are:
(a) The core failed leaving the hollow perimeter shell to fall whilst remaining essentially intact; and
(b) there was approximately free fall for about 7-8 storeys.

Which led me to postulate earlier in the thread (Post #260 and following.) two potential causes... remember the WTC 7 was bridged over a Con-Ed substation and had cantilever bridging trusses THEREFORE two possibilities jump out:
1) Failure of the cantilever portions of those bridging trusses; OR
2) Direct failure by buckling of trusses in that same vicinity.

From looking at different angles of the collapse, it appears to me now that it can't exactly be concluded that WTC 7 fell completely straight down. It appears to have fallen to the south and west.

But does that affect the argument of the topic??

Would it at least be reasonable to say (which was the subject of earlier debate in this thread) that the curtain wall of WTC 7 at least initially came down as a unit at near free fall acceleration without crumpling,

Yes. Those are the broad overview opening premises.

unlike what was seen in NIST simulations of the collapse of WTC 7 (where the modeled curtain wall displayed inwards and downwards crumpling as it fell)?

Too many members are forgetting that the NIST sim is NOT a "look like" accurate geometric representation.

 

[investigating911]

The topic is how buckling led to free fall. The first two (accepted) premises are:
(a) The core failed leaving the hollow perimeter shell to fall whilst remaining essentially intact; and
(b) there was approximately free fall for about 7-8 storeys.

Okay, so the core failing (independent of floor failure) can lead to loss of support for the hollow perimeter shell, causing the hollow perimeter shell and (semi-attached) floors to fall down as one unit?

Which led me to postulate earlier in the thread (Post #260 and following.) two potential causes... remember the WTC 7 was bridged over a Con-Ed substation and had cantilever bridging trusses THEREFORE two possibilities jump out:
1) Failure of the cantilever portions of those bridging trusses; OR
2) Direct failure by buckling of trusses in that same vicinity.

Right, it seems like the initiation of local failure which cascaded into global failure was around the lower portion of the building, where the fires appeared to be concentrated.

But does that affect the argument of the topic??

I guess in the sense that the buckling could have resulted in tilting of WTC 7 to the south and west as it fell, but otherwise it wouldn't really affect the argument of the topic.

Too many members are forgetting that the NIST sim is NOT a "look like" accurate geometric representation.

Was this because the NIST sim had exaggerated movements and deformations of the building elements in order to clearly show failure of the building elements, which resulted in a representation that was not a "look like" accurate geometric representation?

 

[econ41]

Okay, so the core failing (independent of floor failure) can lead to loss of support for the hollow perimeter shell, causing the hollow perimeter shell and (semi-attached) floors to fall down as one unit?

Yes. And that IMO should be the agreed start point for this discussion.

Right, it seems like the initiation of local failure which cascaded into global failure was around the lower portion of the building, where the fires appeared to be concentrated.

Take care that you mean "global" failure of the perimeter shell - the core had already (mostly) failed.

I guess in the sense that the buckling could have resulted in tilting of WTC 7 to the south and west as it fell, but otherwise it wouldn't really affect the argument of the topic.

EXCEPT that engineers digging into details and losing the plot is endemic. "can't see the trees for the forests" syndrome. Or, as I parodied it on another forum about 12 years ago:

"How many leaves on the seventh branch of the fourth tree?" is meaningless when you are in the wrong forest

Was this because the NIST sim had exaggerated movements and deformations of the building elements in order to clearly show failure of the building elements, which resulted in a representation that was not a "look like" accurate geometric representation?

Yes. Remember that Truthers have been "taught" that the NIST sim is wrong because it doesn't "look like" the real event. AND Hulsey was required to fake his sim so that it did "look like".

 

[FatPhil]

So assessed over-G motion of that infamous point or zone on the WTC7 facade can come from THREE sources:
1) Errors of measurement. (The NIST and Chandler measurements were crude and subject to as many as 10 or 12 sources of error.)
2) Precise measurements which are accurate AND over-G actually happened OR
3) IIRC NIST and Chandler claimed "average of G" - and given the nature of "average" that makes it highly probable that it was both oveer and under G.

One likely cause of over G is "torquing" as some rotary motion imparts rotational vectors. Another is linear pulling from other partially still attached internal components which are moving ahead of the falling perimeter and "dragging it after them".

Indeed. And I'd like to see the above chart that supposedly shows over-g but with some error bars on it. The line that spends more time beyond g in particular seems to have a surprising level of precision - how do they justify that?

 

[Abdullah]

Okay, so the core failing (independent of floor failure) can lead to loss of support for the hollow perimeter shell, causing the hollow perimeter shell and (semi-attached) floors to fall down as one unit?

We're missing the point. The point was the loss of north-south restraint.

If the only damage on the tower was a complete removal of the wind girders of the east or west walls, the building would still have been destroyed. Because of wall crippling.

In fact, a partial removal would have been fine. For Euler buckling of the affected columns

Length² × strain = π² × gyradius.

At a rather low stress of 50MPa and with a gyradius of 7 inches or less consistent with the W14 section columns, the critical length would be 35m, or 10 storeys. This is for a fixed-guided boundary condition, which is applicable here as the the undamaged top block of the tower slides in the danaged section without rotating.