Debunked: NIST's Lack of Explanation for WTC7 Freefall [They Have One - Column Buckling]

[Mick West]

In a recent interview with the BBC, Roland Angle claimed that NIST did not have any explanation for the 2.25 seconds of essentially free fall descent observed during the collapse of World Trade Center Building 7, saying:

You've got to understand that the way that that building collapsed was unprecedented. You had a forty seven story steel frame building that collapsed in just under just around seven seconds, which is just about freefall speed. And measurements of the video showed that the first two and a quarter seconds of the collapse was an absolute freefall speed, which means that there was no resistance, which means that all 82 columns in the building were not acting at all for a distance of about eight floors.Now that's an impossibility. The columns were there. What what could possibly have caused those columns not to be acting, not to be giving any resistance to the collapse of the building at all? Well NIST doesn't have an answer for that. They admit, NIST's reports at actually admitted that there was two and a quarter seconds of freefall, and yet they don't have any explanation for how that could occur. It certainly wouldn't occur under the premise that they have, which is that the girder failed and therefore there was a progressive collapse those those collapses even if they did occur would take time there's no way that the building collapse at freefall speed under their scenario. They have no explanation for that.

Content from External Source

But NIST actually DO have an explanation for that, in their report on their collapse, and summarized in the FAQ
https://www.nist.gov/el/faqs-nist-wtc-7-investigation

11. In a video, it appears that WTC 7 is descending in free fall, something that would not occur in the structural collapse that you describe. How can NIST ignore basic laws of physics?

In the draft WTC 7 report (released Aug. 21, 2008; available at http://www.nist.gov/el/disasterstudies/wtc/wtc_draftreports.cfm), NIST stated that the north face of the building descended 18 stories (the portion of the collapse visible in the video) in 5.4 seconds, based on video analysis of the building collapse. This time period is 40 percent longer than the 3.9 seconds this process would have taken if the north face of the building had descended solely under free fall conditions. During the public comment period on the draft report, NIST was asked to confirm this time difference and define the reasons for it in greater detail.

To further clarify the descent of the north face, NIST recorded the downward displacement of a point near the center of the roofline from first movement until the north face was no longer visible in the video. Numerical analyses were conducted to calculate the velocity and acceleration of the roofline point from the time-dependent displacement data. The instant at which vertical motion of the roofline first occurred was determined by tracking the numerical value of the brightness of a pixel (a single element in the video image) at the roofline. This pixel became brighter as the roofline began to descend because the color of the pixel started to change from that of the building façade to the lighter color of the sky.

The approach taken by NIST is summarized in NIST NCSTAR Report 1A, Section 3.6, and detailed in NIST NCSTAR Report 1-9, Section 12.5.3.

The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline) revealed three distinct stages characterizing the 5.4 seconds of collapse:

• Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
• Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall)
• Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity

This analysis showed that the 40 percent longer descent time—compared to the 3.9 second free fall time—was due primarily to Stage 1, which corresponded to the buckling of the exterior columns in the lower stories of the north face. During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below. This is consistent with the structural analysis model, which showed the exterior columns buckling and losing their capacity to support the loads from the structure above. In Stage 3, the acceleration decreased as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below.

Content from External Source

Their explanation is the buckling of the exterior columns (which happened after the collapse and buckling of the interior columns). Since a buckled column supplies only a tiny fraction of its designed resistance to vertical load, the result is very close to freefall.

AE911 Truth must be aware of this, so it's rather misleading on their part to suggest otherwise. It's fine to disagree with a complex analysis, but not to claim it does not exist.

 

[Cantonear1968]

The worst thing NIST ever did in regards to their WTC7 Report was including this break down of the collapse stages. At that point they were acknowledging the conspiracy theories and giving them legitimacy in the eyes of the conspiracy theorists. If their hope was to be open and honest to appease their concerns it obviously had the exact opposite effect.

They wrote an engineering report that is far above the heads of most people. They should have kept it for the audience they wrote it for and ignored the small rumblings of those only looking for a sound bite.

 

[deirdre]

giving them legitimacy in the eyes of the conspiracy theorists.

everything you do or don't do, gives CTers 'legitimacy' in their own eyes.

 

[Cantonear1968]

everything you do or don't do, gives CTers 'legitimacy' in their own eyes.

Which doesn't mean you have to play by their rules. This was a professional engineering report that is truly understood by maybe 15% of the population. And was written for that 15%. Stooping to the bottom 5% in the hopes of appeasing them was a fool's errand.

 

[deirdre]

Stooping to the bottom 5% in the hopes of appeasing them was a fool's errand.

maybe they wrote it so they can just reply to the hundreds of repetitive emails asking questions, by saying "the information is online. here's a link. "

 

[Geoff Forbes]

I agree that the AE 911 Truth position that NIST did not address free fall of Building 7 is correct.

However, here are the two problems I have. NIST says that there was 'negligible' resistance during free fall. There can be zero resistance, not 'negligible' resistance, if gravity is really the only force involved. Your response, Mick, is also a problem. You say it was 'very close to free fall'. Where did you get that from? It was free fall. NIST admitted it. Period. And the only way for that to happen is for additional energy to be applied.

 

[Mick West]

You say it was 'very close to free fall'. Where did you get that from? It was free fall.

In a pedantic sense it could not be free fall, as there would be air resistance.

In a less pedantic sense there would also be resistance from the buckled columns. However if this was less than around 2% of the original resistance then it would not be measurable on the videos.

So if the remaining resistance was a few tons of force (a lot, when it's put like that) then it could still result in an acceleration of approximately (say 98% to 99%) of free fall, when the original resistance was a few hundred tons.

Basically "negligible" her can mean tons, but since you need hundreds of tons, it's negligible. Like braking in a car by pressing the brake pedal 1/100th the normal amount.

 

[Geoff Forbes]

"less than 2%"? Unless they melted like butter, they didn't lose 98% of their resistance.

 

[Mick West]

"less than 2%"? Unless they melted like butter, they didn't lose 98% of their resistance.

They didn't melt at all, nor were they particularly weakened by the fire.

They buckled.

 

[Jeffrey Orling]

I agree that the AE 911 Truth position that NIST did not address free fall of Building 7 is correct.

However, here are the two problems I have. NIST says that there was 'negligible' resistance during free fall. There can be zero resistance, not 'negligible' resistance, if gravity is really the only force involved. Your response, Mick, is also a problem. You say it was 'very close to free fall'. Where did you get that from? It was free fall. NIST admitted it. Period. And the only way for that to happen is for additional energy to be applied.

How do YOU know it was free fall? What was measured as "free falling'? how many data points were there? What was the margin of error? Perhaps you missed the fact that the tower was distorted as it fell, a huge kink developed and wouldn't you admit that the motion was complex and not like Newton dropping an apple?

 

[econ41]

How do YOU know it was free fall? What was measured as "free falling'?

There is actually a fundamental issue of definition adding to the confusion both here and in the wider debate of this and related or similar issues.

It is the difference between:
(a) the STATE of free fall which can be defined as 'no support' - object free to fall under gravity. (And more correctly should be defined as 'zero net external force on the system under consideration'. The pedantic distinction is actually critical in the explanation of WTC7 façade movement. ); AND
(b) the 'free fall ACCELERATION' ['FFA'] which results from a state of free fall (and disregarding air resistance.)

The problem of confusion is compounded because even professionals do not distinguish the two. They often say 'free fall' when they mean 'free fall acceleration'. Sort of excusable when discusing with other professionals - we usually know which is intended in the context. In fact it is only in recent months that I have become aware of the issue as a source of confusion.

So Jeffrey Orling actually asks the wrong question with 'What was measured as "free falling'? It should be 'What was measured as free fall acceleration'? We cannot measure 'free fall' per se. And we need to take care imputing that FFA is caused by free fall because there are other causes.

Jeffrey like me and possibly other members are aware that the NIST (and Chandler) measurements of WTC 7 façade acceleration were quite crude approximations. Far more rigorous measurements have been performed which I accept as valid. And they show that a portion of the contested period of free fall acceleration actualy exceeded 'G' for a brief period. The issue has been missed in most on line debate - but - if those more precise measurements are correct the existence of brief 'over"G"' proves that there was an external force applied to the subsystem in question. And it was NOT free fall. Hence the need for pedantic accuracy of definition for this WTC7 situation.

 

[Geoff Forbes]

How do YOU know it was free fall?

Just quoting NIST

 

[Jeffrey Orling]

Just quoting NIST

That is part of the problem everyone faces in understanding. We refer to other people's work as gospel. We trust but don't, won't or can't verify. There has been some very rigorous and detailed time motion studies by independent researches who published their findings on the 911 forum and the work includes critiques of Chandler and NIST on this matter.

 

[econ41]

Just quoting NIST

I suggest that you are not quoting NIST - rather presenting your words which do not accurately reflect NIST.

And part of the confusion is addressed by my previous post - the reason why is is necessary to be clear whether we are referring to 'free fall' - or 'free fall acceleration'.

The core of Mr Angles assertions is these two claims "[NIST] admitted that there was two and a quarter seconds of free fall and yet they don't have any explanation for how that could occur".

As Mick has shown the second claim 'they don't have any explanation' is false.
The first claim is also mendaciously false. NIST was explicit that what was measured was accelerations which they concluded were the result of a status that was 'essentially free-fall'. i.e. it was equivalent to a state of free fall - zero net support.

From there discussion has referred to the NIST explanation about the state of near zero net support arising from column buckling. I have no problem - those explanations are valid as far as they go. But both Jeffrey Orling and I have referred to some additional factors - other research - which contribute to the local acceleration of that point or zone on the façade. It may be drifting too far from the OP topic but I can explain further if appropriate.

The net effect for this current topic is that it shows that low level column buckling was not the only factor controlling the acceleration of that subject point on the façade and the LOCAL free fall accelerations of the façade did not result from a state that was free fall or even 'essentially free fall'. And those LOCAL effects were not solely the result of the GLOBAL effects of low level column buckling as per the NIST explanations.

NIST did explain it. There is a potentially better - more comprehensive explanation.

 

[Jeffrey Orling]

When the building came down it had clearly lost the axial support provided by the columns coupling the building's structure to the foundations. As the collapse represented a 2+ second of accelerating drop it seems that there was no crushing of columns or other structures which could have slowed the rate of descent. The height of the structure that was "non performing" was 100'+ and an object dropping in the air from that height would take 2+ seconds to reach the ground... only thing to slow the decent would be air resistance.

The building of course was 95% or more air and the column area was a small fraction of 1% of the entire foot print. And this cross sectional area was in fact less from floor 7 to grade because multiple columns from above floor 7 were terminated on load transfers which move the axial loads above laterally.

There were 57 perimeter columns in the moment frame above floor 7 and there were only 26 of those which continued directly in line to the foundation. Obviously 31 columns above floor 7 had their axial loads transferred at floor 7.

Likewise several interior columns were supported on transfer structures in the region of floors 5 through 7.

When the collapse became naked eye observable... those transfers and the remaining 26 perimeter columns had "failed" and no longer provided any coupling to the foundations. It's likely that when they were overloaded the more or less intact upper part not only descended, pulled down by gravity but rotated or twisting a bit because the it's unlikely that the remaining 26 failed simultaneously. But the failure likely rapidly spread through / to all the columns. I believe the videos show the tower DOES twist or rotate counter clockwise when viewed from above suggesting to me that the NE corner columns were the first to go and were what induced the counter clockwise motion. The vertical kink was also at the northeast region. Hard to read precisely, but the NE corner could have been pushed northward, the north west side held and the kink developed. That is to say the kinky may not have been an inward bowing with respect to the original orientation of the north facade... as much as a northward movement of the NE quadrant.. giving the appearance of an inward bow or folding.

Regardless, the building did not collapse without showing distortion, twisting or torquing and so the motion was more complex than an apple being dropped through the air.

 

[Geoff Forbes]

I suggest that you are not quoting NIST - rather presenting your words which do not accurately reflect NIST.

NIST did explain it. There is a potentially better - more comprehensive explanation.

I guess I'm wondering why it takes so much effort to explain the end results of an office fire -- so much so that the group that was tasked to do it over a number of years couldn't even do it very effectively.

Doesn't it make it clear why people question what really happened that day? And, to be honest, this is only one of many things that occurred on 9/11 that require a great deal of explanation, beyond what they typically would.

I have looked, but have yet to find, another example of an office fire (even far more extensive) causing a steel structure to come down so quickly -- and, at least initially, so uniformly. If anyone has a link, or another example, I'd love to see it.

But for now, with my limited knowledge of physics, I still have to question if there's something else to the story.

 

[deirdre]

And, to be honest, this is only one of many things that occurred on 9/11 that require a great deal of explanation,beyond what they typically would.

This was a terrorist attack and 3,000 people died. An unprecedented event. So what do you mean by "typically"?

I have looked, but have yet to find, another example of an office fire (even far more extensive) causing a steel structure to come down so quickly -- and, at least initially, so uniformly. If anyone has a link, or another example, I'd love to see it.

You can look for threads on "Plasco" fire.

 

[Jeffrey Orling]

I guess I'm wondering why it takes so much effort to explain the end results of an office fire -- so much so that the group that was tasked to do it over a number of years couldn't even do it very effectively.

Doesn't it make it clear why people question what really happened that day? And, to be honest, this is only one of many things that occurred on 9/11 that require a great deal of explanation, beyond what they typically would.

I have looked, but have yet to find, another example of an office fire (even far more extensive) causing a steel structure to come down so quickly -- and, at least initially, so uniformly. If anyone has a link, or another example, I'd love to see it.

But for now, with my limited knowledge of physics, I still have to question if there's something else to the story.

Why do you find it extraordinary that a robust structure would take many hours to reach the point of structural collapse when there was no fire fighting and the building had some rather unusual structural design features which made it unlike 99.999 of all steel high rise buildings?

What I and others believe was that the failure of the transfer structures led to the total collapse which NIST calls global collapse. Unfortunately NIST kinda dropped the ball in explaining how the transfers had to fail for the tower to collapse as it did. They can up with an explanation consistent with the observed collapse of the EPH. The failure of the region around col 79 may have been what triggered the collapse/failure of the transfers which were interconnected across the entire northern part of the plan. For some reason they did not want to go into the engineering design which was necessitated by having to build a 40 story tower over a Con Ed sub station... clearly a very unique feature for a steel hi rise tower. Even their animation ignores the transfer structures.

 

[econ41]

I guess I'm wondering why it takes so much effort to explain the end results of an office fire -- so much so that the group that was tasked to do it over a number of years couldn't even do it very effectively.

Neither the official reports nor the many professional papers are targeted at the average "lay person in the street" OR the conspiracy fringe of the population. They are targeted at professionals and informed lay persons. Whether there should be explanations targeting minority groups with alternate agendas is a question of policy - and it goes too far "off topic" for this thread on this forum. If you want to discuss the issues "Should Statutory Reporting Process Target Minority Interests" or some such topic why not start a new thread?

Your assertion "couldn't even do it very effectively" is not supported by members of the target audience.

Doesn't it make it clear why people question what really happened that day?

Why SOME people question >> the issue already addressed - "Which people are the target of these types of reports?" .

 

[Jeffrey Orling]

One aspect of the discussions about the collapse of the WTC towers is that they are not simple matters that the average person can grasp. They are very complex engineering, chemistry, physics and so on aspects to understanding them. Therefore to explain this complexity to a lay person... there needs to be a need to do that... and someone(s) who can do justice to the complex technical arguments making them accessible.

I don't believe NIST saw this as part of their mission. Yet they were at some point questioned by reporters who were not technically qualified and likely in over their depth. If they were part of the media their role is to communicate to the lowest common denominator. And one should note how reporters described the collapses - using analogies to things people might know... "It looked like a CD," for example.

My experience with truthers is that they are not technically educated or experience in structural matters to understand what they had seen. They use their experience for reading and seeing fictional things in the media in many cases as references for what happened. They use the only examples of building collapses documented in the media - CD. When they cite other burning high rises which did not fall, they fail to consider how different those were from the WTC buildings. It's a dumbed down approach... high rise on fire, steel frame... it did not fall... so if the WTC buildings fell it had to be from an engineered cause.

Is there a legitimate purpose in making explanations which the lowest common denominator can understand?

Even so, anything resembling a "proof" explaining an event will by necessity be a complex technical one so there may be no way to satisfy those who don't have the technical background to understand the WTC events.

I use the analogy of weather and clouds which we all experience but those who have not studied meteorology simply cannot understand... except things like dark clouds mean rain and white ones don't.

The getting people to understand seems like a no win situation.

 

[deirdre]

Even so, anything resembling a "proof" explaining an event will by necessity be a complex technical one so there may be no way to satisfy those who don't have the technical background to understand the WTC events.

It's not that hard to understand. They just have to read the MB threads. I did and i get it.


(edit add: this does not mean i 'get' most of what you say Jeff, and econ41 i only follow like 30% of what he says... but if you read all the MB threads you can understand the technical stuff.. even if you're an airhead like me)

 

[Oystein]

A friend of mine, years ago, broke both his ankles in complex ways, which cost him a year in hospitals, rehab and on crotches, because he climbed a ladder that he suspected was borderline stable, but he figured that, if it began to give way, he should easily be able to descend soon enough, before total collapse.

Well, he figured wrong. When that metal structure collapsed, it collapsed far more sudden and quickly than my friend, who as a craftman was no stranger to ladders and climbed them almost daily, anticipated.

So if a professional ladder climber's intuition of the collapse behavior of ladders fails, how much more must all our intuition fail when we try to imagine how 47+ story skyscrapers might collapse?

 

[deirdre]

because he climbed a ladder that he suspected was borderline stable, but he figured that, if it began to give way, he should easily be able to descend soon enough, before total collapse.

ok i stand corrected. guess i'm not as big of an airhead as i thought. So nix my above comment.

 

[Jeffrey Orling]

Deirdre... your point is well taken. There are several things operative I believe. Proofs, which is what truthers demand are technical and precise by nature. Then their understanding is informed by analogies... if it looks like a duck it is a duck thinking. A simple structural solution to spanning is a truss. But it hard to understand for someone who doesn't understand compression, tension and maxwell force diagrams.

The twin towers not only had to stand up ... carry their own weight and the imposed loading of the floors, but had to resist lateral wind loads which can be enormous considering the total area that the winds would "push" on. And wind loading... resisting lateral forces is a consideration for tall buildings and especially very tall ones like the towers. There are a few strategies that engineers use to make the towers stiff. Winds will like not push a tower over but it will cause them to sway and make it uncomfortable for the occupants. Movement will also work connections and cause materials and finishes to crack. So there is a limit to what amount of motion is acceptable. We know that bridges have to have expansion joints because heat will expand the materials and the road bed. If these joints were not there road would buckle! This sort of design is not intuitive. Engineers willl iuse diagonal members to provide stiffness because a triangle is stiff and rigid whereas a rectangular or 3 sided shape is not. Very rigid joints make right angle connections stiff. But these are expensive. Engineering is also value engineering... getting the job done for the least cost and making the solution aesthetic and practical for the building's functions.

The mech floors were structurally stronger and stiffer than the tenant floors and were part of the strategy to limit the towers from flexing too much. The exterior columns in panel configurations with tall spandral beams was a second strategy.. The exterior was like 4 rigid plates with the side aligned with the wind force acting like beams. The side normal to the wind were sails and would defect and this was resisted by the floor plates and the damping system employed in the floor trusses. The core itself was a moment connected stiff spine which resisted the lateral forces as well

 

[benthamitemetric]

What I and others believe was that the failure of the transfer structures led to the total collapse which NIST calls global collapse. Unfortunately NIST kinda dropped the ball in explaining how the transfers had to fail for the tower to collapse as it did. They can up with an explanation consistent with the observed collapse of the EPH. The failure of the region around col 79 may have been what triggered the collapse/failure of the transfers which were interconnected across the entire northern part of the plan. For some reason they did not want to go into the engineering design which was necessitated by having to build a 40 story tower over a Con Ed sub station... clearly a very unique feature for a steel hi rise tower. Even their animation ignores the transfer structures.

While NIST did not prepare a fulsome narrative as to the role the transfer trusses played in the horizontal propagation of the collapse, the NIST report, and related work done by certain of the NIST authors, did make it clear that the transfer trusses were studied in depth and fully considered by NIST, as I noted to you previously on metabunk here. And, given that the transfer trusses were modeled in NIST's global model, I'm not sure what you mean when you say that "[e]ven their animation ignores the transfer structures." At the end of the day, NIST's lack of focus on the narrative concerning the transfer trusses does not mean they were in any way ignored where it counted (the modeling).

 

[Jeffrey Orling]

While NIST did not prepare a fulsome narrative as to the role the transfer trusses played in the horizontal propagation of the collapse, the NIST report, and related work done by certain of the NIST authors, did make it clear that the transfer trusses were studied in depth and fully considered by NIST, as I noted to you previously on metabunk here. And, given that the transfer trusses were modeled in NIST's global model, I'm not sure what you mean when you say that "[e]ven their animation ignores the transfer structures." At the end of the day, NIST's lack of focus on the narrative concerning the transfer trusses does not mean they were in any way ignored where it counted (the modeling).

Hold on...
I think that there needed to be some more discussion how a single column failure in a high rise leads to the total collapse of the building. In actual fact it would not and the collapse would be local to the column even it it was full height of the building.

Their simulation did not come anywhere near matching observations so it was rubbish.

To note that they "looked at the transfers" essentially means nothing because if they had and understood that it was THEIR failure which caused what they called "global" collapse and its form. There is no other explanation which explains the way the tower came down. The transfers failed and with them the structure had no axial connection to the foundations from floor 7 to grade which precisely aligns with the time frame of the so called free fall phase of the collapse.

Ignoring the role they transfers play... CRUCIAL one... is in my opinion a huge blunder and one has to wonder how they could make it.

Unless there is a better explanation for their ignoring the crucial defining role in the form of the collapse of the transfers... my hunch is that they did not want to explore design engineering choices... nor even the wisdom of erecting an office tower over a main power sub station. Those decisions had nothing to do with the terrorist attack, but perhaps if the designs of all three towers were different they might not have completely collapsed. I believe this is true and engineers will agree. The design decisions were of course driven by economics more than anything else.

 

[econ41]

It's not that hard to understand.

Agreed. Tho there are differences of level of understanding some of the complexity.

...and econ41 i only follow like 30% of what he says...

...It's the risk I take. There are many debunkers capable of moderately high level explanations. So I don't often repeat the medium level stuff when others do a good job. But - take this thread as an example. The NIST failing columns explanation subject of the OP was accurately recalled by Mick. It is good enough for most. EXCEPT it does not explain the specific detail of the high accelerations of that specific point on the façade. In fact it doesn't even address the reality that both* NIST and* Chandler did not identify a specific point. So I simply identified the other technical aspects. Fully anticipating that I would be ignored. It goes with the turf.

...but if you read all the MB threads you can understand the technical stuff.. even if you're an airhead like me)

Don't put yourself down - you are no airhead. We - you and I specifically - have enjoyed pushing discussion into some more complicated stuff on previous occasions. And this forum is one of the best resources for a wide range of comprehensive explanations.

Then - my other failing - I'm not an apologist for NIST or Bazant so I often - usually - address the technical issue - not whether the authorities explained it correctly or not.

* EDITs - my grammar was 'orrible.

 

[Jeffrey Orling]

Econ touches on an interesting aspect of the collapse discussions... it should not ONLY be NIST vs crazy conspiracies. The so called ROOSD was not a NIST defined concept and it rings true. Same for the core led collapse as opposed to the floor truss less collapse which also rings true. Neither require placed devices. As noted the collapse of 7wtc clearly suggests that the transfer structures played a key role in the "global" collapse....whether or not the collapse was triggered in the transfer region or not.

What I found interesting was the discussions with explanations driven by careful and in some cases precisely measured movements and observations before and during and even after the collapse which tell a lot of the story of how the buildings fell. NIST's work and explanations though possible were simply not the only possible ones and in my opinion not the most likely ones. And individuals with way way way fewer resources produced those explanations. Truthers of course seem to be focused on showing NIST was wrong concluding that they were hiding a CD type explanation.

 

[econ41]

Econ touches on an interesting aspect of the collapse discussions... it should not ONLY be NIST vs crazy conspiracies.

Yes - but I am keenly aware of the strict line that this forum takes about "on-topic" and the OP was specific to NIST so I was pushing the bounds by saying - in effect - "Yes but NIST doesn't go far enough in explaining that specific point".

 

[deirdre]

"Yes but NIST doesn't go far enough in explaining that specific point".

is a more detailed explanation of "that specific point" helpful to redesigning buildings for safety? because wasnt that sort of the whole point of the NIST report, to find out what happened to try to prevent the occurance in future building construction.. and/or to know to check out other buildings that might be constructed the same way?

 

[econ41]

Well spotted. No. The issue arose out of NIST's releases of the draft reports for public comment.

It is not of technical value for consideration in future designs. In fact most of the free fall debate is only of incidental interest for professional designers.

But the issue arose as a consequence of NIST being open and transparent and responding to concerns. The core policy issue is "How far should a statutory agency go in anticipating and pre-empting or responding after the event to conspiracy theory issues". That itself is a valid topic foir debate but I doubt you will see it discussed in technically oriented online forums.

For what it is worth - I have worked at that same level of "advising Government" (In my case on environmental matters but...there are many CT's in that arena) and I would have made the same sort of call as NIST did responding to Chandler. Even in full knowledge that he would spin and misrepresent the issue for conspiracy related purposes.

 

[benthamitemetric]

Hold on...
I think that there needed to be some more discussion how a single column failure in a high rise leads to the total collapse of the building. In actual fact it would not and the collapse would be local to the column even it it was full height of the building.

You seems to be assuming that, if not for the truss system, the building would not have undergone a global failure following the buckling of column 79 or that there is some incredible universal lesson about transfer trusses to be drawn from the failure of WTC7 that NIST needed to detail for future building designs. But I don't know why you think that other than that it's a pet theory you happened to land on. The NIST authors studied the truss system and its role in the collapse extensively, but, don't take my word for, or even links that you apparently can't be bothered to read, let's look at what the NIST report itself actually says. I currently only have a local copy of the summary NIST report (NCSTAR 1-9A), but even it has ample detail on their findings re the horizontal propagation. First, let's take a step back and see how columns 79, 80 and 81 all failed before the truss system began to fail:



Now here's the clear narrative (with illustrations) about the role the trusses did play in the horizontal propagation of the collapse. You can find that under the section helpfully titled "Horizontal Progression of Collapse":








NIST makes it very clear that in the case of WTC7, the loss of truss 2 triggered the collapse of column 77. NIST also makes it clear, however, that the primary driver of the column collapse propagation throughout the entire building was (1) loss of lateral support, (2) debris impact, and (3) load redistribution. So the transfer truss systemn did play a role in the collapse and that role was clearly detailed. But was that role determinative? It seems likely not. And was it generalizable into an important lesson about transfer trusses? It seems likely not (truss 1 survived the initial collapse wave, for example).

Their simulation did not come anywhere near matching observations so it was rubbish.

I disagree. Their models experience a bit more distortion in parts of the building far away from the key initiating areas--e.g., along the roofline--but the models are remarkable for how they so closely capture the timing of the collapse sequence and otherwise show the internal-to-external dynamic of the collapse.

To note that they "looked at the transfers" essentially means nothing because if they had and understood that it was THEIR failure which caused what they called "global" collapse and its form. There is no other explanation which explains the way the tower came down. The transfers failed and with them the structure had no axial connection to the foundations from floor 7 to grade which precisely aligns with the time frame of the so called free fall phase of the collapse.

Ignoring the role they transfers play... CRUCIAL one... is in my opinion a huge blunder and one has to wonder how they could make it.

Unless there is a better explanation for their ignoring the crucial defining role in the form of the collapse of the transfers... my hunch is that they did not want to explore design engineering choices... nor even the wisdom of erecting an office tower over a main power sub station. Those decisions had nothing to do with the terrorist attack, but perhaps if the designs of all three towers were different they might not have completely collapsed. I believe this is true and engineers will agree. The design decisions were of course driven by economics more than anything else.

Again, you want to believe the transfer trusses played some special, determinative role, but you do not know that. I've shown you that, without doubt, that the NIST authors spent a great deal of time theorizing about, and actually modeling and studying, the transfer trusses. They ultimately detailed the role they found the system played in the collapse very clearly in their report.

And this isn't about NIST versus conspiracy theories or me saying the NIST report is unimpeachable. I've made it clear time and time again that I of course understand the NIST report is not perfect. I am not posting to defend the NIST report; I posting to correct you as you misrepresent what it actually contains.

 

[Jeffrey Orling]

I will continue to believe as probably most engineers, including THE engineer of the building that the transfer structures enabled to global collapse. NIST focuses on the initiation location... which they believe to be at column 79

if there were no transfer structures and all the columns on the tower were directly coupled with the foundation the collapse would have been confined to the NE quadrant.

I am not going to argue this because my understanding is driven by observation... and some BASIC engineering.

The 104 collapse from floor 7 down can only be attributed to the transfers failing across the footprint.

You are defending NIST... but that's fine. You're not convincing me.

 

[benthamitemetric]

I will continue to believe as probably most engineers, including THE engineer of the building that the transfer structures enabled to global collapse. NIST focuses on the initiation location... which they believe to be at column 79

if there were no transfer structures and all the columns on the tower were directly coupled with the foundation the collapse would have been confined to the NE quadrant.

I am not going to argue this because my understanding is driven by observation... and some BASIC engineering.

The 104 collapse from floor 7 down can only be attributed to the transfers failing across the footprint.

You are defending NIST... but that's fine. You're not convincing me.

Again, I am not defending NIST. I pointing out what NIST actually did, which you first mischaracterized and now still do not even attempt to address. It seems like you have some cognitive dissonance to work through. You settled on your preferred theory re the transfer trusses playing a very significant role in the collapse and seemingly assumed that NIST never considered it. To the contrary, however, NIST clearly did fully consider the role of the transfer trusses and very clearly described what that role was ultimately found to be in their collapse models, but it's just not the highly significant role you expected.

NIST does not give a play-by-play of the entire failure of the transfer truss system across the building, but that's because it wasn't material to their analysis given the foregoing. Could they have provided more narrative? Sure, and, if you want to criticize what NIST actually did on the merits, then feel free, but do so on a factual basis and don't use this forum to spread the false notion that NIST did not thoroughly analyze the transfer trusses and their role in the collapse. You were wrong in how you characterized the NIST report in this thread and you are wrong to not reevaluate your thinking now that you know what the NIST report actually says.

 

[deirdre]

Moderation Note: deirdre

Ok gents. everyone has had their say, we don't need to hear the same thing over and over again.

 

[benthamitemetric]

...

... The analyses of the video (both the estimation of the instant the roofline began to descend and the calculated velocity and acceleration of a point on the roofline) revealed three distinct stages characterizing the 5.4 seconds of collapse:

• Stage 1 (0 to 1.75 seconds): acceleration less than that of gravity (i.e., slower than free fall).
• Stage 2 (1.75 to 4.0 seconds): gravitational acceleration (free fall)
• Stage 3 (4.0 to 5.4 seconds): decreased acceleration, again less than that of gravity

This analysis showed that the 40 percent longer descent time—compared to the 3.9 second free fall time—was due primarily to Stage 1, which corresponded to the buckling of the exterior columns in the lower stories of the north face. During Stage 2, the north face descended essentially in free fall, indicating negligible support from the structure below. This is consistent with the structural analysis model, which showed the exterior columns buckling and losing their capacity to support the loads from the structure above. In Stage 3, the acceleration decreased as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below.

Content from External Source

Their explanation is the buckling of the exterior columns (which happened after the collapse and buckling of the interior columns). Since a buckled column supplies only a tiny fraction of its designed resistance to vertical load, the result is very close to freefall.

AE911 Truth must be aware of this, so it's rather misleading on their part to suggest otherwise. It's fine to disagree with a complex analysis, but not to claim it does not exist.

To expand on Mick's post, I think the follow page excerpted from the NIST report is helpful:



NIST's global model is a finite element model and not a perfect representation of reality. NIST never pretends otherwise and painstakingly describes the model's assumptions and limitations throughout its report. Moreover, all finite element models are simplifications of reality and the cumulative simplifications across a model with millions of elements will lead it to diverge from reality. On top of the calculation simplifications inherent in any finite element model, in the case of WTC7, there was a whole lot of information that was unknown and unknowable about the internal condition of WTC7 (e.g., the exact load distributions of furniture, the exact fire damage, the exact damage from the other collapses, the even exact pre-damage state of the building, etc.) that would also lead to additional path-dependent calculation errors. Additionally, at the time the NIST report was authored, the global model took weeks of compute time to run output a complete sequence, which of course limited how much it could be tweaked and refined. (It is also worth pointing out that neither we nor NIST can see the vast majority of the building for the vast majority of the collapse and so we cannot truly assess the accuracy NIST model with respect to the predicted behavior of most of the exterior of the building.)

Given all of that, not only does the NIST model seem to reasonably accurately predict the observed collapse sequence, but it also seems to provide a very powerful explanation for the "free fall" period in particular, among other things. As the above excerpt shows, the NIST model predicted that the external columns of the building between floors 7 and 14 (just enough distance to allow for approximately 100 feet of rapid acceleration as observed in the the "free fall period") would buckle in rapid succession and lead to the "free fall" period for the northern face. Maybe the NIST model is wrong and some other chain of events did occur, but AE911Truth is clearly wrong here as NIST did offer a very clear and powerful explanation for the "free fall period".

 

[Tomi]

The remarkable thing I see in the two images above is that the North facing facade is basically flat. And a few from the North would be remarkably like the view of the actual collapse at the start. Pity they did not display that view since this is where nearly all the photos were taken from. If Nist had a northern view and had shown the solidity of the facade, then that would have been less confusing to those who are deliberately or accidentally misinterpreting the images.

I agree the collapse does not look unreasonable although I do find the large out of plane movements of the East facade at level 30 quite surprisingly large. I am not sure if there were any pictures of that part of the wall. And of course there would be many ways to model the non-linear behaviour of the diaphragm that would result in smaller in plane movements.

 

[benthamitemetric]

The remarkable thing I see in the two images above is that the North facing facade is basically flat. And a few from the North would be remarkably like the view of the actual collapse at the start. Pity they did not display that view since this is where nearly all the photos were taken from. If Nist had a northern view and had shown the solidity of the facade, then that would have been less confusing to those who are deliberately or accidentally misinterpreting the images.

I agree the collapse does not look unreasonable although I do find the large out of plane movements of the East facade at level 30 quite surprisingly large. I am not sure if there were any pictures of that part of the wall. And of course there would be many ways to model the non-linear behaviour of the diaphragm that would result in smaller in plane movements.

In the real collapse, the north east corner of the building did actually detach while still within the limited area visible from known camera angles:


What happened along the east face itself, however, is unknown as there is only one camera angles that shows that face for any time during the collapse and the face itself was very quickly obscured by dust and smoke during the collapse.

Here's the video:


There was a rather involved discussion about the accuracy of the NIST model versus reality in a related thread on Metabunk here, so I'm hesitant to rehash the specifics much more here lest the threads become redundant.

 

[SR1419]

I have looked, but have yet to find, another example of an office fire (even far more extensive) causing a steel structure to come down so quickly -- and, at least initially, so uniformly. If anyone has a link, or another example, I'd love to see it