Was Column 79 the Achilles' Heel of WTC7?

[Thomas B]

This something that was suggested by @Jedo a few years ago:

In several of NIST's simulations they could achieve a global collapse of WTC7 by removing just column 79 only on two stories. (Btw, they used this as their minimal controlled demolition scenario for checking window breakage and sound levels of the charges would make for just removing the column 79 on two stories, all incompatible with the observed facts.)

Column 79 was apparently the Achilles heel of the building. Probably columns 80 and 81 were also, but they were not tested afaik.

https://www.metabunk.org/threads/ho...more-visible-exterior-damage.9523/post-219373

I raised the issue again the other day in our discussion of Kostack's simulation, also tying it to the minimal controlled demolition scenario.
https://www.metabunk.org/threads/ka...lation-using-bcb-and-blender.9084/post-269170

Does anyone know whether NIST has said something directly about this? I know Sunder described it as a new kind of progressive collapse, but did NIST say that they had identified a particular point of vulnerability in an otherwise strong building?

I ask because it might give us some sense of how unfortunate (i.e. how unlikely) the collapse of WTC7 was. If the fires had not lingered around column 79 quite as long, and exactly that girder had therefore not walked off, would the building still be standing? Given only uncontrolled fire in the building, what was the developing probability of global collapse after N hours? And was it simply identical to the likelihood of removing lateral support around column 79 over the height of a few floors?

Also, how fortunate (again, how unlikely) was the relatively symmetrical nature of the collapse, i.e., that it fell more or less straight down, rather than into the neighboring buildings?

Does anyone know NIST's view on this?

 

[FatPhil]

One of the columns was going to be the one with the lowest safety factor. One of the kids in class is going to be the shortest. These are just truisms.

 

[Thomas B]

One of the columns was going to be the one with the lowest safety factor. One of the kids in class is going to be the shortest. These are just truisms.

I don't think that's NIST's view, or quite captures the issue I'm raising. According to NIST, "[C79]'s failure under any circumstances" (not just overloading, but by, for example, a "blast event") would have brought down the entire building (also if all the other columns had been intact).

In response to comments from the building community, NIST conducted an additional computer analysis. The goal was to see if the loss of WTC 7's Column 79—the structural component identified as the one whose failure on 9/11 started the progressive collapse—would still have led to a complete loss of the building if fire or damage from the falling debris of the nearby WTC 1 tower were not factors. The investigation team concluded that the column's failure under any circumstance would have initiated the same sequence of events.
Content from External Source

https://www.nist.gov/world-trade-center-investigation/study-faqs/wtc-7-investigation

Moreover, we know that this isn't true of at least some of the other columns, because they were in fact severed by the debris damage from the other collapses.

I don't think it's trivially true that in any building one of them will be "critical" in this sense. Though it is of course true that if we imagine applying a constantly increasing load, like stronger and stronger wind, one column will ultimately fail first. That does not mean that it had the lowest safety factor. It will very much depend on the direction that the loads are applied in.

Or that's at least how I understand it.

 

[Jeffrey Orling]

My sense is that the "failure" of columns 79, 80 and 81 led to "problems" with the load transfer structures on floors 5-7.

Column 79's failure seems to have caused transfer truss 1 to displace the girders which ran east-west at the north side of the core connecting columns 61, 64, 67, 70, & 73 and the 8 MG27 north-south cantilever girders bearing on columns 47 - 54 on the north side of the structure. The failure PROGRESSED from col 79 to TT#1 to girders at 61, 64, 67, 70, & 73 causing all MG27s to failure... and with that the north facade / moment frame as well as the slabs north (and east of the core) and TT#3 which began the collapse of the WPH.
Collapsed debris from 47 stories at the base undermined the the 27 columns of the perimeter and the the moment frame had no support.

Col 79 (80 & 81) were what failed TT#1... and that failed the MG27s and TT#3 and the floor slabs north of the core collapsed along with the moment frame.
The elevator "core" also collapsed brought down by the failure of cols 61, 64, 67, 70, & 73 ... and that took down the floor slabs south and west of the core.

 

[Marc Powell]

Also, how fortunate (again, how unlikely) was the relatively symmetrical nature of the collapse, i.e., that it fell more or less straight down, rather than into the neighboring buildings?

The "symmetry" of WTC 7's collapse was not unlikely at all. It was a result of the building's massive inertia and its relatively small ability to resist dynamic forces. WTC 7 was not a monolith and could not possibly topple over like a tree felled by a lumberjack. This is something that most conspiracy theorists and even a few physics teachers and college professors (David Chandler, Leroy Hulsey, et al.) just cannot seem to grasp. As a civil engineering student, one of the first things I learned in structural design classes is that massive steel frame buildings will collapse essentially into a heap regardless of how or where they may be fatally damaged. This is because, upon the initiation of collapse, inertia tries to keep the building where it is while developing dynamic forces work to move it. Loads almost instantly transfer from failed members to others which also successively become overloaded and fail resulting in the building coming straight down.

A well-documented example of how inertia prevents damaged steel frame buildings from toppling sideways would be the collapse initiation of WTC 2. As can clearly be seen in video of the event, the top block of the building (about 28 stories in height) started to tip in the direction of the failed east exterior wall. Then, as dynamic forces built up, the building sheared in two at the plane crash/fire floor so that the top block completely disconnected from the bottom block, slid sideways over it and fell straight down onto it. The mass centroid of the top block did not, and could not, move laterally any appreciable distance. The building did not have adequate strength to resist the horizontal component of the gravitational forces working to topple the top block. In the same way, the inertia of WTC 7 would resist any dynamic forces and cause it to come straight down with minimal tipping regardless of how asymmetrically it may have been damaged.

I am a civil engineer, not a physicist. Perhaps another Metabunk member can offer a more eloquent explanation of the physics at play that resulted in the vertical symmetry of WTC 7's collapse, but suffice it to say that it was certainly not a fluke or an indication of controlled demolition, as conspiracy theorists would have you believe.

 

[Oystein]

My sense is that the "failure" of columns 79, 80 and 81 led to "problems" with the load transfer structures on floors 5-7.

We know that this is your sense, as you have been telling us your sense in every available thread, repeatedly every time, almost regardless of actual topic, for many many years.

Will it ever stop?
No qualified expert who ever studied the structure in any depth agrees with you, and you have not been able to support your hunch with actual evidence (which would require calculations). It's just your personal "sense".

 

[Jeffrey Orling]

We know that this is your sense, as you have been telling us your sense in every available thread, repeatedly every time, almost regardless of actual topic, for many many years.

Will it ever stop?
No qualified expert who ever studied the structure in any depth agrees with you, and you have not been able to support your hunch with actual evidence (which would require calculations). It's just your personal "sense".

Not true... I think you are thinking I am claiming the collapse originated with the transfer trusses. I believe that they failed after failures on higher floors of cols 79, 80 & 81 caused massive debris to affect the transfers structures on the NE and N side of the building.
In order for the entire building to become "involved" and collapse there needed to be a lateral progression of failures. I believe it was the the transfer structures especially the girders on the north side of the core.

NIST punts by saying that after the failures of col 79..... global collapse ensued.

Explain this please.

+++++

Why should it stop?

 

[Oystein]

... The mass centroid of the top block did not, and could not, move laterally any appreciable distance. The building did not have adequate strength to resist the horizontal component of the gravitational forces working to topple the top block. ...

I am a civil engineer, not a physicist. Perhaps another Metabunk member can offer a more eloquent explanation of the physics at play that resulted in the vertical symmetry of WTC 7's collapse, but suffice it to say that it was certainly not a fluke or an indication of controlled demolition, as conspiracy theorists would have you believe.

The first sentence in the quote above is key.

For a building (or anything) to move laterally some distance from rest within a limited time interval (the time it takes to reach the ground), it would have to experience some minimum, easily calculable acceleration.
Acceleration times Mass = Force.
That minimum force will surely be far larger than the lateral loads such a structure is designed to resist. Such a lateral force towards the direction the building's center of mass would have to come entirely from structural connections as forces that, in sum, are equal in size but opposite in direction are exerted on the structure below - i.e. from bending resistance of columns.

I propose a thought experiment:
Suppose a building is designed to resist laterally some hurricane force wind, but not more.
Suppose you could put the building on ice, or a bed of ball bearings, such that it can move laterally with neglible resistance.
Suppose our time interval, the time it takes for WTC7 to collapse completely to the gound, is 10 seconds
Imagine a hurricane blowing against the building while it glides effortlessly on the ground, for 10 seconds.
Ask yourself: How far would the building move laterally within those 10 seconds?

It appears to me that we are talking, at most, a few meters. Perhaps much less, as such a large building has a lot of mass inertia, relative to "sail" area.

Now that thought experiment is for a building that retains its full lateral load bearing capacity and full sail area the whole time.
But since the building is breaking up, it is losing lateral capacity rapidly, and also it gets shorter, diminishing its potential to topple very much.

So that is why the building, as a whole, must drop almost vertically.

It is at most parts of the structure, like walls detaching from floors, that may topple and reach farther laterally.

---

WTC7 had two belts of strong moment frames, and it appears that much of the walls remained unbroken well into the collapse sequence. As a result, that "skin" was not free to fall outward in all directions. Instead, it twisted and turned counter-clockwise, turning around a pivot probably not far from mass centroid, and as a result, the North-East corner fell towards North (and knocked out Fiterman Hall), while the South-West corner fell towards South.

 

[Oystein]

...According to NIST, "[C79]'s failure under any circumstances" (not just overloading, but by, for example, a "blast event") would have brought down the entire building (also if all the other columns had been intact).

Yes, that is my reading of NIST's conclusions also.

https://www.nist.gov/world-trade-center-investigation/study-faqs/wtc-7-investigation

Moreover, we know that this isn't true of at least some of the other columns, because they were in fact severed by the debris damage from the other collapses.

Hmm that's a bit of a different issue. Surely no one claims that any column would be critical.
NIST claims that total collapse would have been triggered by fire damage regardless of whether or not some columns had been severed or damaged previously by impact.

What we have identified here earlier as possibly a point of contention vs NIST is the prevalence of many structural points of failure preceding the floor collapse around c79 and the subsequent buckling of c79. It's not clear (at least to me) whether NIST has run a dynamic (LS-DYNA) model with damage to c79 only, i.e. without any of the other damage accrued by hours of fire.

I don't think it's trivially true that in any building one of them will be "critical" in this sense.

Indeed, and one would think that actually no column alone should ever be critical, that there should be redundancy to any and all structural elements.

If I remember correctly, the Weidlinger study identified floor failures around column 80 or 81, and subsequent buckling of that column, as the trigger event, suggesting that c80 and/or c81 also were "critical" in the sense that taking out any one of them would have triggered global collapse. This appears to me to be a serious design flaw, although this was not brought up in the court case against Silverstein as developer. Instead (again, IIRC), other design features were identified as culpable by the engineers supporting plaintiffs. So perhaps (likely) my personal feeling is incorrect according to an expertly consideration of what codes would or wouldn't demand concerning "criticality" of columns.

Though it is of course true that if we imagine applying a constantly increasing load, like stronger and stronger wind, one column will ultimately fail first. That does not mean that it had the lowest safety factor. It will very much depend on the direction that the loads are applied in.

Or that's at least how I understand it.

The way it should go under increasing load is not that all is fine ... all is fine ... all is still fine ... and then suddenly a single failure triggers collapse. Rather, when some element fails first, there should be load distribution and the structure settles to a new equilibrium without collapse, then the next weakest element fails, with load distribution to yet a new equilibrium... until eventually too many elements have failed and no more distribution finds a viable equilibrium. It will then not be the first, nor second nor third structural failure that initiates collapse, but rather "some", perhaps random, element.
That's anyway what obviously happened at the twin towers.

 

[Thomas B]

The "symmetry" of WTC 7's collapse was not unlikely at all. It was a result of the building's massive inertia and its relatively small ability to resist dynamic forces. WTC 7 was not a monolith and could not possibly topple over like a tree felled by a lumberjack. This is something that most conspiracy theorists and even a few physics teachers and college professors (David Chandler, Leroy Hulsey, et al.) just cannot seem to grasp. As a civil engineering student, one of the first things I learned in structural design classes is that massive steel frame buildings will collapse essentially into a heap regardless of how or where they may be fatally damaged. This is because, upon the initiation of collapse, inertia tries to keep the building where it is while developing dynamic forces work to move it. Loads almost instantly transfer from failed members to others which also successively become overloaded and fail resulting in the building coming straight down.

@Mendel has moved this issue to another thread. I've answered there.
https://www.metabunk.org/threads/can-skyscrapers-fall-over-on-their-side.12394/post-269333

 

[Thomas B]

This appears to me to be a serious design flaw, although this was not brought up in the court case

I've had the same thought, but I find it hard to believe that any of the buildings failed because of a design flaw that hasn't been publicly acknowledged. If that were the case, it must have been covered up. And, like controlled demolition, the cover up would have needed to silence all of official engineering. It leaves us in the uncomfortable position of rejecting one conspiracy theory only to propose another. Or am I misunderstanding your suggestion?

 

[Mendel]

@Mendel has moved this issue to another thread.

I'm not a moderator, so I couldn't have.

 

[Landru]

I'm not a moderator, so I couldn't have.

I moved it as it was off topic in this thread.

 

[Thomas B]

While looking for something else I found what looks a like a definitve answer to this thread's question. At the end of section 3.4.6, on page 42 of the NIST report on WTC, there's this:

The fourth LS-DYNA analysis showed that, following the removal of Column 79 between Floors 11 and 13, vertical and horizontal progression of failure occurred. This was followed by downward movement at the roofline due to buckling of exterior columns, which led to the collapse of the entire building. This analysis showed that WTC 7 was prone to classic progressive collapse associated with the local failure of Column 79.

The simulation is described on page 39 as follows:

In the fourth simulation, the building experienced no debris or fire-induced damage. A section of Column 79 between Floors 11 and 13 was removed. The purpose of this analysis was to determine the potential for a classic progressive collapse, i.e., disproportionate structural damage from a single failure, regardless of the cause of that failure.

https://nvlpubs.nist.gov/nistpubs/Legacy/NCSTAR/ncstar1a.pdf

So the answer seems to be yes, Column 79 was the Achilles' Heel of WTC7. According to NIST, removing only a two-story section of column 79 (and leaving everything else as is) would have caused "the collapse of the entire building" on any given day. It was "prone" to this kind of collapse.

 

[Jeffrey Orling]

While looking for something else I found what looks a like a definitve answer to this thread's question. At the end of section 3.4.6, on page 42 of the NIST report on WTC, there's this:

The fourth LS-DYNA analysis showed that, following the removal of Column 79 between Floors 11 and 13, vertical and horizontal progression of failure occurred. This was followed by downward movement at the roofline due to buckling of exterior columns, which led to the collapse of the entire building. This analysis showed that WTC 7 was prone to classic progressive collapse associated with the local failure of Column 79.

The simulation is described on page 39 as follows:

In the fourth simulation, the building experienced no debris or fire-induced damage. A section of Column 79 between Floors 11 and 13 was removed. The purpose of this analysis was to determine the potential for a classic progressive collapse, i.e., disproportionate structural damage from a single failure, regardless of the cause of that failure.

https://nvlpubs.nist.gov/nistpubs/Legacy/NCSTAR/ncstar1a.pdf

So the answer seems to be yes, Column 79 was the Achilles' Heel of WTC7. According to NIST, removing only a two-story section of column 79 (and leaving everything else as is) would have caused "the collapse of the entire building" on any given day. It was "prone" to this kind of collapse.

Same would apply to col 80 and 81

 

[Thomas B]

Same would apply to col 80 and 81

I would still really like to see a simplified load path diagram to explain to me how this is possible. I just don't get it.

 

[Jeffrey Orling]

look

 

[Thomas B]

Thanks @Jeffrey Orling, I think I need it simplified a little more and broken down into comic book frames. But I think I'm starting to understand the important difference between moment and shear connections.

 

[econ41]

So the answer seems to be yes, Column 79 was the Achilles' Heel of WTC7. According to NIST, removing only a two-story section of column 79 (and leaving everything else as is) would have caused "the collapse of the entire building" on any given day. It was "prone" to this kind of collapse.

Same would apply to col 80 and 81

Again I suggest caution. You are forming a false conclusion for the same error as I identified in another thread. The issue of levels of vulnerability.

Level #1: Neither Col 79 nor cols 80, 81 nor any combination of them is any form of "Achilles Heel" within the envelope of design parameters for "normal" conditions.

Level #2: We have no proof either way but it is probable that the same would apply in the situation of a"normal" fire - the fire scenario designed for. i.e. one, possibly two, points of origin, fire sprinklers operational and active fire fighting commenced within the fire rating time frame.

Level #3: We are probably convinced that Cols 79, 80 and 81 were a form of "Achille's Heel" in the gross trauma of the 9/11 event which went way beyond what the building was designed for.

So the conflation of differing scenarios into one oversimplified comment is both wrong and misleading.

And the underlying issue - the need to define the level of trauma and associated vulnerability is of critical importance to arriving at legitimate conclusions.

 

[Thomas B]

We are probably convinced that Cols 79, 80 and 81 were a form of "Achille's Heel" in the gross trauma of the 9/11 event which went way beyond what the building was designed for.

That's not how I read the part of the report I quoted. In NIST's fourth simulation the only "trauma" was the removal of two stories worth of column 79 as far as I can tell. The result was total progressive collapse.

 

[Mendel]

That's not how I read the part of the report I quoted. In NIST's fourth simulation the only "trauma" was the removal of two stories worth of column 79 as far as I can tell. The result was total progressive collapse.

Yes. Removal of a load-bearing column is "gross trauma", and it occurs exceedingly rarely outside of war zones and building demolitions.

Do not remove a load-bearing column from any building and expect it to remain structurally sound.

 

[Thomas B]

Yes. Removal of a load-bearing column is "gross trauma"

I was responding to Econ's suggestion that the gross trauma of the 9/11 event was necessary. That doesn't seem to be the case. As the NIST FAQ puts it,

The investigation team concluded that the column's failure under any circumstance would have initiated the same sequence of events.

https://www.nist.gov/world-trade-center-investigation/study-faqs/wtc-7-investigation

You raise a good point though about how likely such a failure would be "on any given day" (as I put it). The question here is whether the fire around C79 was particularly extreme and perhaps whether the other damage to the building somehow allowed the thermal expanstion and "walk-off" that caused C79 to fail. In that case, we may not be talking about an Achilles' Heel, but just a very unfortunate combination of unlucky events (initial damage, location of fires, lack of sprinklers and water to fight them).

I'll think a bit more about that.

 

[econ41]

That's not how I read the part of the report I quoted. In NIST's fourth simulation the only "trauma" was the removal of two stories worth of column 79 as far as I can tell. The result was total progressive collapse.

I have now on three recent occasions explicitly spelled out the three distinct levels of vulnerability. And you once again ignore my comments.

The discussion between you and I is NOT about what NIST said in its reports or your interpretation of NIST. I am discussing reality. Not confusing reality with what NIST may or may not have said. Not even concerned with whether to not NIST got the explanation correct. And I certainly have zero intention oa accepting NIST as an absolute authority. Not relevant in this case because the reference to NIST is not relevant to the logic of the reasoned argument.

Let me try just once more - I'll come at the argument from a different direction:
(a) Do you agree that WTC7 collapsed? Do you agree that some combination of Columns 79, 80 and 81 was causally critical in the collapse? Do you agree that in that situation the combination of those columns including Col 79 alone was an "Achille's Heel" << AND If you don't agree that I'll refer you to all the recent discussions.
(b) Do you agree that the trauma on 9/11 was "gross" and of a magnitude greater than the building was designed for? IF you need a reminder the four ways that 9/11 exceeded the design envelope were (i) multiple floor initiation of fires; (ii) Sprinklers essentially disabled; (iii) Deliberate choice to not progress active fire fighting AND - (iv) the primary objective had been met - all occupants had escaped.

I may as well explicitly debunk your derailing excursion into "NIST".
(c) IF you have agreed with me on a and b above Do you now see that NIST's artificial scenario with Col 79 trauma essentially agrees with. Possible adds more detail to what I have said many times. The NIST statement adds nothing to the three levels of vulnerability I have attempted to explain and "we" should be either going along with OR proving me wrong.

 

[Thomas B]

The discussion between you and I is NOT about what NIST said in its reports or your interpretation of NIST. I am discussing reality.

I'm not quite sure how to respond. I don't imagine myself (or you, for that matter) to have better access to the "reality" of the WTC7 collapse than NIST. Most of my efforts are concentrated on understanding what the report says.

a) Do you agree that WTC7 collapsed? Do you agree that some combination of Columns 79, 80 and 81 was causally critical in the collapse?

Yes (of course) and yes (because NIST says so).

Do you now see that NIST's artificial scenario with Col 79 trauma essentially agrees with. Possible adds more detail to what I have said many times. The NIST statement adds nothing to the three levels of vulnerability I have attempted to explain and "we" should be either going along with OR proving me wrong.

I don't understand this, sorry. (I think there may be a few typos.) I'm in no position to "prove" you wrong about "reality" since I have no independent evidence bring to the discussion. I have only the NIST report to interpret. My understanding of it is different from yours (you seem to disagree with it on some points that I'm not clear about). And I don't think I have any issues with your "three levels of vulnerabilty". I'm just trying to understand the specific, critical vulnerability that NIST's fourth simulation apparently identifies.

It, in itself, puzzles me.

But if it makes sense to build buildings that have such a vulnerability, then the total collapse of WTC7 makes sense too. So I'm trying to get it to make sense.

 

[econ41]

I'm not quite sure how to respond. I don't imagine myself (or you, for that matter) to have better access to the "reality" of the WTC7 collapse than NIST.

Utter hogwash. The strategic framework of argument I've outlined includes but is not dependent on NIST

Most of my efforts are concentrated on understanding what the report says.

Yes. Your preference for "Published Books" from authorities which leads you to ignore reasoned arguments from qualified professionals who are not "published". We have discussed the issue to stalemate many times.

Yes (of course) and yes (because NIST says so).

Try thinking - I've presented you spoon-fed bits of argument and facts which you choose to ignore.

But if it makes sense to build buildings that have such a vulnerability, then the total collapse of WTC7 makes sense too. So I'm trying to get it to make sense.

I'm not convinced. Remember this must be about the 5th or 6th time we have reached a stalemate. If you are not prepared to respond to what is put to you there is little more that I can do.

 

[Thomas B]

Try thinking

How could I possibly arrive at the conclusion that the collapse of WTC7 began with the failure of column 79 by "thinking" (about anything other than what NIST reports)?

What "arguments and facts" are you talking about?

 

[Thomas B]

The strategic framework of argument I've outlined includes but is not dependent on NIST

What, then, is it dependent on? What is the basis of your framework? What can we know about the collapse of WTC7 without relying on the NIST report? In cases where these sources conflict, with NIST or with each other, how can we decide among them?

I just don't see where you get the authority to outline a "strategic framework of argument". It's certainly not an authority I would feel comfortable invoking in a debate with a truther.

 

[econ41]

What, then, is it dependent on? What is the basis of your framework?

A reasoned argument supported with necessary factual evidence. What other basis is there for rational discussion?

What can we know about the collapse of WTC7 without relying on the NIST report?

We can examine the facts and construct explanations. It is the very simple basis of rational discussion. You (I, we) construct a reasoned argument or explanation. It consists of steps of logic supported by necessary evidential facts. We are discussing what actually happened - not whether the NIST report is correct or not. Once we know what happened we can check to see if NIST "got it right" or even identified issues of fact that we missed. Read NIST first if you prefer BUT build your own logic. Remember you should be arguing for what happened at WTC on 9/11. Not arguing about what NIST said unless you are NOT pressing your own argument and are simply accepting NIST as infallibly omniscient. Acting as an uncritical agent?

In cases where these sources conflict, with NIST or with each other, how can we decide among them?

Pick the one that is correct. If you are not competent ask someone who is.

I just don't see where you get the authority to outline a "strategic framework of argument".

I am presenting explanations in a discussion forum. ANY sensible argument MUST have a logical framework. A "strategic" framework to set the overall plan of the intended explanation. Then such "tactical" subsets as may be needed. It is not an authority per se. It is a mandatory requirement for a successful explanation or valid argumentation.

It's certainly not an authority I would feel comfortable invoking in a debate with a truther.

Do you mean that YOU can't formulate a valid argument to press to a truther? If so - don't attempt to argue.


I have many times met that obligation - presenting reasoned arguments supported by claimed evidence. You persist in ignoring any such presentation. Why don't you try proving me wrong? Show me the faults in my argument. Stop meandering all around the topic when I have given you some clear points to address.

I have given you answers to four (sets of) questions in this post. Do you agree with my answers?

 

[Thomas B]

Why don't you try proving me wrong? Show me the faults in my argument. Stop meandering all around the topic when I have given you some clear points to address.

I feel like we talk past each other a little, and, as I think I've said before, I think you underestimate how much I get out of our exchanges. I have no way of deciding whether you're right or wrong, but some of the things you say send me back to the various publications and I sometimes find things that are clarifying.

I realize it's frustrating for you when I refuse to take your logic over NIST's conclusions. But as a non-expert I have no way of assessing your expertise, whereas I feel I can take NIST's authority more or less for granted. The challenge for me is simply to square NIST's account with my lay-understanding of physics.

I really do appreciate your input. I just can't always keep up. Cheers.

 

[Mendel]

I was responding to Econ's suggestion that the gross trauma of the 9/11 event was necessary. That doesn't seem to be the case.

@econ41 suggested 3 levels, 1) normal conditions, 2) normal fire, 3) gross trauma

You raise a good point though about how likely such a failure would be "on any given day" (as I put it). The question here is whether the fire around C79 was particularly extreme and perhaps whether the other damage to the building somehow allowed the thermal expanstion and "walk-off" that caused C79 to fail. In that case, we may not be talking about an Achilles' Heel, but just a very unfortunate combination of unlucky events (initial damage, location of fires, lack of sprinklers and water to fight them).

Yes.

There's always something so rare and so costly to counteract that you won't want to include protection against that scenario in your design. Take "detonate a truck filled with a fertilizer bomb in front of the building"; the Alfred P. Murrah Federal Building in Oklahoma wasn't designed for that eventuality, and few other structures would be. On the other hand, you don't want an office copier catching fire to be able to bring the building down. So, somewhere between these two extremes is the line for which you design a building. (And it's not even a hard line, because there'a a gray area of "evacuate safely before it collapses".)

Achilles was dunked in the river Styx, which made him invulnerable; unexpectedly, his heel, where his mother held him, remained dry, so Achilles had an unexpected vulnerability that could be exploited with a poison arrow. I'm not really comfortable with using that phrase for obvious vulnerabilities, such as truck bombs, uncontrolled fires raging in the building for hours, or the removal of load-bearing structural elements. If you remove something that's not load-bearing and the building then collapses, that'd be a legitimate Achilles heel, because the building is not expected to be vulnerable to that. But I don't think it applies to WTC7.

 

[Thomas B]

There's always something so rare and so costly to counteract that you won't want to include protection against that scenario in your design. ... On the other hand, you don't want an office copier catching fire to be able to bring the building down.

I think this is what I'm trying to say. Suppose the FDNY shows up to a 50-story office building that was built in the mid-1980s this weekend because an office copier has caught fire. It's the weekend, so there's not a lot of people in the building, and maybe the fire alarms weren't working properly in the room where the copier was (maybe it shouldn't have been there), and lets say the sprinklers failed or were ineffective for some reason (maybe too much paper or dangerous chemicals foolishly stored around the fire hazard).

In any case, they get to the scene a bit later than they would have liked and after a few hours, half a floor is engulfed in flames and it's spreading up to the next floor. The building has been completely evacuated. At some point (five hours into fighting this thing), the fire chief realizes that they're basically in the same situation as WTC7 in the fourth NIST simulation. If the fire happens to be around a critical column and causing the same thermal expansion, there's a risk that the whole building will come down.

Do they withdraw and evacuate all the surrounding blocks until the fire burns itself out? Or do they fight the fire with still "greater urgency", hoping to avoid a major building collapse in downtown Manhattan? I'm leaning to the second option (I'm happy to hear why I'm wrong about that). And I would think that 9/11 did sharpen the fire chief's sensitivity to the possibility of collapse, i.e., gave them a greater sense of urgency. (Maybe I'm wrong about that too. And the FDNY has always seen their work as a race against time to keep skyscrapers from total collapse?)

 

[Mendel]

maybe the fire alarms weren't working properly in the room where the copier was (maybe it shouldn't have been there), and lets say the sprinklers failed or were ineffective for some reason (maybe too much paper or dangerous chemicals foolishly stored around the fire hazard).

That's 3 things aligning, with 2 of your conditions already exceeding my "office copier fire" scenario.

At some point (five hours into fighting this thing), the fire chief realizes that they're basically in the same situation as WTC7 in the fourth NIST simulation.

After 5 hours of effort and the fire still not close to being contained, and the "risk that the whole building will come down" being present?
I expect the decision would be similar. They'd try to keep the fire from spreading to neighboring buildings.

But typically the sprinkler system would prevent the fire from spreading too much before the fire fighters arrive, even in an unoccupied building.

 

[Thomas B]

and the "risk that the whole building will come down" being present?

I'm not positing this as something they know in its own right. I'm saying they might reason from analogy to the WTC7 collapse that they're now in this situation, that this risk is now present. And I'm suggesting that this thought wouldn't have occurred to them under the same conditions pre-9/11.

Indeed, even after 9/11 they might have thought those circumstances were exceptional, special. With the publication of the NIST report, however, the possibility that skyscrapers are "prone" to this kind of thing was established.

This study has identified thermal expansion as a new phenomenon that can cause structural collapse. For the first time we have shown that fire can induce a progressive collapse. (P. 3)

https://www.nist.gov/system/files/documents/2017/05/09/RemarksSunderAug212008briefing.pdf

That's 3 things aligning, with 2 of your conditions already exceeding my "office copier fire" scenario.

None these facts would be available to them at this time either. I'm just saying there are many reasons an office fire might burn for five hours after starting from a single accidental source. That does happen, right?

 

[Jeffrey Orling]

I'm not positing this as something they know in its own right. I'm saying they might reason from analogy to the WTC7 collapse that they're now in this situation, that this risk is now present. And I'm suggesting that this thought wouldn't have occurred to them under the same conditions pre-9/11.

Indeed, even after 9/11 they might have thought those circumstances were exceptional, special. With the publication of the NIST report, however, the possibility that skyscrapers are "prone" to this kind of thing was established.

This study has identified thermal expansion as a new phenomenon that can cause structural collapse. For the first time we have shown that fire can induce a progressive collapse. (P. 3)

https://www.nist.gov/system/files/documents/2017/05/09/RemarksSunderAug212008briefing.pdf


None these facts would be available to them at this time either. I'm just saying there are many reasons an office fire might burn for five hours after starting from a single accidental source. That does happen, right?

The effect of fire/heat on steel is settled science/engineering. I suppose the issue is how much heat ie expansion of steel... what steel is expanding... and does the structure have a design which can redistribute loads.
The collapse of 7wtc was not because col 79 failed... but because the floor slabs around it over multiple floors collapsed and set off a runaway collapse through out the entire building.
The destructive impact of heat was more from expansion of lateral steel, than weakening of columns. So not only did the columns support the lateral steel... but the columns depended on the lateral steel for some of their strength through bracing.

 

[Gamolon]

This study has identified thermal expansion as a new phenomenon that can cause structural collapse. For the first time we have shown that fire can induce a progressive collapse. (P. 3)

https://www.nist.gov/system/files/documents/2017/05/09/RemarksSunderAug212008briefing.pdf

The point you are missing is that the collapse of a structure, whether partial or not, is always a concern for firefighters. The CAUSE is what was new, not the fact that buildings may collapse.

Start reading at 6.5.2 within the document linked below (page 298).

https://www.govinfo.gov/content/pkg...VPUB-C13-9a037c68b30199701298c007e0910dc4.pdf

Start at the bottom of page 300 which shows WHY the firefighters indicated they needed to clear a few blocks around WTC7 because they thought the building was going to collapse. The thought that WTC7 was going to possibly collapse was reached prior to this thermal expansion/new phenomenon.

I'm pretty sure thermal expansion was happening prior to 9/11 and that the visual and auditory clues caused by thermal expansion (even though they may not have known that's what caused them at the time) existed for firefighters to make decisions regarding the probability of a building collapsing.

 

[Thomas B]

The thought that WTC7 was going to possibly collapse was reached prior to this thermal expansion/new phenomenon.

I'm pretty sure thermal expansion was happening prior to 9/11 and that the visual and auditory clues caused by thermal expansion (even though they may not have known that's what caused them at the time) existed for firefighters to make decisions regarding the probability of a building collapsing.

Yes, I agree with this. That's why I'm saying that the explanation for the collapse of WTC7 that NIST finally arrived at (and not just the events they witnessed on 9/11) might cause firefighters to feel greater urgency about reducing the possibility of a thermal-expansion-induced total progressive collapse. They were used to other signs that a floor or section might collapse. Now, even in the absence of such signs, (or in the presence of signs they would normally take as a sign of possible local collapse), they are going to worry about girders "walking off" locally with global consequences. (Like I say, that's what the fourth simulation seems to warn about.)

 

[Gamolon]

Yes, I agree with this. That's why I'm saying that the explanation for the collapse of WTC7 that NIST finally arrived at (and not just the events they witnessed on 9/11) might cause firefighters to feel greater urgency about reducing the possibility of a thermal-expansion-induced total progressive collapse.

I'm still not understanding your "greater urgency" take. Can you explain what you think firefighters should do going forward to show "greater urgency" in their firefighting techniques?

In hindsight, what do you think the firefighters could have done with regards to WTC7 to have shown "greater urgency"?

 

[Mendel]

For the first time we have shown that fire can induce a progressive collapse.

Yes.
So previously, if the East side of the building was burning, the fire fighters might have reconned that the West side staircase would still be a safe path to attack the fire. This is now not a safe assumption for steel frame buildings.

Even if the building collapsed only partially, burning debris could be ejected some distance. And remember, we found no instance of a steel-frame building tipping over.

And honestly, fire fighters are often fine with letting buildings burn out and collapse. Please educate yourself.(I recommend against using TV dramas/"docuseries" for that purpose.)

 

[Gamolon]

Yes, I agree with this. That's why I'm saying that the explanation for the collapse of WTC7 that NIST finally arrived at (and not just the events they witnessed on 9/11) might cause firefighters to feel greater urgency about reducing the possibility of a thermal-expansion-induced total progressive collapse.

How would a firefighter determine an Achilles' Heel of a particular building during a fire?

Would they need to look at building blueprints to locate a column constructed like WTC7's column 79 and connected/surrounding long span floor beams and then focus on that area?

 

[Mendel]

In hindsight, what do you think the firefighters could have done with regards to WTC7 to have shown "greater urgency"?

they should have developed ad-hoc "creative solutions" that would have allowed them to extinguish a burning 40-storey building with no water mains within a few hours; the fact that they didn't is attributed to them being stupid and lazy, I think?

if he can answer this question, he should apply for a job as NYPD fire chief