Buildings collapse into their own footprint

[Thermal]

...This is not to diminish the knowledge and expertise of professions such as physicist or software developer or construction manager or whatever. But absent additional relevant training and experience, those professions are not positions from which one can speak condescendingly about one's allegedly superior understanding of engineering. Intuition does not help here, and therefore it's better to stay in one's lane.

This is the thing. From working in construction, I understand firsthand some of the structural issues involved in a collapse, but I need to defer to experts when the technicalities go beyond my narrow wheelhouse. While I can see truss failure allowing the 'pancaking' of floors very easily, the core quickly going down was something that I had to be walked through. It would seem reasonable that the core would stand longer, till the increasing mass of dropping floors would overwhelm it. And the abrupt failure of WTC7 seemed truly bizzare, but after renditions were shown of the infrastructure and how it got compromised, it made sense and was consistent with my understanding of the Jenga Model of ◊◊◊◊ going south; structures can be designed with redundancy that keeps them standing even when crippled, right up until there just ain't enough solid to keep it vertical.

Eta: I also get the 'footprint' suspicions. It seems reasonable that one element would fail on one side/corner first, and the structure would naturally list towards the failing side. And that's the beauty of the pros getting involved- they don't rely on what 'seems right', but break out the design till they determine what forces act where and when, and can explain what actually takes place

 

[JayUtah]

Which is why medicine is really just a branch of engineering.

Among physicists—whether properly educated or self-credentialled—there are a few who exhibit the "God complex" found in so many other professions including engineering and medicine. Because classical physics forms the basis of so many specialized professions and occupations, there is the notion that having learned "the king of sciences" qualifies one to practice in the various occupations that rely upon it. When I taught engineering and computer science classes at our university, my saying (or rather, advice to students) was, "Don't work on your own cars."

From working in construction, I understand firsthand some of the structural issues involved in a collapse...

This cannot be overstated. The engineering of a structure necessarily includes the engineering of building the structure. When I said above that the response of a structure to various loads is largely a matter of geometry, this necessarily includes the temporary geometries that support the structure while it is being composed. The simplest example of this is the need to support both sides of an arch until its keystone is placed. Until the structure is complete and includes all the designed-in redundant load paths, the structure is often vulnerable. Construction engineering can point to some unfortunate collapses of partially-completed structures. Understanding the vulnerability of structures during construction and how to accommodate and mitigate them is an important part of understanding how highly efficient structures can fail.

A lay person can note that a highly efficient steel structure falls at roughly the same rate as one that has been prepared for controlled demolition using explosives. That observation is within the ken of any reasonably intelligent person. But an engineer can tell you why it appears to do that even when no controlled demolition was used. But also anyone who has had to follow construction procedures for any reasonably complex structure can provide similar rigor behind the explanation.

While I can see truss failure allowing the 'pancaking' of floors very easily, the core quickly going down was something that I had to be walked through.

But then once you were walked through it, you can see the same elements of classical mechanics at work there as in any other building. The gravity load was meant to be shared between the core and the perimeter. As the perimeter failed, the gravity load transfered and core became overloaded and failed for fairly unremarkable reasons. The Twin Towers relied upon a structural system composed of many elements. That system offered fewer redundancies than you find in a typical 25-foot-bay PEMB of the kind that dots my urban area. It is more costly and difficult to build, but it's the only way you can have a 1,300-foot tower with the skills available at the time.

Structures can be designed with redundancy that keeps them standing even when crippled, right up until there just ain't enough solid to keep it vertical.

High-rise structures must generally be high-efficiency structures in order to minimize the dead load, which eventually becomes the limiting parameter in the design. High-efficiency structures must often also be low-redundancy structures too. More of the structural strength is found in the geometry of the design rather than in its materials. We ask more of the materials by not allowing them to be stressed in their weakest ways. And we do this by arranging them in clever geometries.

Your standard 25-foot-bay PEMB relies on a fairly redundant system in which colums are braced redundantly in any given plane. But that redundancy increases the dead load and severely limits the height of buildings built this way. The tradeoff is cost. You can almost literally just order the prefabricated steel members off a web site and assemble it using simple machinery and plentiful workers. In a system such as used in the Twin Towers, when enough of the system has been compromised by the loss of key members (i.e., floor trusses), the unbraced structure simply falls victim to Euler. In the case of WTC 7, the loss of redundancy in having to accommodate the power station led to a similar (albeit lesser) vulnerability. For all of that you have to write a pretty expensive check to a structural engineering firm because these are unique and/or one-off designs. With PEMB, the structural engineering has been done. The steel comes out of the catalog and the assembly manual comes with it.

You can make a PEMB just as structurally efficient as the WTC system by removing redundant members. That's what happens in a controlled demolition before the "blasters" go in. The structure is analyzed to identify what subset of it constitutes the minimum self-supporting structure. All the live load and as much of the dead load as possible is removed. The resulting structure—shorn of its redundancy—is now vulnerable to failure by quickly and carefully failing the last remaining elements in a sequence using explosives to cut and kick away pieces of the remaining columns.

Failure is rapid not just because you've weakened the structure down to its bare minimum, but also because steel compositions just aren't as robust at that scale as you would intuitively guess. Not all structural elements can sustain the strain rates imposed by global collapse. The really do crumple as fast as the empty soda can under the school student. But whether you've rendered the structure precariously efficient via a contrived lightweight structural system or by removing redundancy from a heavier system, the result in both cases is the rapid failure of the structure when it finally yields.

 

[ahhell]

I largely agree with Jay but I'll add couple of notes. Based on memory not recent research mind you.

Part of the tower collapse mechanism was the connections between the beams and columns where not designed for tension. The collapse was initiated when the trussed sagged imparting tension on the connections. The first floor failed due to that then the impact from that sudden failure resulted in the floor below failing and so on and so forth.

WTC 7 is often the building of last result for the truthers. They base that mostly on what it looked like from the front. A mostly intact building with some fire suddenly collapsing. They never show the footage from the other side where you can see a large chunk of structural damage due to debris from the towers.

On PEMB, they also get a lot of there price savings because the folks that design and build them only do that. They can engineer to within a nat's ass of weight because they designed and built hundreds of very similar and sometimes identical buildings. I hate those guys. Get them outside of that little comfort zone and they give up. They won't even design the anchors of foundations and sometimes designs column connections that are unworkable. Also, I don't care how close your anchors are, four anchors is not actually pinned. Sure, its not exactly fixed but do what everyone else does and envelope that ◊◊◊◊.

 

[Oystein]

Everything has been said already, but not yet by everyone. So may i pile on:

Unless you are a physicist or an engineer, your opinions may lack the necessary foundation.

Keep that principle in mind - we will use it later!

1) No steel-framed building has ever collapsed at near free-fall speed

Whoops! And here you betray that you are not a physicist!
There IS no such thing as "free-fall speed". There is "free-fall acceleration". No physicist would EVER use one of these terms in place of the other, because it is plain WRONG, and cringe-worthily, too!

So keep in mind: You, Óðinn, are NOT A PHYSICIST! (And you have insufficient understanding of the bare minimum of high school physics. You truly suck at physics, period.)

3) The steel structure at each floor is designed to support the weight of all floors above it. Therefore, a steel-framed building cannot "pancake" unless the steel frame is compromised in some manner.

Whoops! And here you betray that you are not an engineer, either!
You see, the "pancaking" that you speak of here is a failure mechanism where floor slabs detach from their vertical supports as they get overloaded.
Did you know that before you composed your long post? If you didn't, you cannot be an engineer - an engineer would have known and understood that.
Now the thing is: Each floor slab was not - I repeat: NOT!!! - "designed to support the weight of all floors above it". Instead, each floor was designed to ONLY support its OWN weight - plus some margin or factor of safety.

It turns out that under theoretical, ideal circumstances, the little plates welded to vertical supports (perimeter columns; core beams) that held the floor trusses up, which were the steel structure that in turn held the floor slabs up, could supoort at most the weight of six floor slabs - and in practice, less than that.
But when the collapses began, it was 12 and more floor slabs, PLUS the roof and the walls and the cores, that impacted the floors below.

Now, did you read, and do you remember, that each floor was designed to hold up only itself, with a factor of safety of no more than six?

12 is greater than 6.

Engineers understand this.

You understood NONE of that when you wrote your post. Which reveals that YOU ARE NOT AN ENGINEER!!!

----------------------------

Let us summarize what we have so far:
A) You are not a physicist
B) You are not an engineer

Óðinn, let me ask you straight-out:
Are you a physicist - yes or no?
Are you an engineer - yes or no?

-----------------------------

Ok, let's move on:

[long tracts of snipped text that pretends to present arguments based on physics and engineerig]

Here is the problem with your entire post:

A) You are not a physicist
B) You are not an engineer

And as you yourself said it right in the beginning of the very same post:

Unless you are a physicist or an engineer, your opinions may lack the necessary foundation. Here is what we know:

Here is what we know indeed:

A) You are not a physicist
B) You are not an engineer

Therefore, your opinions may lack the necessary foundation.

Óðinn, let me ask you straight-out: Are you prepared to retract your ENTIRE post, because your opinions lack the necessary foundation, given that you are neither a physicist nor an engineer?

 

[Thermal]

You know, all you cocky guys with all your 'reality' and 'engineering' think you've got all the answers, don't you? What about The Plain Unvarnished Twoof? You don't need experts and professionals when a regular guy can see what it looks like.

 

[ahhell]

You know, all you cocky guys with all your 'reality' and 'engineering' think you've got all the answers, don't you? What about The Plain Unvarnished Twoof? You don't need experts and professionals when a regular guy can see what it looks like.

You know, I totally get why folks look at the WTC failures and think, that looks like a controlled demo. But when there are relatively easy to understand reasons for why it looks that way and folks persist in saying, "controlled demo..." This conversation is all for the truther adjacent really. The true believers will never give it up but he folks that have only recently been exposed to it, there's a chance I think.

Of course the real problem with the controlled demo isn't that there are straight forward engineering explanations for what it didn't happen, its the vast conspiracy needed to make it happen in the first place. This isn't a half dozen close associates of the president breaking into a hotel. This would require hundreds of folks directly involved and thousands indirectly involved. Either the explosive had been there for decades or there was a small army of demolition experts combing the buildings in the year or so leading up to 9/11.

There is a version of the CT that is possible, Bush knew it and did nothing is possible. Improbable but possible. The nefarious forces were planning this for years and did it in the most complicated way possible instead of just having planes fly into the buildings? Why? Even if the buildings weren't destroyed that would have been enough to start the wars. Similar to the WWII FDR knew it was coming. Could have and they could have ordered the fleet out of pearl harbor before the attack and gotten the same results without hamstringing the pacific fleet.

 

[kookbreaker]

I also can point out that even before the NIST report came out building engineers knew exactly what the fault was with the tower's designs. Steel just isn't that good in fire. I've heard plenty an architect point out that big timbers of wood that old warehouses are constructed from are more stable in a fire* despite being flammable.

The Comcast tower in Philadelphia has always been my goto on this. I don't know what the initial design was like but there were a couple of revisions made between the announcement and the start of construction. And the final design included the building having a concrete core to prevent failure like the WTC. Construction on the Comcast tower started in March of 2005, and NIST released its report in October of that same year.

There was a price of course: A skyscraper with a concrete core needs a lot of concrete. There were actually shortages in Philadelphia as a result. Many utility and Street Department projects faced huge delays because they had to wait for concrete.




* Of course you cannot build very tall buildings with timbers.

 

[ahhell]

I'm dubious about timber and fire, I'm not alone, its mostly limited to 5* stories in the US mostly due to fire concerns. You might notice a 5 over 2 building in your town. 5 stories of wood over 2 of concrete. That's a style of construction driven entirely by building code limits and economics. Wood is cheap but limited to 5 stories.

*in the US, this is changing as there have been some buildings permitted recently under special considerations.

 

[JayUtah]

I mentioned we had a major fire last week in our tavern district that claimed some well-beloved restaurant-bar businesses in historic old buildings. I have to point out that the heavy roof timbers in one of the locations survived even though the interior was completely gutted. Heavy timbers perform better under fire conditions than trusses. But we use trusses for reasons. These were single-story structures. The roof timbers only had to hold up the roof.

Part of the tower collapse mechanism was the connections between the beams and columns where not designed for tension. The collapse was initiated when the trussed sagged imparting tension on the connections.

Yes. As the floor trusses sagged under the combination of gravity and thermal loading, they pulled inward on the truss seats and pulled the perimeter columns out of plumb. The rest is Euler.

WTC 7 is often the building of last result for the truthers. They base that mostly on what it looked like from the front. A mostly intact building with some fire suddenly collapsing. They never show the footage from the other side where you can see a large chunk of structural damage due to debris from the towers.

That debris damage argues against the proposition that the towers fell into their footprint. They largely did, but importantly did not fall exactly into their footprint.

On PEMB, they also get a lot of there price savings because the folks that design and build them only do that. They can engineer to within a nat's ass of weight because they designed and built hundreds of very similar and sometimes identical buildings. I hate those guys. Get them outside of that little comfort zone and they give up. They won't even design the anchors of foundations and sometimes designs column connections that are unworkable. Also, I don't care how close your anchors are, four anchors is not actually pinned. Sure, its not exactly fixed but do what everyone else does and envelope that ◊◊◊◊.

Indeed, it's commodity engineering and construction. It's not a good representation of the art, and therefore not a good yardstick against which to measure innovative one-of-a-kind engineering such as in the towers.

As useless as those guys are outside their niche, we need the niche. At the aerospace museum where I volunteer, a substantial number of our large airframes have been displayed outside. Now you might think that the Utah arid climate is great for that, except that our part of Utah is temperate most of the year. We finally had to send our B-47 (an airplane I loved dearly) off to be scrapped because it was just straight-up falling apart beyond our ability to restore it. Our desire to move the collection indoors as much as possible necessitated the construction of a third hangar on a budget provided mostly by private donations. We simply had no choice but to get the bog-standard hangar structure that could be put together on the cheap. As of now, only the B-52, the B-1B, the C-130, the KC-135, and the C-124 remain outdoors.

But that's by no means prestige architecture. If you're going to build the world's tallest buildings, you will probably need some exotic engineering. And that means designs that rely more on the systemic nature of the structural system than the robustness of any one component. The safety of the WTC 1 & 2 engineering was based on presumptions of what loads and damage would conceivably arise. When that comes up against engineer-trained attackers who know to bank the airplanes in order to inflict damage across as much vertical extent as possible, no system survives for long.

You know, I totally get why folks look at the WTC failures and think, that looks like a controlled demo. But when there are relatively easy to understand reasons for why it looks that way and folks persist in saying, "controlled demo..." This conversation is all for the truther adjacent really. The true believers will never give it up but he folks that have only recently been exposed to it, there's a chance I think.

This just invokes the common mindset among fringe claimants. They really want to believe that they intuitively got it right when all the "so-called" experts got it wrong. Fringe claims are more about establishing the claimant as some kind of hero genius than about figuring out what really happened.

 

[junkshop]

I think he's gone back into hibernation.

 

[Thermal]

I think he's gone back into hibernation.

Check back in 2035 when he spontaneously remembers how They murdered Diana.

 

[Axxman300]

In conclusion, the evidence presented in the NIST Report indicates that the three WTC buildings collapsed in a manner consistent with controlled demolition, as no steel-framed skyscraper in history has ever collapsed at free-fall speed into its footprint as a result of fire, which is physically implausible. It is important to note that free-fall implies zero resistance. A steel-framed building cannot even experience a pancake collapse, let alone collapse with no resistance. On September 11, we observed three such unprecedented occurrences. While these implications do not necessarily suggest an inside job, they do indicate that the truth has been obscured.

You Sir, are a goose.

 

[bknight]

You Sir, are a goose.

You sir, are too kind.

 

[Oystein]

I'm dubious about timber and fire, ...

My understanding is that timber, while it burns and will eventually be destroyed by fire, will retain a sufficient percentage of design strength longer than a similarly strong steel element. That's because a) It burns mostly only on the outside, with a limited burn rate and b) conducts heat much slower than steel, so it stays cooler inside for longer (also, a wooden beam of same strengh is thicker.
Put conversely: The heat of fire is conducted to the core of a steel element pretty fast - and steel is thus susceptible to yielding within a short amount of time. My structural engineering friend has also explained to me that when a steel structure fails from fire, it is liable to do so with little to no warning, whereas wooden structures somehow broadcast coming failure a bit more ahead of time.

I like to show the effects of a fire in a workshop of a local cooky factury that burned several years ago at a time when they were producing a christmas variety of chocolate cake - photo is my own:



True, this a simple one-story workshop, the steel structure had only to hold up the roof, and obviously no fire proofing.
One could say that "chocolate cake fires melt steel!"
Point is: JayUtah had an anecdote about timbers holding up such a roof even after a fire - steel does not.

 

[ahhell]

To be clear, I'm willing to give a wood a chance. As JayUtah, says, heavy timber performs better than light frame and trusses. Anyone that's lit a campfire can understand why. Still, wood does burn, even the heavy stuff. I suspect that a fire hot enough to weaken steel is hot enough to light up even heavy timber. Of course the intent of the building code in the US is for things to fail slowly*, so maybe heavy timber is ok on that score in a fire.

*Actually, its mostly dustily but that nit picking. The idea is that you can see the beam bending so you know and have time to get out.

 

[Myriad]

That probably explains why steel trees haven't replaced wooden ones in most of the world's forests.

 

[JayUtah]

Point is: JayUtah had an anecdote about timbers holding up such a roof even after a fire - steel does not.

If this link works, it's a walkthrough of one of the bars destroyed by fire a couple of weeks ago. You can see the surviving roof members.

View this content on Instagram

View this content on Instagram

My understanding is that timber, while it burns and will eventually be destroyed by fire, will retain a sufficient percentage of design strength longer than a similarly strong steel element. That's because a) It burns mostly only on the outside, with a limited burn rate and b) conducts heat much slower than steel, so it stays cooler inside for longer (also, a wooden beam of same strengh is thicker.

The latter is really the key principle. It's important to remember that in most cases we're talking about steel in the form of a truss or an I-beam.

Put conversely: The heat of fire is conducted to the core of a steel element pretty fast - and steel is thus susceptible to yielding within a short amount of time.

That's the main factor as far as time is concerned. Metal conducts heat far more readily than wood.

But returning to the key principle, classic wood timbers have a robust cross section. They rely on the sheer bulk of material to carry gravity loads. This thick bulk is primarily why they take so long to burn to the point of failure. There is a substantial reserve strength from the sheer excess of material. This would be most apparent in, say, the previous roofing of Notre-Dame de Paris.

Thinner wooden joists are the next step down. As any carpenter knows, the joist has to be kept from rolling because its strength depends on the width dimension staying in the vertical plane. You can block them or bridge them, as your skill, time, and lumber provides for.

A steel I-beam carries this principle further. The web of the beam must be kept in-plane, but in contrast can be made surprisingly thin. The strength of an I-beam when loaded in the plane of the web is significantly greater than its strength perpendicular to the web. The flanges help keep the beam in-plane. But I-beams are notoriously weak in torsion, which eventually comes into play as standard steel structures warp under thermal load.

Finally the truss must be kept strictly in-plane. The truss minimizes the weight of the structural member while preserving most of its strength by means of thin members, some acting in tension and some acting in compression. A through-truss bridge, such as the one that failed in Baltimore, derives its strength from the portion of the structure above the roadway acting in compression. When a thin member acts in axial compression, it is again subject to Euler's law which limits the strength as a function of the cross section and the square of the unbraced length.

Obviously heat loading applied to truss members acting in compression reduces the critical strength. Any out-of-plane condition will cause those compression members to buckle—more easily since the steel is softened by heat. The practical formulation of Euler's law for design includes essential materials properties such as modulus of elasticity and presumes ambient-temperature conditions. Since the load paths in a truss are minimized, there is no redundant load paths—no marginal strength. And since the thermal loading reduces the essential strength of the material, the effective unbraced length is shortened.

My structural engineering friend has also explained to me that when a steel structure fails from fire, it is liable to do so with little to no warning, whereas wooden structures somehow broadcast coming failure a bit more ahead of time.

As I recall, @sts60 is or was a volunteer firefighter and had much to say about the failure modes of trusses. But yes, just like the empty soda can fails in a small fraction of a second when its composition is compromised, a truss will fail suddenly and completely. You don't get observable sagging or audible cracking as you do in a wooden beam.

I like to show the effects of a fire in a workshop of a local cooky factury that burned several years ago at a time when they were producing a christmas variety of chocolate cake - photo is my own:

In that photo you can see that failure included steel beams being bent out-of-plane.

 

[JayUtah]

That probably explains why steel trees haven't replaced wooden ones in most of the world's forests.

It would take far less effort to rake the forests in that case.

 

[Thermal]

It would take far less effort to rake the forests in that case.

Found the guy who never raked up and bagged steel shavings.

 

[JayUtah]

Found the guy who never raked up and bagged steel shavings.

Haha, believe me, I have. And I have the steel splinters to prove it. There is a standing contest in one of our machine shops for who can get the longest curlicue out of a particular NC turning center.