The core remnant: could it have survived?

Abdullah

Active Member
We are familiar with the core remnant of the Twin Towers, which stood in excess of 40 stories several seconds after the main collapse had reached the foundations. I think question of how it collapsed under it's own weight deserves a separate thread.

For reference, here are videos of the core remnants

WTC1

Source: https://youtu.be/7W0-W582fNQ

WTC 2

Source: https://www.youtube.com/watch?v=75X3auwx_kQ


You've implied that after the core columns lost lateral support of the floors, they would fail also, but I don't think you cited anything to back that up. The core columns were laterally braced between themselves.

Here is my preliminary attempt at a response.

First of all, the core columns were mostly 'braced' by floor beams and slabs. These things provide only limited bending resistance compared to the deep spandrel plates of the exterior walls.

With that said, let's demonstrate that the core could collapse without lateral bracing.

Consider a 40 storey, 480ft remnant whose base load is just 10% of normal at 1.8 to 2.1ksi. I don't know the gyradius, but it's probably in the range of 1-2ft. So let's use this handy derivative of the self-buckling equation

Slenderness² × base strain = 7.8373

Slenderness = 361 to 335

So gyradius = 1.3 to 1.4ft

FOOTNOTES
1. Wikipedia gives self buckling equation

Length³ × density × gravity × area = 7.8373 × elastic modulus × area moment of inertia

Length × area = volume

Volume × density = mass

Area moment of inertia = gyradius² × area

Mass × gravity / area = base stress

Base stress) elastic modulus = base strain

Length / gyradius = slenderness

Therefore we get

Skenderness² × base strain = 7.8373

2.The core columns were mostly made of 36 and 42 ksi steel, with a load factor of 50%. Hence the base stress values

But I need more information for a more complete response. Specifically, I need core column specs to calculate gyradius. I also need beam specifications to calculate bending resistance.

Also, it would be better to have criticism from experts like @econ41
 
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Mendel

Senior Member.
First of all, the core columns were mostly 'braced' by floor beams and slabs. These things provide only limited bending resistance compared to the deep spandrel plates of the exterior walls.
Could you elaborate on that?
 

Mick West

Administrator
Staff member
I don't think you can ignore the huge amount of damage that would be done by thousands of tons of steel hitting the general area of the base at 100 mph. It's not like the rest of the structure was gently removed.
 

Henkka

Active Member
Yeah I saw this response in the other thread, but I can't really comment on calculations at all. By the way, a question that is hopefully on-topic enough for this thread... I saw someone make the argument that the core remnant, or the "spire", should not have fallen in the way that it did. They were making the argument that if the spire were to collapse at all, it should have tipped over like a tree, rather than falling rather straight down. What do people think about that? I feel like this thread should have videos of the spires for reference:

WTC 1
Source: https://www.youtube.com/watch?v=EVxSJ2VLktU

WTC 2
Source: https://www.youtube.com/watch?v=75X3auwx_kQ

Source: https://www.youtube.com/watch?v=24FPI-c4Xuc
 

Abdullah

Active Member
I saw someone make the argument that the core remnant, or the "spire", should not have fallen in the way that it did. They were making the argument that if the spire were to collapse at all, it should have tipped over like a tree, rather than falling rather straight down.

Im not sure if the 30 storey spire itself failed at all. It seems to me that ihe core beneath it failed.
 

johnny plectrum

New Member
But I need more information for a more complete response. Specifically, I need core column specs to calculate gyradius. I also need beam specifications to calculate bending resistance.

The core columns specifications are readily available, however, the site J-welds, at 36’ column lengths, that joined the fabricated box-sections together would be what actually dictated the radius of gyration and hence the slenderness of the column.

Similarly they would govern both the tensile and bending resistance of the member.
 

FatPhil

Senior Member.
I saw someone make the argument that the core remnant, or the "spire", should not have fallen in the way that it did. They were making the argument that if the spire were to collapse at all, it should have tipped over like a tree, rather than falling rather straight down. What do people think about that?

To topple like a tree requires the base (including foundations/roots) to be strong enough to counter the sideways component of the force applied to it along the axis of the spire. (To see such a component of force exists, consider a rigid smooth rod on wet ice, as soon as it starts to topple, the bottom, receiving no friction from the ice, will pop out to the opposite side of the toppling (I just made a shitty video of a bic pen on a glass table doing this, but it's not very good, it's probably best that you convince yourself it exists by seeing it with your own eyes).)

To buckle requires the bottom to not be strong enough to resist the forces applied to it - and if there are any nett sideways moments, the quicker you yield (edit: yielding is a sign of not being strong, I want to emphasise that) to those forces, the quicker their moment decreases, as you've moved the pivot point towards the downward load, and any tendency to topple will be reduced.

You can see in the first video that the "spire" made a brief sideways motion as it started falling, some part of it indeed did rotate downwards, but only a small part. Classic signs of buckling and very much not of toppling - which is what we'd expect from a very slender structure. Everything's pretty much textbook here.
 
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Mendel

Senior Member.
To see such a component of force exists, consider a rigid smooth rod on wet ice, as soon as it starts to topple, the bottom, receiving no friction from the ice, will pop out to the opposite side of the toppling
everyone who has ever tried to step out of a small unmoored boat may have experienced this. What you hope to become your own lateral motion can work out to leave your own center of gravity not as much propelled forward as you would want it to, as the feet planted firm on the frictionless boat "pop out to the opposite side". Splash.
 

Abdullah

Active Member
the site J-welds, at 36’ column lengths, that joined the fabricated box-sections together would be what actually dictated the radius of gyration
How do I find their specs?

Edit: they are in the book, but I don't yet know how to interpret them to get strength and elastic modulus
 
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econ41

Senior Member
But I need more information for a more complete response. Specifically, I need core column specs to calculate gyradius. I also need beam specifications to calculate bending resistance.

Also, it would be better to have criticism from experts like @econ41
Thank you for the comment but I don't know that I can help @Abdullah. My own focus on this issue is on how the situation arose that there were spires. Which requires understanding of the full collapse mechanism(s) for both Twin Towers. The "spires" existed, briefly , as a consequence of a massive collapse mechanism. The fact that they were "left behind" is of interest to me. Because it is proof of my own explanations of the collapse mechanisms. Specifically the "established progression|" stage.

I see it as inevitable that such tall spires would collapse. Euler buckling instability is obvious. But @Mick West as already commented on the enveloping chaos of other massive processes.

So why are you studying the buckling collapse details? I comprehend that it is an interesting technical challenge. But I'm sure that the surrounding chaos would dominate the ultimate collapse. Whether or not Euler Buckling was the main factor. @FatPhil has identified some possible factors. So what? The spires were bound to collapse. I doubt we will ever know the exact trigger. So exploring the Euler Buckling aspect may not lead us to better understanding. I could be wrong.
 
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Abdullah

Active Member
It would be nice if we could have many of the "deleted" posts deleted as well. They take up the same space as s normal post.

As I said earlier, I don't know how exactly to translarevtyrvweld dimensions into strength and stiffness.
the site J-welds, at 36’ column lengths, that joined the fabricated box-sections together would be what actually dictated the radius of gyration and hence the
Suggests thatbrgevwelds were significantly weaker. But thebweld dimensions appear to cover most of the column cross section.

This is detail 301, the most common in box columns by far.
Screenshot_2022-08-07-08-30-31-963_com.microsoft.office.word-01.jpeg
And here's column 605, a lightweight box column
Screenshot_2022-08-07-08-46-54-501_com.microsoft.office.word.jpg
 

johnny plectrum

New Member
As I said earlier, I don't know how exactly to translarevtyrvweld dimensions into strength and stiffness.
Suggests thatbrgevwelds were significantly weaker. But thebweld dimensions appear to cover most of the column cross section.
If you use weld type 301H as a nice example, as it's a 1" weld, and look at column 502A between floors 27-30 you'll see that the column is fabricated from 3-1/4" and 3-1/8" plate.
As the column depth at 52" is greater than 28" then the weld is only undertaken along the long side on the 3-1/8" plate.
So, from a bending, tension and radius of gyration perspective your column consists of only 2, 1" welds, 52" long spaced 21" apart.
They're the numbers you can use to determine those geometric parameters.
To determine strength you'd need the steel strength but I haven't spotted an electrode spec for the welds anywhere.
Presumably they'd be lower strength than the A36 steel.

Here's a picture of a failed column.
Note how much weld there was.

WTC column.jpg
 

johnny plectrum

New Member
I thought that number referred to some amount of space that would not be welded. 301A says "Full penetration" surely that doesn't mean a zero inch weld??
301A, ie column 904A level 72 to 75 is made from 11/16 plate. Full penetration weld will be simply the same thickness as the materials joined for those 'lesser' columns.
 

johnny plectrum

New Member
How would I see that?
are you referring to the thin line along the long edge?
Yes. You can see the failed weld along the long edge.
You can see the similar failure on the square box column behind too.

Slenderness for the core columns is dictated by their welded connections - not their plate geometry.
 

Mick West

Administrator
Staff member
Related:
I asked structural engineer Donald Friedman (who worked on the WTC cleanup) about this issue. At 15:10

Source: https://www.youtube.com/watch?v=ezHc9x75808&t=910s



Mick:
I'm gonna make sure I understand like, when you're talking about lateral loads, like, like, say, if you were to remove it, delicately remove the exterior and the floors. And you're just left with the core. Would that core column, be able to, the core of the building, be self-supporting?

Donald Friedman:
It would be self-supporting for gravity, but it would basically be a house of cards, we have very little lateral rigidity. So a wind load much less than the code required wind load in 1968, or now would be enough to destroy that. So yeah, it's what's called a tube structure. It was invented in the 60s, where you have closely spaced columns and beams at the exterior of the building. And that serves as your lateral bracing. John Hancock and Chicago's tube structure. There are any number of them here in New York. So it's a, it was for a while, sort of the in-vogue method of structural design for tall buildings.
Content from External Source
In other words, the later bracing is both the core and the out walls. They need to be connected. I didn't even ask him if the outer walls would be self-supporting because it never really occurred to me that people might think that.

While he's talking about the full height core, I think it would apply even more to the heavily damaged "spire."

In his book, Freidman (a structural engineer who worked at the WTC site from 9/12,) made observations about the welds:

Donald Friedman, one of the privately contracted engineers from LZA Technology (a division of the Thornton-Tomasetti Group) who helped oversee cleanup operations in consultation with the City of New York, provided a few notes on the column connections and how they appeared to have failed in his book After 9-11, An Engineer's Work at the World Trade Center:

upload_2018-1-30_17-53-49.png
... (pg 47)

upload_2018-1-30_17-55-51.png
...(pg 89)



upload_2018-2-5_13-40-8.png

upload_2018-2-5_13-39-46.png


... (pgs 104-105)

I had misgivings about the core columns I was seeing. I was sure the dunnage design would work—Kyle and Chris know their stuff—but I was unhappy that the columns I saw lying on West Street seemed to be in too-good condition. These huge columns—the largest weighed more than one ton per running foot—were almost all straight, with clean edges at both ends. There were some dents here and there, but I expected a piece of steel that had been wrenched out of a building to be bent. I examined the ends of the columns every chance I got. Every welded splice at the column ends I saw had failed the same way: by ripping out of the steel. The plates that had been assembled into boxes for the core columns varied from a couple of inches at the top to five inches at the bottom. The top and bottom ends of each column were flat and had been spliced with a partial-penetration groove weld: the upper column’s four sides were beveled about an inch and a half. When the upper column was erected over the already in-place column below, the bevel and the flat top surface of the lower column formed a lopsided “V” shaped groove, which was then filled with weld. Partial penetration welds are not as strong as full-penetration welds, where the groove is the same depth as the steel is thick, but they are far stronger than is needed for most purposes. Under the extraordinary loads imposed during the collapse, the columns were free to buckle after the the welds ripped off of the flat surface of the groove. Like a lot of the structural damage I saw, this was not a normal phenomenon and it was hard to accept. I spent a lot of time noting such issues and trying not to learn too much from them. It would be easy to stop trusting my knowledge of building design, and weld performance, and steel strength, and so on. I felt that by understanding what had physically happened on September 11, I could contrast it with the ordinary engineering problems I dealt with on my projects.]
Content from External Source
 

Abdullah

Active Member
Could you elaborate on that
Compared to the floor beams, the spandrels were:
1. Shorter
2. Deeper
3. Probably of larger section as well, particularly towards the base of the tower.
4. All had fixed boundary conditions, unlike many of the beams, which were pinned.
 

Abdullah

Active Member
So, from a bending, tension and radius of gyration perspective your column consists of only 2, 1" welds, 52" long spaced 21" apart.
They're the numbers you can use to determine those geometric parameters.
To determine strength you'd need the steel strength but I haven't spotted an electrode spec for the welds anywhere.
I would expect the weld dimensions to be more useful for determining tension and bending strength than stiffness. I would expect the sheer heught of the column section to dominate over the short weld height?
 

Mendel

Senior Member.
Compared to the floor beams, the spandrels were:
1. Shorter
2. Deeper
3. Probably of larger section as well, particularly towards the base of the tower.
4. All had fixed boundary conditions, unlike many of the beams, which were pinned.
but aren't the floor beams/trusses long in the direction that the spandrels are deep?

We're talking about core columns bending/buckling.

The facade columns are connected via the spandrel plates, which provide excellent in-plane stiffness to the facade (allowing it to bridge the airliner-shaped holes), but the facade would bend more easily from orthogonal forces such as wind if not braced by the floor. Would you say the floor connections provide less bracing against wind loads than the spandrels do?

I believe that, on each floor, the core columns were tied together with a belt truss on the outside, and connected to the floor beams, such that lateral motion of a core column relative to the other core columns would only be possible if some floor beams and slabs buckled or broke.

You claim that the capacity of the floor system to resist this lateral motion of core columns was less than the capacity of the spandrel plates to resist the inward lateral motion of facade columns, particularly below a certain floor when the spandrel plates became very strong?

Or do you claim that once the floors had been destroyed both inside and outside the core, only in-plane truss connections between the core columns remained, which were weaker than the spandrel plates?

(apologies if I got stuff wrong, which is likely)
 
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Abdullah

Active Member
but aren't the floor beams/trusses long in the direction that the

No. The spandrels are deep in the vertical direction.

the core columns were tied together with a belt truss

Really? The closest thing to your belt truss is the channel girder whose function is well known to be supporting the inner ends of the trusses.

What I am saying is that the floor cannot provide the out of plane resistance needed to make the core columns act as a rigid frame.
 

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