WTC Towers Core Columns - What Held Them Together? Welds.

Mick West

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20171118-160100-eyp9t.jpg

The core columns of the world trade center were incredibly strong box columns with inches thick walls. But most of their strength was just for support. They were not designed to be resist much lateral forces. It seems they largely came apart where they were joined at the ends.

But what actually held them together?

I think it would be very helpful to get a clear picture of the collapse to have a clear picture of the strength of the connection between column sections. You see a lot of weld marks on the face of the column at the end, like in the above: [UPDATE: Those are actually cut marks, where the lifting tabs were cut off after the columns were lifted into place.]
20171118-160401-o2u41.jpg

Also here:



That looks like where the column failed. But what was actually welded there? A plate? A seat? Are there photos or plans of these connections?
 
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Mick West

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This famous bent column in the memorial museum has them at both ends (same column as in the pic above, just the other way up)
20171118-161413-kg9bv.jpg
 

Mick West

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This shows a transition to a smaller beam, at the top of the building somewhere. The upper beam is welded to the lower, and there the same angled weld mark.
20171118-162131-x7gxi.jpg
 

Mick West

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This looks like two beams end to end with matching angled weld marks. But what is holding them together?
20171118-162508-a66ce.jpg
(FEMAphoto_WTC - 181)
 

Tomi

Member
The columns were designed as bearing connections with the joints about 3ft above floor level . The plates that you see in the middle of the column are lifting lugs and alignment plates. The long plate at the top is a lifting lug and the short plate at the bottom is an alignment plate. They are used to fix the column in the temporary location and are cut off

The final connection between the column is a partial penetration butt weld. Which is essentially like a fillet weld probably about 8 to 12mm. And you can see the indent of the welds where the columns have been ripped apart.

Interestingly they seem to have only run the fillet weld on the two long sides of the column and the short sides are just in bearing.

So if the columns are going to fail they are likely to fail at the joint

I think the construction photo you can just about see the shadow of the lifting lugs. You can see what looks like a white paint line at some of the column positions. This could be part of the Magnetic Particle Inspection that would look for cracks in welds and this would be the time the lifting lugs would be removed. These welds are quite difficult to do in practice because the columns act as a heat sink. However there is little concern in practice about the adequacy of the welds because at the time the columns were only designed to transfer axial compression, nothing else.
 

Mick West

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Staff member
The final connection between the column is a partial penetration butt weld. Which is essentially like a fillet weld probably about 8 to 12mm. And you can see the indent of the welds where the columns have been ripped apart.

Interestingly they seem to have only run the fillet weld on the two long sides of the column and the short sides are just in bearing.
Very interesting. For some reason I had it in my head that the core columns had splice plates, which could potentially be stronger than (or in addition to) a butt weld.

Here's a brightened version of the photo I took at the museum.
20171119-085052-2nqdr.jpg
As you say, the welds are just on the long face.

So it seems this image here does actually show the weld.
20171119-090042-8iq38.jpg

There was one column that shows up in a few photos, roughly in it's original location. You can see the connection between the upper section and a somewhat thicker lower section. You can also see the connection point for floors. These are not symmetrical, with a shorter gap at the top. (Maybe useful in figuring out which way up a beam was)
20171119-090824-s76sy.jpg

What I'm trying to do here is figure out a way of effectively explaining to people why the columns failed. Always tricky, but maybe an analogy could be something like slender blocks glued together. Like if you were to take an 8 foot long 2x4 piece of wood. It's very strong. But if you saw it in two four foot sections and then glue the ends together with a strong glue epoxy along the longer edges, then you can see how it might come apart with far less lateral load.

Then there's the welds themselves. People perhaps think of welded connections being incredibly strong, but we can see that there are lots of columns that seem to have simply snapped at the welds with very little deformation of the columns themselves. Weld material can be brittle. It's probably a bit technical to get into with most people, but the images are useful to show just how little needed to fail.

Also of interest is the welding on two sides only (the long sides), which would suggest that one side would fail under tension, then the other would essentially act briefly like a hinge

20171119-115601-0efk1.jpg

20171119-115801-k6o4t.jpg
 
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Tomi

Member
Your pics show welds on two sides only. However I would not be surprised if they welded all round on some of the smaller sections. So I assume it was a mixture of some side welded, while others welded all round.

Now imagine the force needed to bend one of these column elements. The forces increase as the radius of bend increase. You could work out the force needed to put a permanent bend into the box. But you will see that the weld will fail before the box starts to yield. So the box breaks at the connections.

Now imagine the long spires of steel that stood after the towers fell. These were long core columns, but appeared too long even to support their own weight. And that's because its easier to bend longer elements rather than shorter ones. Its like a tall lego tower. They fell over and just like lego they break at the connections rather than in the middle of an element.

Now the cutters on the recovery team probably knew it was easier to arc gouge out the fillet weld rather than cut through main box. So they probably used that to speed the clean up. And they only cut them mid-length where the columns were hooked on other pieces of debris, which was most of the time.
 

Mick West

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Staff member
Here is a core column being lifted into place. From this video.
20171119-145547-6e4k1.jpg

You can see the two angle brackets at each end. So they were used for initial alignment, then were removed when the columns were fixed in place and welded along the seam.

Some similar alignment flanges here:
20171119-150420-nb54i.jpg

20171120-085700-0x6ce.jpg
 
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Mick West

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20171126-153238-rgf4o.jpg

This shows the similar weld patterns, but appears to be exterior columns. What part of the building is this?
 

Tomi

Member
View attachment 30375

This shows the similar weld patterns, but appears to be exterior columns. What part of the building is this?
These do not look like perimeter columns to me, since they are not welded panels. The external columns were prefabricated into a triple column sections 400mm apart with bolted ends. Not surprisingly they mostly failed at the bolts

However they could be the mega columns at the base of the building and they look square because of the perspective of the photo. They do appear to have cladding around them
 

Jeffrey Orling

Senior Member
View attachment 30171

The core columns of the world trade center were incredibly strong box columns with inches thick walls. But most of their strength was just for support. They were not designed to be resist much lateral forces. It seems they largely came apart where they were joined at the ends.

But what actually held them together?

I think it would be very helpful to get a clear picture of the collapse to have a clear picture of the strength of the connection between column sections. You see a lot of weld marks on the face of the column at the end, like in the above:
View attachment 30172

Also here:



That looks like where the column failed. But what was actually welded there? A plate? A seat? Are there photos or plans of these connections?
First of all not all the core columns were box columns at every level. Columns start out at the top as rolled wide flange sections designed for the roof load... and 2 attached floor loads... live and dead.... then the next 3 story tall column supports the axial load from the column above it plus the attached live and dead loads of 3 floors and so on. Column cross sectional area grows in proportion to the load... but at some load value there are no more rolled sections with large enough cross sectional area... so they columns were fabricated as box sections

The 36' tall 3 story columns were aligned with welded on tabs at the ends... but held laterally by the beams attached to them with knife connections at 4', 16', 28' The tabs were probably tack welded and were not relied on for lateral support, but for positioning in erection.

They were essentially more like jenga block erections... but the beams were welded making the steel into a composite frame.
 

Jeffrey Orling

Senior Member

Mick West

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Staff member
no visual evidence that the columns ends were welded one to the other... other than the alignment tabs.
No? So what is that area along the long edge? It looks like a weld failure to me. There's also images above of core columns still welded together.

The issue of weld failure is interesting, as observing how the welds failed would tell us something about the stress put on the joint. It seems like in most or all of the ends I've looked at there's a void where I'd expect weld material - which suggests the weld "popped out". This seems unlikely. Have I just not seen the ends where you can see weld material?

Here's a cross section of a (non 9/11) failed weld. What I see appears to be like the left side. Why don't we see anything like the right side? Was the penetrations so bad that it failed on both sides? Did the weld material get knocked loose in subsequent impacts?
upload_2017-11-27_8-22-24.png
 

Jeffrey Orling

Senior Member
No? So what is that area along the long edge? It looks like a weld failure to me. There's also images above of core columns still welded together.

The issue of weld failure is interesting, as observing how the welds failed would tell us something about the stress put on the joint. It seems like in most or all of the ends I've looked at there's a void where I'd expect weld material - which suggests the weld "popped out". This seems unlikely. Have I just not seen the ends where you can see weld material?

Here's a cross section of a (non 9/11) failed weld. What I see appears to be like the left side. Why don't we see anything like the right side? Was the penetrations so bad that it failed on both sides? Did the weld material get knocked loose in subsequent impacts?
View attachment 30383
Mick... you need to think logically here. Take a column made of 4 - 5" thick plates... factory assembled to be a box column.

The welds had to be perhaps fillet welds running the full length of the 5" plates. This was probably strong enough to hold the assembly together for shipment and erection. But these welds are flimsy WEAK compared to the 5" thick material they are welding together. The column forces are AXIAL and the bending resistance was likely provided INDEPENDENTLY from each member..

The same logical applies to the end to end attachments. No weld would be strong enough to keep these monsters from acting as a continuous long column. The end to end joint would crack open easy peasy if the upper column had a significant lateral force levering it .These columns were restrained by the beams.... but the unsupported column stack... which only existed during the collapse.. . would with topple like one huge pole or break apart at easy end of the connection.

Also recall that the facade columns were staggered... so there were no weak horizontal lines a except maybe at the mech floors... But those were reinforced.
 

Mick West

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Staff member
Mick... you need to think logically here.
I'm unclear what you are referring to. You said:
no visual evidence that the columns ends were welded one to the other... other than the alignment tabs.
But there is visual evidence of this. In fact there were column sections still welded together along the edge in the debris pile.

And the alignment tabs do not seem to be flush with the ends, so could not have been welded there.

This (presumably lower exterior) column shows a good view:
20171127-091333-ytvnt.jpg
 

Jeffrey Orling

Senior Member
Mick I don't know... they play no role in structurally preventing much of any movement... with a force large enough to displace those columns.
 

Mick West

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Staff member
Mick I don't know... they play no role in structurally preventing much of any movement... with a force large enough to displace those columns.
Right, but that's what I'm trying to determine, so I can explain it to truthers. My goal here is to provide better explanations for the things they think are impossible.
 

Mick West

Administrator
Staff member
Mick, the often forgotten FEMA report on the WTC towers has a reasonably detailed chapter on steel connections that has some pretty good overviews of the connections between the WTC1&2 columns and the basic formulas that can be used to understand their failure conditions. The graphics and explanations in this report may be of some help to you in this thread if you haven't looked at them recently.
Thanks. It does not really cover box-box column welds though, except for some discussion about welds in general.
 

Tomi

Member
Wrong Tomi.... don't you see the aluminum facade cladding????
wtc collapse.jpg


I have highlighted a typical perimeter column behind the mega column. And that typical column has a standard spandrel connection. These columns are 400mm apart. that's the window size . And that means the columns in the foreground are the mega columns from the base of the building.
 

Jeffrey Orling

Senior Member
View attachment 30390


I have highlighted a typical perimeter column behind the mega column. And that typical column has a standard spandrel connection. These columns are 400mm apart. that's the window size . And that means the columns in the foreground are the mega columns from the base of the building.
Tomi.... that sure looks like WOOD
 

benthamitemetric

Senior Member
At the base level, I think it's hard to distinguish the center columns from the perimeter merely by reference to what the prefabricated exterior panels that included columns looked like on higher floors. Here is what the lobby of WTC 2 looked like for reference:



The easily identifiable pre-fabricated pieces weren't used for the first couple of floors.

And from the outside:

upload_2017-11-27_18-17-48.png
 
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Gamolon

New Member
Tomi.... that sure looks like WOOD
I agree with cloudspotter that what is in the red box is one of the perimeter assemblies. I also think that the white "cladding" seen on the larger columns in the foreground of that photo is actually the gypsum planking.
 

Jeffrey Orling

Senior Member
Scale is hard to read... it's a long lens with lots of foreshortening... The 2 columns in the foreground may be the tree columns... They WOULD have fireproofing and cladding... just as the facade panels.
 

Gamolon

New Member
Scale is hard to read... it's a long lens with lots of foreshortening... The 2 columns in the foreground may be the tree columns... They WOULD have fireproofing and cladding... just as the facade panels.
Great point. Also, the distance between them would seem to favor that they ARE the tree columns.
 

Svartbjørn

Senior Member.
Just for clarification (because Im not an engineer, welder or metallurgist)

and 2 attached floor loads... live and dead.
A Dead Load would be the concrete floors/office equipment etc (dead weight, in lay terms Im guessing)
A Live Load would be all of that plus the people?

The welds had to be perhaps fillet welds running the full length of the 5" plates. This was probably strong enough to hold the assembly together for shipment and erection.
Would this be a tack weld in smaller applications?
 

Mick West

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Staff member

Svartbjørn

Senior Member.

Tomi

Member
Great video. It appears to show that the large welds (perhaps full penetration) are on the outside columns. There is a long shot view of the columns

The core columns have smaller welds and are partial penetration. Again there is a wide view of a series of core columns and you can see small gaps.

This perhaps makes sense if the core columns are in compression only and the perimeter core columns are long and have some bending in them because of their slenderness
 

benthamitemetric

Senior Member
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)

 
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