Effects of fire on the structural capabilities of steel structures

Mick West

Administrator
Staff member
This is not what would have happened to 47 core columns on the 98th floor to cause them to be perceived to fail simultaneously. All three cans already exhibit local buckling as soon as your weight was applied. Who was it that recently was talking about something from "some guy on the Internet". Oh, that was Mick. Nice try though.

At the risk of taking an analogy too far - the cans we not buckling. Very roughly they were all at about 50% of their peak static load (which experimentally is around 100 pounds).
 

Tony Szamboti

Active Member
At the risk of taking an analogy too far - the cans we not buckling. Very roughly they were all at about 50% of their peak static load (which experimentally is around 100 pounds).
Mick, do you understand what local buckling in a column concerns and why in your little experiment it shows the failure of the second two columns was not due to load redistribution from the first column after it failed, but to the original load you put on them?
 
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Mick West

Administrator
Staff member
Mick, do you understand what local buckling in a column concerns and why in your little experiment it shows the failure of the second two columns was not due to load redistribution from the first column after it failed, but to the original load you put on them?

Two cans can easily support a total of 160lb (my weight), if it were directly above them. But here it's not when the third is removed, so the wooden "girder" I am stood on puts excessive compressive load on the center can, which collapses, followed by the remaining can.

I understand there's a complex dynamic redistribution of forces when a support is lost, especially an edge support. If I dented the center can, then the structure would likely survived.

Hmm, sounds like I have found my experiment of the day.
 

Jeffrey Orling

Senior Member
All the columns did not have to buckle... once the load on the remaining ones became asymmetrical... there would be some lateral forces introduced and the column to column connections would shear and the meeting ends slide past one another and drop with no resistance. You recall the at the column ends were unrestrained doncha?

Just sayin'
 

911247

gerrycan
I don't believe that the hat truss would have been done as it was if there was no antenna
You need to speak to a building professional about this. These outrigger trusses are designed to transfer load between the external to the internal columns and are a crucial design feature in the towers. If neither tower had an antenna, there would still be a very real need for these trusses, they are an integral part of the design regardless of antennas. To think that they would not have been required absent an antenna demonstrates that you do not understand how these towers worked in terms of load redistribution.
 

Jeffrey Orling

Senior Member
You need to speak to a building professional about this. These outrigger trusses are designed to transfer load between the external to the internal columns and are a crucial design feature in the towers. If neither tower had an antenna, there would still be a very real need for these trusses, they are an integral part of the design regardless of antennas. To think that they would not have been required absent an antenna demonstrates that you do not understand how these towers worked in terms of load redistribution.

Oh really? Please explain. What loads were the hat trusses redistributing?

I am a building professional and have a license in NYS since 1982 to design buildings. By the way I worked for the architects of the twin towers back in 1970... How about that for a coincidence?
 
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911247

gerrycan
I am a building professional and have a license in NYS since 1982 to design buildings. By the way I worked for the architects of the twin towers back in 1970... How about that for a coincidence?

That makes your assertion that the trusses would not have been required absent an antenna very perplexing. I would again urge you to read the article that I posted for you previously.
 

Mick West

Administrator
Staff member

This is not what would have happened to 47 core columns on the 98th floor to cause them to be perceived to fail simultaneously. All three cans already exhibit local buckling as soon as your weight was applied. Who was it that recently was talking about something from "some guy on the Internet". Oh, that was Mick. Nice try though.

Just did the update. Unfortunately I did not have three identical cans, and did not realize the coke can was slightly taller than the other two, so when I applied the load, the center support failed instantly. However, as the load redistribution was symmetrical, the two cans were able to take the quite sudden adjustment, unlike in the original experiment above.


Interesting how you can see the cans actually deform a little there

I then redistributed my weight to the heel end, and the local collapse followed.
 
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Jeffrey Orling

Senior Member
A good way to think about its function is that it ties the columns together. The inclusion of an outrigger will reduce drift in a tower, it is explained fully here http://www.engr.psu.edu/ae/thesis/portfolios/2005/plh139/finalreport/StructuralDepth_Hat Truss.pdf

This has nothing to do with load redistribution and the twin towers did not have a braced core. Each of the 107 floors acted as means to transfer wind shear loads to the rather stiff, but unbraced core. Yes the hat truss add stiffness at the top of the tower as it linked several of the floors together into a deeper end plate for the square tube. I am not sure that this was needed or did much.

Perhaps you can explain instead of referring to some article which barely applies and is theoretical and related to the design on another building. Nice try.

The main purpose of the twin tower hat truss was to carry and redistribute the antenna loads among many columns and provide proper anchoring and stiffness to its mooring.
 

Jeffrey Orling

Senior Member
That makes your assertion that the trusses would not have been required absent an antenna very perplexing. I would again urge you to read the article that I posted for you previously.

I read it.

Perhaps you can explain drift in the twin towers? and how the hotel design applies? What sort of building professional are you?
 

Jeffrey Orling

Senior Member



Just did the update. Unfortunately I did not have three identical cans, and did not realize the coke can was slightly taller than the other two, so when I applied the load, the center support failed instantly. However, as the load redistribution was symmetrical, the two cans were able to take the quite sudden adjustment, unlike in the original experiment above.


Interesting how you can see the cans actually deform a little there

I then redistributed my weight to the heel end, and the local collapse followed.

better looking footwear! Run out of Coke?
 

911247

gerrycan
The main purpose of the twin tower hat truss was to carry and redistribute the antenna loads among many columns and provide proper anchoring and stiffness to its mooring.

As I said, you need to talk to a building professional about this issue.

This from Dr Mike Byfield, from Southampton’s School of Civil Engineering,
"The severed columns on the impact face of the towers effectively hung like a shirt on a coat hanger (the hat truss). Without this system the towers would have probably collapsed immediately following the impacts. Dr Byfield strongly advocates the installation of hat trusses or similar systems into all new towers in the UK, although the Department from Communities and Local Government does not have mandatory strengthening measures for new towers on its agenda."
https://www.southampton.ac.uk/research/southamptonstories/engscimaths/blastproofing_britain.html

Or this from Kamran Moazami, P.E., M. ASCE and Ahmad Rahimian, Ph. D., P.E., S.E
"The building's movement is not restricted with any form of damping. Extra rigidity has been attained by employing a 'hat truss' at the building's top, which comprises outriggers that connect the perimeter columns to the core." # Discussing the Shard, a tower with no antenna. http://www.structuremag.org/article.aspx?articleID=694

I could list lots of building with nothing at all atop them that have hat or outrigger trusses, but it may be straying too far from the topic of this thread. The architect (your former employer) and at least one of the SEs for the towers disagree with you, in "city in the sky"p133
 

Jeffrey Orling

Senior Member
As I said, you need to talk to a building professional about this issue.

This from Dr Mike Byfield, from Southampton’s School of Civil Engineering,
"The severed columns on the impact face of the towers effectively hung like a shirt on a coat hanger (the hat truss). Without this system the towers would have probably collapsed immediately following the impacts. Dr Byfield strongly advocates the installation of hat trusses or similar systems into all new towers in the UK, although the Department from Communities and Local Government does not have mandatory strengthening measures for new towers on its agenda."
https://www.southampton.ac.uk/research/southamptonstories/engscimaths/blastproofing_britain.html

Or this from Kamran Moazami, P.E., M. ASCE and Ahmad Rahimian, Ph. D., P.E., S.E
"The building's movement is not restricted with any form of damping. Extra rigidity has been attained by employing a 'hat truss' at the building's top, which comprises outriggers that connect the perimeter columns to the core." # Discussing the Shard, a tower with no antenna. http://www.structuremag.org/article.aspx?articleID=694

I could list lots of building with nothing at all atop them that have hat or outrigger trusses, but it may be straying too far from the topic of this thread. The architect (your former employer) and at least one of the SEs for the towers disagree with you, in "city in the sky"p133

I stand by my statements not withstanding the additional benefits of more structure at the top mentioned.. The purpose ofr the hat truss was to support the antenna. PERIOD.
 

911247

gerrycan
I read it.

Perhaps you can explain drift in the twin towers? and how the hotel design applies? What sort of building professional are you?
Is it ok to do that here? I don't want to get off topic, but I would be more than happy to explain it as best I can.
 

Mick West

Administrator
Staff member
Is it ok to do that here? I don't want to get off topic, but I would be more than happy to explain it as best I can.

I'd prefer to keep threads to single topics. Momentary related digressions are sometime fine if quickly curtailed. Threads can be split if needed.
 

Mick West

Administrator
Staff member
And you can use the PM (Conversation) system for personal discussions of no general interest.
 

Redwood

Active Member
Mick, did you just demonstrate how perimeter columns of the Twin Towers were hurled beyond their footprints?




Just did the update. Unfortunately I did not have three identical cans, and did not realize the coke can was slightly taller than the other two, so when I applied the load, the center support failed instantly. However, as the load redistribution was symmetrical, the two cans were able to take the quite sudden adjustment, unlike in the original experiment above.


Interesting how you can see the cans actually deform a little there

I then redistributed my weight to the heel end, and the local collapse followed.
 

xenon

Active Member
Are you familiar with the Sight and Sound Theater Fire, The Delft University of Technology's Architecture Building Fire, the Madrid's Windsor Building fire, the One Meridian Plaza fire, or the Kader Industrial fire ? These fires al have 1 thing in common Steel Framed structures which collapsed due to fire load.

Do you dispute the fact that steel begins to lose its load bearing ability at 300-400 degrees ?

You are calling this a "Collapse"? (click here for larger image)
The Torre Windsor in Madrid



Your list doesn't include the Beijing TV Cultural Center Fire.





Six hundred firefighters arrived on the scene to fight the blaze, which lasted five hours and caused one death and seven injuries...

...The initial images of the blaze suggested that the tower might be nearly destroyed. However Rem Koolhaas said that "they are simply rebuilding it as it was, because there was no structural damage."

Here is that building after the fire:

 

jaydeehess

Senior Member.
The Windsor IS an example that structural steel members can, and have failed due to fire alone.
What seems to be missing in the discussion is that fire damage was the proximate cause of collapse initiation in the towers and WTC 7. It was not the cause of the progression to global collapse. All the pictures of structures suffering fires over a greater percentage of their volume are irrellevent. We have many examples of local failures due to fire damage and it was local, fire floor, failures that initiated collapse. It was building design that allowed progression to global failure.

The Delft University fire is a very good example of this. The fire was halfway or so up the building but when local collapse occurred on the floor(s) on fire at the time, the collapse progressed through all floors below the local collapse. It was this structure's design that disallowed this collapse to progress through the remaining structure.

In the case of the towers when local collapse took place the column sections, by definition, could not be aligned or taking load at that level, collapse indicates they have failed.
Thus when this dynamic load of the falling upper section moves down it cannot impinge greatly on the column tops of the next lower floor and therefore must be on the floor pans. The floor pans are designed to transfer the load expected to be on a single floor through the truss seats, to the columns. They are not designed to transfer the entire gravity load of ten+ storeys of the structure let alone the dynamic load of that mass being dropped on it.
From that point on there is simply not a mechanism in place that would stop this progression.
 
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jaydeehess

Senior Member.
It can be noted that the Delft University collapse was a reinforced concrete structure, normally less prone to fire damage failure. It is however, a very good example of progression of local collapse to a larger collapse.
 

Hitstirrer

Active Member
A relevant experiment I did this morning:

Relevant? Really? You consider such a ludicrously slender model as relevant, and as evidence of anything remotely equivalent to organic office fires affecting integrated and massively laterally braced steel?

I'm afraid that you have just embarrassed yourself Mick.
 

Mick West

Administrator
Staff member
Relevant? Really? You consider such a ludicrously slender model as relevant, and as evidence of anything remotely equivalent to organic office fires affecting integrated and massively laterally braced steel?.

It's relevant in that it's steel losing it's strength under heat and load. The topic here is the "Effects of fire on the structural capabilities of steel structures". A part of that is the general effect that high temperatures have on the structural strength of steel - which is demonstrated here.

I think it goes without saying that this is different in MANY ways to the World Trade Center. I'm bemused that anyone would think I was saying this was "equivalent" in anything other than the limited sense I described.
 

Redwood

Active Member
Relevant? Really? You consider such a ludicrously slender model as relevant, and as evidence of anything remotely equivalent to organic office fires affecting integrated and massively laterally braced steel?

I'm afraid that you have just embarrassed yourself Mick.


Actually, it was the failure of the "lateral bracing", i.e. the sagging of the floor trusses, that caused the collapse! :D
 

Whitebeard

Senior Member.
Another example of an iron and steal structure seriously deformed and compromised by fire.

The Britannia railway bridge over the Menai Straits in north Wales was designed by Robert Stephenson and built 1846-50. In its original form the tracks were carried inside two iron and steel tubes. It looked like this...

...note the flat bottom of the tubes.

During the evening of 23 May 1970 two young local boys were exploring the bridge using a medieval type burning torch. They set the thing on fire. Due to the location of the bridge and the 'horizontal chimney' effect of the iron tubes fighting the fire proved difficult, and the fire took around six hours to burn itself out. After six hours the tubes looked like this...

... and was in danger of collapse. Note the 'sag' of the tubes.

Now in this case the load bearing parts of the structure was the masonry pillars, not the tubes, which were designed to hold just themselves and passing trains, so they didn't fail. (All rail traffic being halted as soon as the fire was detected)
 

Tomi

Member
I think that you are forgetting that the main impact of fire on steel is the effect of thermal expansion and the forces that these can induce. The graph below shows how

The main point is that steel is rarely fully restrained. However you can see how partial restrained can cause local buckling.

upload_2018-5-20_22-40-8.png
 
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