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Steel Exposed to Fire

TRAK.Structural

TRAK.Structural

Structural
Dec 27, 2023
321
Looking for folks with experience evaluating steel structures that have been exposed to fire/heat. I've read some reference material/articles about general evaluation processes but its a bit of a mixed bag. Some say more or less that if it hasn't deformed than its generally ok. Others say that coupons should be taken for testing. Is there more recent info out there that is generally accepted that offers an evaluation procedure?

Additionally, when looking for deformation, is the standard of care just a visual inspection or are we talking about Lidar scanning to detect miniscule variations?

For reference, I've been asked if I want to evaluate a local auto body shop that had a recent fire. Its a couple bay PEMB structure so nothing super crazy.
 
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We do this a lot. You end up paying less attention to the steel and more attention to everything else. You can get a good spatial idea of how hot the fire burned by examining all involved materials. Cable jacketing melts at one temperature; concrete can turn pink at another. There have been times we used LiDAR, yes. It just depends on what you’re seeing out there and how critical the member is in terms of redundancy. Most recently, we were setting up to scan after a major fire in a trash warehouse when the thing collapsed. (We wanted to monitor ongoing movement.) Haven’t had to take coupons yet. The steel is usually spaghetti by the time I get out there, so who cares what the material properties are at that point.

I’ve read something or other about how a fire can accelerate corrosion, but I defer to the corrosion experts for the veracity of that claim. I’ve only ever seen coating damage.
 
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Spagetti beams look so neat... If the structure has not deformed, it is likely OK. Take some coupons and do some tests to confirm.
 
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ANE91 said:
I unfortunately can't post pics [shadessad]
Click to expand...
I remember as a kid (before teens), seeing the burned warehouse across the street from my grandparents place. The beams were draped over the columns and some of the columns had collapsed. I don't recall what happened to the OPSJs. I was amazed at the sight.
 
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ANE91 said:
I unfortunately can't post pics [shadessad]
Click to expand...
So I don't work with structual steel.
But did research on A36 steel
It is received as hot rolled. With required
U and y tensile properties. High temperatures will have an effect on surface decarburization.

So questions, how deep is decarb?
What effect does the higher temperature have on tensile properties?
Is it resistant enough for external forces causing moment.
I am not a structual engineer. But if I was I were i want to know the above data to run the numbers.

Sent a sample to a Matlab and have them provide answers to thise questions.
Does the steel meet the original specification.
Will the reduction in hardness due to decarburation affect tensile strength.
Provide actual hard data.

Unknown factor temperature of the fire.
 
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Actually I can share 1
 

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ANE91 said:
We do this a lot. You end up paying less attention to the steel and more attention to everything else. You can get a good spatial idea of how hot the fire burned by examining all involved materials. Cable jacketing melts at one temperature; concrete can turn pink at another. There have been times we used LiDAR, yes. It just depends on what you’re seeing out there and how critical the member is in terms of redundancy. Most recently, we were setting up to scan after a major fire in a trash warehouse when the thing collapsed. (We wanted to monitor ongoing movement.) Haven’t had to take coupons yet. The steel is usually spaghetti by the time I get out there, so who cares what the material properties are at that point.

I’ve read something or other about how a fire can accelerate corrosion, but I defer to the corrosion experts for the veracity of that claim. I’ve only ever seen coating damage.
Click to expand...

Twisted spaghetti beams would be a dead giveaway, I don't expect that in my particular case but I haven't seen any photos yet.

What about bolted connections? Take a few bolts for testing?

Is there something in between Lidar and visual to get measurements? Maybe a building survey to check for straightness/levelness?
 
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TRAK.Structural said:
Maybe a building survey to check for straightness/levelness?
Click to expand...
I’m skeptical that a survey could aid in parsing minor fire-related deformation from original erection misalignment, thermal movement, and foundation movement beyond what you could discern visually to begin with. Fire-related movement will generally be obvious and spatially correlated with heat and load.

If Column A is 0.1 degree out of plumb, and no burn debris indicates temperatures capable of plastically deforming the column, then you cannot attribute that to the fire. It’s possible that localized, temporary steel softening resulted in additional deflections from in-situ loads, but so what? Part I is establishing whether the structure “changed” or not because of the fire, and Part II is determining whether the structure can continue as-is or needs to be modified/strengthened.

I like the 1998 paper by Tide. It talks about how tests are generally not useful and goes into specifics.

I’d love to see a metallurgist chime in on this. We used to have one…
 
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I'll lastly point out that we as engineers have a tendency to rely on calcs and tests to even our own detriment at times. It's no coincidence that fresh engineers say things like, "but the model told me that the building would do this..." Let the building speak for itself.
 
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