Fire Rating & Precast Column Corbels 1

[KootK]

I've got a precast column corbel in a parking garage with a required fire rating of 3HRS. And I'm interested in the fire rating requirements for:

1) The beam bearing plates on the corbel below and, more so;

2) The primary corbel rebar welded to the bearing plates which, somewhat by definition, doesn't have a ton of cover.

I assume that there's some "out" for not having to fireproof the bearing plates but I'm unsure about the corbel reinforcing. Do I need to use 2" bearing plates just to get the rebar cover for 3HR? Does that even make sense given that any heat on the exposed bearing plate is likely to transmit that heat straight to the rebar anyhow?

Rationally, I feel that these element do not need to be fire rated the same way that primary members do because the odds of fire getting to and locally taxing these elements seems unlikely. That said, I'd like to see an exclusion in the codes rather than making this stuff up myself. Does anybody know of such an exclusion? Or can anyone confirm that I'm out to lunch in thinking that such an exclusion exists?

 

[TehMightyEngineer]

You're not alone in the concern apparently:

https://www.fireengineering.com/articles/2009/02/construction-concerns-prestressed-concrete.html

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[TehMightyEngineer]

Some annecdotal evidence:

https://carnews.types-cars.com/2018/01/1400-cars-destroyed-in-massive-parking.html

Hard to beat a real-world test like that. Hot enough to melt aluminum rims; that's likely well over 1000 °F. See lots of spalling and thermal cracking but no obvious distress at the corbels or beam connections.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Brad805]

It is cases like this where it would be nice to have better access to Piekko Hangers.

https://www.peikko.com/products/product/pc-beam-shoe/photos-and-videos/

You might want to give Wayne Kaissan a call. He is the technical consultant for many of the CPCI details.

 

[TLHS]

Yeah, I was just thinking out loud to decrease the demand. Maybe with the lower load combination and a shifted bearing point you could justify the lower ties. I trying to buy you more development length and less moment by shifting over.
Probably wouldn't work though.

Everywhere I find a reference to this sort of thing, it seems to be saying that the embedment in concrete helps but you should probably fire proof embedded items anyway. They aren't particularly specific references though. Barring a good reference the other way I'm pretty sure there's enough of a best practices argument that I could shoot down a vendor's argument that fire proofing isn't required if I were an owner's rep.

On the other hand, there's also stuff like this (Link) that has a built up steel corbel that's got a one hour rating and some bearing angles with nelson studs that have two hour fire ratings without additional insulation. If they can test that out, you could probably test out the typical detail you've got and that I agree is all over the place. Knowing that isn't particularly helpful without a reference, though.

 

[GC_Hopi]

Some annecdotal evidence...

That is pretty impressive. However without knowing the exact connection details it's hard to hang your hat it.

If its a plastic failure then maybe the load is redistributed and whatever capacity that is lost is taken by the lower ties. Which is what TLHS was saying but that can only be truly confirmed with testing.

 

[GC_Hopi]

KootK: What manual did you get the dabbed beam detail from?

I emailed this question to PCI and got the following response on the dabbed beam. Thought?

The 4-#5 bars reinforce the bearing. They are encapsulated in concrete and thus protected. The plate only provides anchorage for the bars. The embed plate bears on concrete along its edge for anchorage resistance. This bearing strength is not really reduced at elevated temperatures.

 

[TLHS]

My thought is that the weld is directly fused to the back of the plate and is in shear... so the face of that weld is at the same temperature as the rear of the plate. Perhaps the heat sink of the concrete helps significantly and there's a big thermal delta from the front to the back of the plate, but just saying 'the plate is just in bearing' seems reductive.

Again, I'm not that worried about this location, but I've been wrong about things that I don't think I need to be worried about before.

 

[KootK]

KootK: What manual did you get the dabbed beam detail from?

PCI Manual 7ed.

I emailed this question to PCI and got the following response on the dabbed beam. Thought?

1) Man, thanks for doing that. Truly, above and beyond even if you are just curious for your own purposes.

2) I think that whomever you spoke to either didn't understand the question or is grossly incompetent.

The 4-#5 bars reinforce the bearing.

3) Nonsense. For bearing reinforcement you'd want vertical bars and/or confinement. The 4-#5 are primary tension bars.

They are encapsulated in concrete and thus protected.

4) With the weld to the exposed plate, I fail to see how the bars aren't essentially exposed themselves via thermal conductivity. The heat sink argument holds but that's a different thing.

The embed plate bears on concrete along its edge for anchorage resistance.

5) This isn't just wrong, it's bass-ackwards. The plate edge is pulling away from the concrete, not pushing against it. Plus, as any fool could guess, you wouldn't get squat out of concrete bearing over such a small area even if that was what was happening.

 

[JLNJ]

Can you paint on some intumescent paint or create a crevice for some other type of fire safing?

 

[WARose]

Ok, I ran some heat transfer numbers today. I'm not sure what the verdict here is, but I found (at minimum) you are talking the first 6 to 8 inches of the embedded re-bar (connected to a plate exposed to fire) being subjected to temperatures that greatly reduce yield strength. So I'm not sure what that means with the corbel....but for that beam detail (posted by Kootk on 30 Apr 19 17:19), I'd think that would greatly impact (if not eliminate) the ability to transfer axial load.

It was a steady-state solution (i.e. not considering time). So there might be an argument for that.

 

[TehMightyEngineer]

What kind temperatures are we talking about WAR?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[WARose]

What kind temperatures are we talking about WAR?

I started with 1100 F. The temperatures ranged (in about the first foot of re-bar) from that to about 700 F. It’s around that point (700-800 F) when you start getting reduced yield strength. AISC 9th edition gives reduced yield strength (see p.6-3) as 77% (of Fy) at 800F, 63% at 1000F, and 37% at 1100.

 

[KootK]

Can you paint on some intumescent paint or create a crevice for some other type of fire safing?

Thanks for the suggestions. I could do those things but I'd be running a significant risk of pushing my client into the arms of another engineer. For the corbel, I'm switching anchorage to a welded end bar as mentioned above.

Ok, I ran some heat transfer numbers today.

Thanks for that. Interesting if disconcerting.

In my travels exploring this, I've noticed that much is made of reinforced concrete shear failures seemingly not being an issue for fire protection. Apparently, shear failure under fire doesn't happen in real fires or in the lab. And this is bolstered by the nature of the prescriptive provisions which only ever seem to address cover to main flexural bars.

This is, admittedly, a bit of a logical reach but I wonder if one might make the argument that the fancy stuff going on at these bearing conditions really constitutes shear situations and therefor falls into the category of things not representing serous fire issues. Disturbed region design is often thought of as essentially addressing localized shear. Maybe it's truly the Bernoulli flexural situations that are really deserving of the extra fire protection considerations. Like I said, a bit of a logical reach around.

 

[WARose]

[blue](Kootk)[/blue]

Thanks for that. Interesting if disconcerting.

I think it’s important to qualify/explain my calculations in that I did the following steps:

1. Fully heated a 50 lbs plate to 1100 F.

2. Conducted that heat to 4 #5 bars physically attached to the plate.

3. Figured temperatures based on losses from conduction via the plate to the bars, and then the losses along the bars embedded in concrete. (Concrete is a pretty good insulator....which is something I was talking about before with the wire analogy.) No other losses considered. (But with a constant heat input into the plate, not sure there would be too many others.)

4. It was done as a steady-state problem: the heat input into the plate was constant and time was ignored. (For simplicity.)

So with my model, we are talking a completely exposed plate instantaneously heated to a temperature and then conducting it pronto. In reality, it’s going to take a while to heat that plate up and then conduct the heat. So I may run it again as a transient heat transfer problem.

[blue](Kootk)[/blue]

In my travels exploring this, I've noticed that much is made of reinforced concrete shear failures seemingly not being an issue for fire protection. Apparently, shear failure under fire doesn't happen in real fires or in the lab. And this is bolstered by the nature of the prescriptive provisions which only ever seem to address cover to main flexural bars.

It’s hard to say why some things fail and some don’t. Failure is tough to predict. For shear, we are talking about it acting across a partially compressed zone. So that probably helps. (Thinking out loud.)