Fire rating of steel

[ajk1]

We need to strengthen an existing concrete beam by providing a supplementary support steel beam below it packed tight, in a parking garage whose size is such that the structural members require a fire rating. Is there any way to avoid the fireproofing of the steel beam by designing it to have more strength than required?

 

[frv]

Yes, but it's really complicated and usually involves hiring some very specialized engineers who charge a lot of money. And even then it's a toss-up as to whether the building official will accept the analysis.

What's the reason to avoid fireproofing?


If it's aesthetics (for a parking garage, I doubt it), look into intumescent paint. But it is expensive.

 

[dcarr82775]

If space is the problem, I agree with FRV, go with intumescent paint

 

[ajk1]

Thanks frv. Yes I want to use intumescent paint and that is one of the systems that I have suggested. But the project manager wants to encase the beam in cement board and build a curb at the floor to prevent vehicle impact. I have my doubts about cement board being robust enoough to withstand vandalism - like a kid with a hockey stick. The one concern I do have about intumescent paint is whether it will by itself be sufficient to protect against corrosion; or do we have to provide a zinc-rich paint first, and if we do, will the intumescent paint fire rating be affected?

Concrete fireproofing around the beams I suppose would solve all problems, by a) providing corrosion protection, b) robustness against damage and c) fire protection.

Any comment?

 

[dik]

Talk to the paint supplier for information about the intumescent coating...

Dik

 

[DST148]

@ajk1: due to high cost of intumescent paint, it is generally used for architecturally exposed structural steel and rarely for parking garages. If headroom is limited, why not explore the option of providing structural steel on the two sides of the concrete beam?
Given the exterior exposure, both the steel and the intumescent paint would need protection from the environment. Like dik suggested, talk to the paint supplier. For severe exposures, it is better to go in for a weather resisting steel rather than counting on the paint to provide both, the fire and the environmental protection.
If you have the luxury of encasing the new structural steel in concrete, then go for it.

 

[concretemasonry]

Keep in mind that steel fails rapidly from extreme temperatures, while the concrete that it is "protecting" has the thermal mass to resist the high temperatures much longer after the steel has reached a failure point. It is a time dependent protection system that will lose reinforcement early in the crisis.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[shobroco]

I don't think you can use intumecent paint in a parking garage. In my limited experience with it, I seem to recall that high humidity affects it adversely.

 

[Tindy]

Much of the commentary here seems to discuss old standards in intumescent technology. I know that products exist that are not so vulnerable to humidity, they are very thin, can be top coated, are lightweight and have very long latency. All of this is quite different from the products that have been around for a few decades.

The newer technologies are not expensive and can be easily applied using any good primer under them and anything you like as a top coat, so very versatile. Depending on the need, I know of a handful that would fill the bill here, but I don't think I'm supposed to mention names, so just do some homework. Two I can think of are in Indiana, one in in Connecticut and more are coming along. Wait till the new developments in nanotechnology hit. These will deliver products that offered increased performance the longer they're exposed to fire. However, getting back to the here and now, Insurance companies like Grinnell Mutual have been doing research on these new coatings and have made some interesting discoveries.

As for the cement board idea, cement board is an extremely efficient conductor of thermal energy, so that would probably be counterproductive - and an excellent point is made about the fragile nature of cement board from vandals, but also just common impact damage from car doors, etc.

I would recommend finding something newer with more advanced features and treating both the steel and the concrete. Bottom line, there are products out there that solve all of these problems very elegantly and affordably without the problems and hassels that have been accurately described here, but relative to older approaches that are largely outdated.

 

[ajk1]

Thanks DST148 - yes we did have scheme sketched up with steel beams each side of the deeper girder that requires the support but is a little more complex and anyway I believe that we may have enough headroom directly under the girder.

To Tindy - yes I have spoken to a fire protection company and they have intumescent paint and also a sprayed cementitious coating that could be used in parking garages - I will have to double check with them whether the intumescent paint is ok for the garage, but they do know it is an open air garage. Both these sytems can be used with a compatible approved zinc rich paint for corrosion protection. Yes I am very aware that intumescent paint is very costly.

Yes, cement board is not appropriate for a number of reasons cited here and I think that the project manager is finally recognizing that!!

Thanks all for the great comments. Much appreciated.

 

[Tindy]

My pleasure. However, I may have miscommunicated...the new direction in intumescent is NOT expensive - you should be able to find things for less than $4/sq. ft. for materials. Also, depending on the W/D of the steel you have, your coating thickness should be less than 2mm or 70 mils counting the top coat. If not, keep looking.

Be wary of cementatious or spray applied cellulose. The Miami-Dade cultural center, the new convention center in San Juan and other places are experiencing extreme damage to the structural steel post application. The problem is that the damage can be hidden until it has become a very serious problem. Intumescents are now at the point that more obsolete approaches aren't really relevant. It reminds me of when 4' of goose down got replaced by 1/8" of 3M Thinsulate.

 

[ajk1]

Tindy - ok thanks for alerting me to these aspects. I will look into it on Monday. When you say new technology for intumescent paints, in what way does it differ from old technology? Is it a completely different material? Will the supplier know what I am talking about when I refer to it as "new technology"

Andy

 

[Tindy]

Just ask what the price is. Is it in the $4/sq. ft. range? Do they have testing on steel? How thin is it? How much is needed for your steel type (W10x49 for example). Does it need specialized labor or equipment? Can it be top coated? If you get the right answers to those, you have your product. I would just Google "Intumescent" and see what you find. Pick something close to the top of the list not counting the sponsored links at the very top.

 

[ajk1]

Tindy - ok I will try. The thing is that the technical specialist at the supplier to;dme ;last week me he does not know costs, so may take some leg work to find this all out. Also, not all things available in U.S. are necessarily available here in Canada. I think we may have to live with what is available, but I will pursue and see. Thanks again.

 

[DST148]

@Tindy - Thanks for the update on intumscent paint.
@ajk1 - Keep us posted about the solution you finally adopt and also any additional info you may acquire about intumscent paint.

 

[Tindy]

You make an important point - your supplier needs to have ULc S-101. Canada is very persnickety about that - and not just UL, ULc. I think it's just another way to squeeze more money out of manufacturers, but that's off topic for our purposes.

 

[epwh]

I'd go with gypsum wallboard protection. Depending on the M/D and required fire resistance rating, you might only need 1 or 2 layers of 5/8" board. Much cheaper than spray-on cementitious or intumescent materials. If headroom is a problem, increasing the M/D of the beam will reduce the required thickness of GWB - see Appendix D in NBC.

 

[Tindy]

epwh - GWB is a classic, proven solution and a perfect fit in some cases. However, most assemblies are complex and hanging GWB, especially multiple layers, can be complex and expensive. Then there are the black mold issues in some environments as well as the risk of impact damage. This instant seems to be easy and the GWB might give an ideal esthetic result. Another factor is whether or not overhead clearance and/or physical space between members allows for the number of sheet of GWB needed. Also, the prices of the most current intumescent products are lower than you think. A thin film that can be top coated can really open up design options too. Another point, though, is that steel positioned higher than 20' from the deck may not need fireproofing at all - check local codes. Also important to know the W/D or A/P of the steel compared to the rating needed. Heavy, legacy steel might not need fireproofing at all.

 

[ajk1]

How do you feel the gypsum walll board would stand up to a guy with a hockey stick and a few beers?

 

[Tindy]

While I am not a physicist, it technically depends on the size of the guy and how many beers. Assuming he's Canadian, the concept, in general, is fraught with peril.