Fire Resistance Rating of a proposed design

We are proposing to install a "fire barrier" wall that we believe will qualify for at least a 2-hour fire resistance rating.  The existing building has a partial wall in place which has 5" steel poles supporting the structure above.

It is essentially two of a 2" x 4" wood stud, 24 in. centers, wall, set back-to-back, so there are 4 layers of 5/8" Type X gypsum in total.  Studs are staggered 12", and drywall installed vertically so that all seams are supported.  The 5" poles are integrated, and the boundary sealed with fire-rated caulk.  And mineral wool insulation will fill one side.  See the drawing.

I would be interested in comments on whether this passes muster, or if not, why?  We have compared it to a design that is rated for 2 hours. UL Design U360 is posted on this database:  http://database.ul.com/cgi-bin...sistanceWizard.html#

We believe our design would have higher resistance because: 1) it incorporates 4 layers of gypsum (vs. 3), 2) uses mineral wool insulation which is essentially not combustible (vs. sprayed cellulose), and 3) those poles, if properly sealed are more resistant than 2 hours.

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Fire resistance in a wall assembly is a two-way system. Only the membrane facing the fire exposure and its supporting structure are effective in providing fire resistance--the membrane on the opposite side has very little to no affect on the overall fire resistance of an assembly. Therefore, you only have 2 layers of gypsum board, plus the framing, providing fire resistance. With a double framed wall, the insulation only helps the side in which it is installed; thus, you have a nonsymmetrical wall assembly and the overall wall fire-resistance rating is based on the weaker (i.e., noninsulated) side per IBC Section 703.2.1.

However, I think a similar assembly will work based on GA-600 Assembly File No. WP 3820, which is a generic system (thus allowed by the IBC). It will be about an inch wider, though, and the studs will need to be at 16 inches o.c. There is no need for insulation, but it can be added for sound transmission quality purposes as indicated by the assembly.

Thank you for your expertise, time and suggestions.

I have seen the WP 3820 generic system you referenced.  If only the facing surface helps, I don't know why the 1" gap in the middle is relevant.  It would seem the gap could be replaced with anything, or eliminated.

Does pretty much any solution with double 5/8" on the exterior qualify as 2hr, but not much else?  I don't understand how U360 rates as 2hr.  Can you explain that? See the additional attachment.  

Thanks again.

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Based on your comments, I would like to ask for your feedback on an updated version of my original design.  See the attached diagram.  The gypsum in the center has been moved to the outside layer, and Instead of leaving a 1" inch air gap in the middle, it has an 1/8" Hardboard Tempered Panel (something cheap) there.  Something to help constrain bats of insulation.  And to divide the two sections of the wall.

Is this likely to be approved for a 2hr fire resistance rating?

thanks!

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Only if you can prove it via testing. Otherwise, use a system that is prescriptively approved by the code (UL, GA-600, IBC Table 721.1(2), or the calculated method). However, the calculated method for wood frame construction is limited to 1-hour assemblies. Two layers of 5/8-inch Type X gypsum board will generally provide the 2-hour fire resistance.

UL Assembly U360 would probably work if the steel columns did not interrupt the center membrane--they are great thermal conductors, which is not great when it comes to fire resistance.

If width is a problem, then GA-600 File Numbers WP4135 or WP 4136, which are nearly identical, can be used with studs at 16 inches o.c. The studs can be increased in depth (2x6s or 2x8s) per General Explanatory Note #18 in GA-600.

Thank you again for the expert feedback.

The irony of this situation is that the building code doesn't require a 2-hour rating between occupancies like F-1 and B (our situation). It is NFPA.   If NFPA is the source of the requirement, then does the limit of 1-hour calculated assemblies apply?

Also, in 703.3 "Alternative methods for determining fire resistance", it offers the approach of "Engineering analysis based on a comparison of building element, component or assemblies designs having fire-resistance ratings as determined by the test procedures..."

Doesn't this mean that I'm not restricted to only using published designs?  Can't I intelligently extrapolate using components of the tested designs?

 

Hi, we have a meeting with our local building official on Monday AM.  We have decided to propose a variant on the WP 3820 that you directed us to.  Instead of the 1" gap in the generic design, we are substituting a 3/8" layer of gypsum and no gap.  Keeping the mineral insulation in place will be impractical without it.

Thanks again for your help.

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John O., regarding your 9/23/16 10:50 PM post, it depends on who's enforcing the NFPA code or standard (which I assume is the NFPA 101, since you don't mention a specific code or standard).

If the building department enforces the IBC (or an adopted version of it), then they'll review the project under that code and only under that code. If they also adopted the NFPA code/standard, then they'll review the project using both codes. If the former situation is true, then they probably will not care if the wall is 2-hour rated or not; however, if it is marked as 2-hour rated, then they will likely apply IBC criteria in evaluating the fire-resistant performance of the wall. If the latter situation is true (both codes adopted), then it could go either way, but I tend to believe that the building department will use IBC criteria in evaluating the fire-resistant performance.

If the former situation exists (IBC only enforced), then another entity must be requiring the use of the NFPA code/standard. That entity may evaluate the fire-resistant performance based on NFPA's criteria, which appears to be a little more flexible than the IBC.

Regarding IBC Section 703.3, it uses the words "engineering analysis," which implies a review of the system by a qualified engineer. You could probably get a fire protection engineer or a materials engineer to provide such judgments (many manufacturers of fire-resistant products, such as firestopping, provide this service, but I'm not sure if gypsum board companies do).

I'd like to hear what the outcome is of your meeting.

Hi, 

We had a "preliminary review" chat with the building official yesterday.  We did not get into the details of what we are proposing, but got a read on how our proposal would be evaluated.  My observations:

- They intend to apply IBC critieria only, as you suggested they would.  The official seems to be unaware of the differences in NFPA 101 regarding separation ratings for specific occupancies.  I don't think he cares.

- He will evaluate our proposed design in comparison to other approved reference designs.  I got the impression that he was primarily thinking what I might find on the UL listing of wall assemblies.  We looked broader than that for a good comparison.

- We still intend to submit the design that is most recently listed above.  We reference the designs posted by Georgia-Pacific in their Gypsum Assemblies Library.  http://www.buildgp.com/GypsumA...es.aspx?ShowAll=True  We believe that our design falls within the range of the 3 wood stud, 2-hour rated designs they list.  We also reference Harmathy‚Äôs 10 Rules of Fire Resistance Rating.  Specifically to make the point that adding layers to the wall can only help (Rule #1) and that because the embedded steel pole is at least 1/2" behind the Type X gypsum, it will not meaningfully affect the wall performance (Rule #5).  We added the 3/8" gypsum in the middle to give some way to contain the mineral insulation... otherwise it seems like a mess.

It might be presumptive of me, but as a registered engineer, I am assuming that I'm qualified to make these judgments, and don't plan to ask for a 3rd-party review.  That's our story.  

We expect to submit today, and should find out in the next two weeks whether the BO is on board.

Thanks for your help!

Hi, 

Sorry for the delay in the postmortem.  I'm pleased to say that the design we proposed was accepted for a 2-hour rating.  I'm attaching the explanation of the design logic that we submitted with the architectural drawings.

In a nutshell, we argued that the only meaningful commonality among all 2-hour rated walls is a double layer of 5/8" type X drywall.  All other variants in wall configuration don't seem to be highly relevant.

Thanks again.

 

 

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