Fire-Rated Exterior Wood Wall - Allowable Compressive Stress Reduction?

[abusementpark]

I am designing exterior wood walls on a five-story building (over two stories of concrete podium). The studs are fire-rated to meed code. The studs are 2x6 with two layers of 5/8" gypsum on the inside and 1/2" FRT plywood on the outside. I believe the architect only needs a fire rating from the inside (two hours) and that is provided by the two layers of gyp, but the code requires the studs and the exterior sheathing to be FRT.

I am aware of footnote m in Table 721.1.(2) of IBC 2015, which requires major reduction in allowable compressive stress of the studs in various fire-rated walls. It would basically require me to design the 9' long studs for an unbraced length of over 15' only use 78% of the capacity of that. However, the exact assembly I have isn't listed in this table and this footnote is only applied to stud wall assemblies which establish the ratings from both sides of the wall. There are three assemblies listed for exterior walls rated for fire resistance from the inside only, which don't require an allowable stress reduction as indicated by footnote q (this is closer to what I have). However, these three assemblies are only for 1 hour ratings, so they don't exactly match what I have. So, I really don't know if I am supposed to design the studs for this reduction or not. It's odd to me that such a major design factor would be left to some footnote in IBC and not prominently addressed by NDS.

Can anyone provide some guidance?

 

[Celt83]

Most of the 2x6 exterior wall assemblies were tested to full capacity.

Check American Wood Councils DCA 3:

https://www.awc.org/codes-standards/publications/dca3

For the FRT you need to find the manufacturer specs for the anticipated treatment, LP Flameblock is popular in my area. Most are a pressure treatment and have reduction factors for all the nominal strengths as well as the elastic modulus.

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https://github.com/buddyd16/Structural-Engineering

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[Aesur]

I agree with Celt83, however many times I have taken the 78% capacity and applied the FRT reductions as the architects in my area don't select wall assemblies early enough in the process many times. We are typically at 75% drawings before a system is selected.

From my understanding, the 78% capacity has something to do with how they tested only to 78% load capacity when doing these tests decades ago and no one has funded updating the test to 100% capacity, so it hasn't been done; therefore they limit to 78% capacity.

 

[Celt83]

Aesur:
Yes your understanding is correct the 78% comes from the ASTM testing criteria and it must be clearly noted when an assembly is not tested to 100%. In my experience it's only been the 2x4 assemblies that aren't tested to capacity and usually have good luck steering Architects away from those assemblies.

For FRT just want to stress that the adjust factor don't apply at the end need to included them in every step of the calculations, impacting things like Cp where Emin' and Fc* would include the FRT factors.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[Aesur]

Below are the values I typically use for FRT reduction, this is based upon finding the lowest values from multiple products (note that these may not include all products, it was just the ones I could find at the time the spreadsheet was developed). We use the lowest values we could find because the contractors use different suppliers too frequently.

 

[abusementpark]

Most of the 2x6 exterior wall assemblies were tested to full capacity.

Check American Wood Councils DCA 3:

I've looked at DCA 3, but the only two hour assembly I see tested is the one below, which two layers of 5/8" gypsum on both sides. As described in the OP, I only have the two layers of 5/8" gyp on one side, but it is my understanding that the two hour rating is only needed for interior exposure one side on my project, so am I safe to assume this assembly applies and I don't have to take any reduction other than the fire retardent lumber strength design factors?

 

[Celt83]

Take a peak at some of the UL assemblies believe they follow the same astm testing so if tested to a reduced capacity they should note it.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer