IBC and Gypsum Association Fire-Rated Assemblies and Wall Stud Compressive Design Value Reduction

[SoleSteel]

Many architectural assemblies we see refer to the Gypsum Association (G.A.) Fire Resistance Design Manual. Early on in this manual, there is a note regarding reduction of compressive strength values. It is similar to footnote "m" in the IBC fire-rated assemblies table in Chapter 7. Basically, if the fire-rated assembly is from the G.A. manual or is in the IBC table and has this footnote, then the F'c parallel value has to be reduced by a factor of 0.78 when designing wall studs. This is straightforward, but the part about slenderness ratio makes me nervous. (The excerpt from the G.A. Manual 2009 is below in quotes). If I have a 2x6 wall stud and it's 9' tall, then I have to assume that it's more than 15' tall (for l/d = 33 minimum) in order to calculate a Cp factor and then adjust my Fc parallel to obtain my allowable design value. For a 2x4 wall, I would have to assume 9'6" tall studs, which is not as bad, but it still needs to be checked for most of the platform-framed walls I design.

I'm getting ridiculous stud spacing for 2x6 corridor and demising walls on a 3-story building with #2 Hem Fir studs [(2) 2x6 @ 12" o.c. not working] and it's all being controlled by the low compression parallel to grain design values when checking the wall stud stability. Do others here agree with how I interpret these limitations and how have you handled this? I'm thinking that I need to have a conversation with each architect about which assemblies they should avoid, but that feels like stepping on the client's toes to me and I'd rather not, if possible.

"Unless indicated otherwise, all load-bearing wood
stud systems were tested while being subjected to the
maximum load allowed by design under nationally
recognized design criteria at the time of the test. Due to
an increase in the maximum allowable loading in the
National Design Specifications (1982 and later editions),
the American Forest and Paper Association issued the
following statement:
Where a load-bearing fire rated wood stud wall
assembly contained in this Manual is specifically
designed for structural capacity, the design value in
compression parallel to grain adjusted for slenderness
ratio (Fc') used in such analysis shall be taken as 78
percent of the maximum Fc' value determined in
accordance with normal design practice but shall not
exceed 78 percent of the Fc' value for such member."

 

[Celt83]

SoleSteel this is a good find think the important distinction at least on the IBC end is that these assemblies are prescriptive. Most if not all the Architects we work with specify the wall assemblies using the UL number which based on excerpts from UL BXUV Guide and ASTM D6513 results in capacities as near to the NDS design value as is practical.

BXUV.GuideInfo:

3. Wood Stud Wall Assemblies

Walls of combustible construction should be fireblocked as required by the building code to prevent the free passage of flames and hot gases.

The hourly fire ratings for load-bearing wood stud walls tested before January 1, 2009, were derived with a superimposed load applied to the wall assembly intended to theoretically develop maximum working stresses not exceeding the design values published in the Supplement to the 1991 Edition of the "National Design Specification" for wood when horizontally braced at mid-height. When horizontal bracing is referenced in the design it is mandatory, unless otherwise stated.

For fire-resistive designs based upon data generated after December 31, 2008, the superimposed load applied to the wall assembly was derived from ASTM D6513, "Standard Practice for Calculating the Superimposed Load on Wood-frame Walls for Standard Fire-Resistance Tests," and includes a reference to the edition of the "National Design Specification" used to calculate the design load, the design method, the limiting design factor, and the percentage of the design load applied to the test sample.

Wood stud walls may contain fire-retardant-treated studs as well as untreated wood studs. The use of fire-retardant-treated plywood (wood structural panels) may be used in designs that contain use of untreated plywood when all other specified attributes are equivalent to the wood structural panel in the design.

ASTM D6513:

4.1 Test Methods E119 and E1529, and other standard fire resistance test methods specify that throughout exposures to fire and the hose stream, a constant superimposed axial load be applied to a load-bearing test specimen to simulate a maximum load condition. They specify that this superimposed load shall be as nearly as practicable the maximum load allowed by design under nationally recognized structural design criteria. For this practice, the nationally recognized structural design criteria is the National Design Specification for Wood Construction (NDS).

Open Source Structural Applications:

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

 

[SoleSteel]

Celt, the fact that they are prescriptive is actually the only way I was able to justify not using the strength reductions in my project as the architect had picked a G.A. assembly and then modified it with deeper studs so it's not an assembly coming out of the G.A. manual and the strength reduction doesn't technically apply. Still, that ignores the intent of the strength reductions and I'd like to understand that better.

Some of the architects we work with also refer to UL assemblies, but we have many that refer to the IBC and the G.A. Manual. From what I understand, the UL has removed verbage relating to strength reductions. I think it's a bit strange that I ask the architect to change their assembly references in order to use the full capacity of the stud. It doesn't make sense and I could imagine a few of them being mopey about it.

 

[Celt83]

Digging into the quote from the GA a little deeper the requirement actually came as a statement issued by the American Forest and Paper Association in response to a design specification change that produced increase capacities and assemblies at the time where tested to the older capacities, seems this was a blanket coverage approach essentially knocking the stud design back down to the old standard to avoid retesting the wall assemblies:

Unless indicated otherwise, all load-bearing wood
stud systems were tested while being subjected to the
maximum load allowed by design under nationally
recognized design criteria at the time of the test. Due to
an increase in the maximum allowable loading in the
National Design Specifications (1982 and later editions),
the American Forest and Paper Association issued the
following statement:

Where a load-bearing fire rated wood stud wall
assembly contained in this Manual is specifically
designed for structural capacity, the design value in
compression parallel to grain adjusted for slenderness
ratio (Fc') used in such analysis shall be taken as 78
percent of the maximum Fc' value determined in
accordance with normal design practice but shall not
exceed 78 percent of the Fc' value for such member
having a slenderness ratio (le/d) of 33.

American Wood Council (AWC) - DCA 3 dives into the assemblies a little further and specifically notes which assemblies meet 100% of the design loads, interestingly only the 2x4 wall assembly with 1/2" fiberboard on the exterior takes the 78% reduction.
DCA 3

The manual for engineered wood construction chapter M16 - provides some additional wall assembly information and again only shows the 2x4 wall assembly with 1/2" fiberboard on the exterior takes the 78% reduction.

The "Calculating the Superimposed Load on Wood-Frame Walls" document provided by AWC notes their testing is per ASTM E119 which permits testing at less than the maximum load but the reports must indicate the test were done under restricted loads. UL references ASTM D6513 which also references ASTM E119.

So far nothing I have found indicates why the limit exists but my best guess would be the performance of the sheathing during the flame and soak testing - the fiberboard + covering maybe some local failure in one or both layers, IBC 2x4 @ 24 with type x gyp - 24" may be too far for the wet gyp board to provide adequate bracing as at 16" o.c. this assembly achieves 100% design load WS4-1.1 in DCA or it may just be that these assemblies haven't been retested using the new NDS stud capacities yet.

That's a long winded way of saying we all need to be mindful of what assemblies are being specified and if possible gently steer the ship away from the reduced capacity assemblies as early as is practical.





Open Source Structural Applications:

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