IBC and Gypsum Association Fire-Rated Assemblies and Wall Stud Compressive Design Value Reduction
IBC and Gypsum Association Fire-Rated Assemblies and Wall Stud Compressive Design Value Reduction
(OP)
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."
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."






RE: IBC and Gypsum Association Fire-Rated Assemblies and Wall Stud Compressive Design Value Reduction
BXUV.GuideInfo:
ASTM D6513:
Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering
RE: IBC and Gypsum Association Fire-Rated Assemblies and Wall Stud Compressive Design Value Reduction
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.
RE: IBC and Gypsum Association Fire-Rated Assemblies and Wall Stud Compressive Design Value Reduction
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