Combining Gypsum Board and Wood Structural Panel Diaphragms??

[omoreno1]

I have a rather unique situation. Table 2508.5 note a of the 2006 IBC indicates that gypsum board horizontal diaphragms are not cummulative with wood structural panels. IBC section 2305.3.9 exception allows the summation of wood structural panel and gypsum wall board in shear wall members to be additive (this is for accounted in APA examples). I have an engineer who is indicating to me that IBC provisions under shearwalls are also applicable to roof diphragms in this scenario. I have done extensive research in the building code, APA, AFPA, and wood design textbooks and cannot find a single example combining these two capacities linearly in the case of roof diaphragms. Does anyone have any idea how this may be approached. I realize gypsum is a much stiffer material than plywood and an appropiate analysis would include testing or a FEM model with the relative stiffness of each material and the roof truss assembly. I realize the assembly may have more stiffness, but i cannot see a linear correlation with strength. Can anyone enlighten me on what I am missing here?

 

[msquared48]

I have never heard that gyp is "stiffer" than plywood. If that were true, then the gypboard allowables in the shearwall tables would be greater. That is not the case. They are much less than plywood.

From everything I know, the nails in the gypboard wall will work the gyp in a seismic event, in particular, to the extent that there will be too much play, allowing greater deflection. The gyp will turn to a powder in the area of the nails, reducing the shear value. The gyp shearwall may hold for the first event, but without repair/replacement, will be much weaker in the second event, possibly to the extent of failure.

Previous codes nave not allowed the combo even with shearwalls, except in very specific circumstances, never on a general basis. I have never heard of this applying to horizontal diaphragms. To my knowledge, the codes (UBC or IBC) have never allowed for sheetrock ceilings to function as horizontal diaphragms. However, I have always practiced on the West Coast where the concern for seismic forces has been much greater.

It is possible that in no-risk seismic areas, this is allowed.

Mike McCann
MMC Engineering

 

[KBVT]

I'm assuming in your original post when you refer to gypsum being 'stiffer' than plywood, you are referring to the rigidity of a gypsum shear wall system, or maybe the lack of flexibility is a better term, which is why the R value of such a system is so wall. By allowing small amounts of deflection, the system is susceptible to cracking which will decrease the shear capacity of the system.

I agree with msquared, I believe the IBC shear wall exception which allows the combination of shear capacities (under wind load cases only) is applicable to shear walls only. This is why the exception appears in the shear wall tables specifically, and not the diaphragm tables. I would ask the engineer for a specific example on why he is interpreting the code in that manner.

 

[omoreno1]

Yes by implying "stiffer" I meant lake of flexibility. I apologize for my mis-use of my terminology. Combing gypsum and wood structural panel is allowed in wind loading conditions only and is not allowed in seismic for shearwall assemblies . For a gypsum shearwall the R value would be 2.0, much lower than then 6.0 or 6.5 mentioned in the IBC codes for wood structural panel shearwalls.

Gypsum horizontal diaphragms are allowed in some high seismic regions with considerable reductions of 50% or greater. So as msquared48 mentioned in high seismic regions it would not be very useful.

I agree with both of you on this being applicable to only shearwall assemblies.

The engineer in this case indicates to me that the provision is also applicable to gypsum diaphragms. I find it difficult to understand why this would be the case. I guess I'm looking for a reason to think that this is plausible. I have never tested these assemblies, but I think I have read enough literature to determine that these values cannot be added cummulatively.

 

[msquared48]

If there is an ICBO rated, tested and approved prorietary system that is out there, I would like to hear about it.

Mike McCann
MMC Engineering