Capacity of 2-ply gypsum board shear walls

[structuresguy]

Ok, so I need to calculate the capacity of wood framed shear walls clad in 2 plies of 1/2" gypsum board on each side of the wall. These are interior shear walls between condo units. I had specified 5/8" plywood on one side. The architect wants me to consider the capacity of the 4 plies (total) of gyp board. But I can't find anything which really helps me here. All the tables I can find are one ply on one side of the wall.

can anyone point me in the right direction?

Thanks

 

[structuresguy]

Oh ya, the wall is 2x4 (or sometimes maybe 2x6) with studs @ 24" o.c., and assume unblocked construction.

 

[structuresguy]

Well, I found some info in teh NDS wind & seismic supplement, but only for studs at 16" o.c. In my case, the existing wall has studs at 24" o.c.

 

[sforesman]

In the NDS W&S supplement, look at table 4.3B on page 27. The table provides two-ply values for 5/8" gyp. board. It doesn't look like you can get away with 24" o.c. though. If you go this route you would need to double the value since you have this two-ply construction on each side. Pay special attention to the fastening, looks like there is a lot to specify.

 

[BAretired]

Canadian code CSA 086 does not permit an increase in shear resistance for the second ply on one side of a shearwall.

Moreover, it does not permit gypsum wallboard to carry 100% of the applied shear. The percentage permitted depends on the number of storeys, i.e. for a 3 story building, gypsum wallboard shearwalls can carry 80%, 60% and 40% of the shear in the 3rd, 2nd and 1st storey respectively. The remainder must be carried by wood panels.

BA

 

[Cadair]

Neither the 2005 nor the 2008 SDPWS of the NDS has clear rules for additional plys added on the same side of the shearwall. The closest it gets would be a comparison of its 5/8" single ply to 5/8" double ply, and that shows that it is not a doubling effect. Sadly the only value given for a double ply is for the 5/8", and it is for studs 16" o.c. and a blocked wall.

Probably the best you could do to get a value without adding a wood structural panel, changing your stud spacing, and going blocked, would be to use 5/8" on both sides and have your edge nail spacing at 4". This would at least give you a nominal seismic and wind value of 580 plf.

You can always email the American Wood Council, who make the NDS, and explain your particular situation. Their website is

http://www.awc.org

 

[structuresguy]

Thanks guys. Some of the walls are not heavily loaded, maybe 200 plf maximum. Others are higher, in line with 1/2" plywood capacities. I have decided to make them add studs and blocking, then use the 2-ply capacity from the tables. Or use plywood.

 

[Mrennui]

BS 5628-6 has values but your in the US so this probably doesn't help

 

[jenDL]

My only advice is to read the footnotes of the tables very carefully...you can usually find favorable exceptions although it usually comes with large reductions in strength. There are a few cases when you can use gyp. studs at 24"o.c. and unblocked edges (I'm looking at Table 2306.4.5 from 2003 IBC) However depending on where you are in the country, gypsum may not even be allowed at all..per 2003 section 2306.4.5. Also check 2003 section 2305.3.8 about summing shear capacities on same sides, opposite sides, and dissimilar materials. (Sorry, don't have the latest IBC on my desk at present, but these numbers should get you to the general area)