Plywood Shearwalls and Horizontal Diaphragms

[ofthesea]

I'm looking for allowable shear loads on 3/4" or 1 1/8" plywood shearwalls or horizontal diaphragms. I've checked with APA but they don't go over 5/8" thickness. Where can I find such information?

 

[Aton]

ofthesea, I do not know what is found in codes from other countries, but here in Canada, the "Wood Design Manual" (WDM), put out by the Canadian Wood Council [visit

http://www.cwc.ca/

], and its companion workbook,"Introduction to Wood Design", are used. The WDM is our 'timber design code", CSA 086.1-94, Limit States Design, and Clause 9, "Shearwalls & Diaphragms", along with the relevant paragraphs in the lumber and plywood clauses, will help you get started. The WDM contains tables with plywood capacities (bending, tension, compression, various shears) for "Canadian Softwood Plywood", "Douglas Fir Plywood", and OSB. The two plywood tables have incremental thicknesses from 7.5 mm (approx. 5/16 in.) up to 31.5 mm (@1.25&quot. Hope this helps. Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.

 

[dougantholz]

I believe the reason that the APA doesn't have diaphragms for that thick of plywood is because the attatchment to the wood framing below would fail before the plywood.

 

[Headran]

According to an article that I have regarding "Calculation by Principles of Mechanics" it states "The following nails fully develop the strength of the structural I plywood listed. Using larger nails will not result in higher allowable shears" [ul][li]6d common - 5/16"[li]8d common - 3/8"[li]10d common - 15/32"[/ul]I am assuming that using thicker plywood also will not result in higher allowable design loads. Unless you can justify using higher diaphragm values by using thicker plywood, you will have to use the diaphragm values listed for the thinner plywood.

 

[Aton]

Good point, Headran. These values you presented correspond exactly (after metric translation!) to those given in our WDM (Table 9.5.1A) for horizontal diaphragms over D. Fir/Larch framing. However the final entry in this Canadian table is for 5/8" ply (15.5 mm), using 3" nails (10 D, having 3.7 mm dia.). This table does not go up to even 3/4" ply.

Using larger nails than 3" in these 5/8" panels will not result in greater shear resistance, this is true. But is this not due to the fact that it is the various shear resistances within the panel itself that govern the assembly? While the shear resistance of thicker spikes is greater, what good will it do to "walk the nails to the panel" if Vr(max) has already been reached by the panel itself? Incidentally, the WDM also deals with shearwalls and diaphragms using diagonal 19 mm (3/4&quot and 38 mm (1.5&quot boarding - Clause 9.5.1.1 The 19 mm boarding is specified with 2.5" common nails (8-penny), and the 1.5" boarding uses 3.5" (16-penny).

As far as I can see, the reason the shearwall tables do not specifically deal with configurations using thicker panels is that these applications are rare, and testing costs money. While the data is there for shear in an individual plywood sheet, (refer to my first post), it is not yet available to us for shearwall and diaphragm assemblies. Does anyone else have any information on this? Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.

 

[ofthesea]

Thanks,

OK, so in order to use the thicker plywood or OSB panels to resist vertical or horizontal shear I'll need to insure that the nails used meet the proper penetration and diameter requirements.

This is fine but I have two questions:

1. The longer the nail becomes, the bending stress increases exponentially. This has to have an effect on the overall capacity. The failure mechanism changes from possible panel tear or nail rip, to nail bending and pull out. With 99% of all residential floors being 3/4" panels you would thing somebody like the APA would address this through testing.

2. I need a reference to justify these panels to a building department plan checker. Does anyone have some literature stating that thicker panels may be used but with a topped out upper limit based on testing?

 

[boo1]

The American Forest & Paper Ass. / American Wood Council publishes the "Wood Structural Panel Shear Capacities for Shearwall Assemblies" for Structural Sheathing up to 19/32" thk, nails 10d Species G>0.49 340# 6" oc edge nailing, 510# 4" oc, 665# 3" oc, 870# 2" oc, studs 16 oc blocked.


Additionally the building code books present simular shear capacities. For higher values I specify double studs and plywood both sides. Check foundation attachment methods.