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Re: horiz. diaph.

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Mark, 5-ply plywood construction has higher uniform load capacity (vertical
roof LL and DL) than 4-ply in PANELIZED roof deck construction, with face
grain of plywood parallel to subpurlins.  It is also somewhat more
dimensionally stable since there is more even distribution of stiffness in
the two directions (the linear expansion is the same for both types,
however).  For 15/32 in. panels, 4-ply plywood is minimum number of plies
for Structural I grade.

When the plywood face grain is PERPENDICULAR to supports (conventional
construction, altho 8 ft x 8 ft panelized roof sections also can be built
this way, with blocking at interior joint for blocked diaphragms - a more
costly option, however), the stiffness and uniform load capacity of 3-4-5
ply plywood are all equivalent.

Also, the diaphragm and shear wall values for 4-5 ply (and 3-ply) plywood
are equivalent, regardless of the application.  City of LA currently has a
restriction on shear wall values for 3-ply plywood, limited to 200 lb/ft.
APA's opinion is that this restriction is overly conservative and not
supported by cyclic (reversed) load testing or monotonic testing.

John Rose/APA, Tacoma, WA

Mark D. Baker wrote:

> What is the advantage of 1/2"- 5 ply horizontal diaphragm vs. 1/2"- 4
> ply, both str. 1? Am I correct in saying the allowable diaphragm  shear
> for a given nailing/blocking is the same but the allowable vertical load
> carrying capacity (d+l) is different?
>
> Regards,
> Mark D. Baker
>