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jike (Structural) (OP)
11 Apr 05 18:17
Is there a strong direction and a weak direction of plywood? In other words, is it important to lay the 8 foot direction across the purlins as opposed to the 4 foot direction?

Is there a strong direction and a weak direction of OSB?

Helpful Member!  SperlingPE (Structural)
11 Apr 05 18:43
In regards to vertical load, yes there is.  An APA stamped sheet of plywood or OSB will indicate the strong direction.
whyun (Structural)
11 Apr 05 19:46
For blocked diaphragms, there is no difference as long as appropriate panel edge nailing is provided for various plywood layout (see 1997 UBC Table 23-II-H).

For unblocked diaphragms, there is a reduction in lateral capacity.  Also see same table.

IBC must have similar table with diaphragm shear capacities.
Helpful Member!  SlideRuleEra (Structural)
11 Apr 05 20:03
Most plywood has an odd number of layers. This way the face grain, on both sides, runs in the same direction (the 8 ft dimension for a standard size sheet). Since the direction of the grain in the layers alternate, there is always one more layer running in the 8 ft. dimension than in the 4 ft. dimension. When you position the plywood with the face grain perpendicular to the the purlins more layers are "working in your favor". In addition to the "extra" layer, the geometry of the position of the (8 ft.) layers tends to increase properties such as the effective moment of inertia and section modulus.

There is more to it than this, but the bottom line is you are correct, having the plywood face grain (8 ft length) cross the purlin is significantly stronger (say 50% or so stronger) than the other orientation. Plywood thickness, wood species, number of layers, etc. all effect the exact improvement.

jike (Structural) (OP)
11 Apr 05 20:25
Where would I be able to find ratings for the vertical load capacity in each direction or is it rated only in one direction?

What about OSB? Is it stronger in one direction or is it the same strength in each direction? What about stiffness? Is it the same in each direction?
SlideRuleEra (Structural)
11 Apr 05 21:11
jike - I have a book titled "Wood Engineering" by German Gurfinkel (1973), but it is almost certainly out of print (and out of date). However if you go to the APA website, you should find what you need. Here is a link for plywood

and another link for OSB

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