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Re: doubled plywood sheathing on one side of shear wall to increase capacity?

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Scott,

The two-layers-on-one-side plywood shear wall assembly needs to be tested in
order to know its strength.  My expectation is that it will not provide
twice the strength of a single-layer plywood shear wall, and, unless
carefully detailed, has the potential of causing undetected [and
undetectable] damage to the framing due to excessive nails.

The nails that secure the second layer of plywood to the framing must pass
through the first layer; they will thus be in bending through the first
layer.  Nails in bending lose much of their effectiveness.  On the other
hand, shear capacity in the first layer adequate to transfer to the framing
the shear assigned to both layers would allow the nails in the second layer
to count  for only the second layer. That requires three times the number of
nails as for a single-layer shear wall.  My practice in transferring shear
through multiple layers [I don't do if for plywood shear walls] is to
provide capacity across each shear plane for the shear that is crossing that
shear plane, and, if a set of connectors is provided for one shear plane, it
may not be also counted on to provide shear capacity in the adjacent shear
plane.

I visualize it this way: imagine bolting a deck of cards together with a
single bolt, and then applying opposing shear forces to the outer cards.
Every card slips a little; the outer cards slip a lot in relation to one
another.  In order to be effective, there needs to be a separate shear
connection across each card-contact surface -- 51 total.  I then suppose
that 51 bolts through the deck would be as effective as 51 separate two-card
connections.

Is my approach overkill?  I'd want to see the results of a testing program
before doing otherwise.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net



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