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Overdriven nails

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To:  James Allen

Where diaphragm nailing is severely overdriven and the existing weakness
is unacceptable, consider the following options...

1.  Use light gauge sheet metal strips over the adjoining panel joints
with staples or short screws.  APA has done diaphragm tests using the
sheet metal as a blocking alternative to transfer load sheet-to-sheet,
by-passing the existing framing member.  I'm not sure if 3/8" plywood
was included in the tests.  APA has a technical bulletin on this option.

2.  Use staples through the existing plywood into the existing framing
member.  Staple spacing minimums are very small since splitting is
rarely a concern.  In addition, stapled diaphragms have ICBO research
report backup.  Since I'm not sure about deformation compatibility
between the nail and staple, ignoring the existing nailing would be very
conservative. 

3.  Provide an overlay diaphragm.  This technical (also documented by
APA) involves adding another layer of plywood with the joints staggered
and attached to the existing plywood with nails or staples.  This is
often used to obtain blocked diaphragm values on an existing roof
without blocking.  Should work with a shear wall using the same
principle.

These techniques are not the prettiest, but they'll help in a pinch.

John Lawson
Kramer & Associates, Inc.
jlawson(--nospam--at)kramerinc.com