Proper nailing is critical to shear wall strength because
it is the controlling factor in design. Testing conducted by the American
Plywood Association proved that shear walls fail in one of three
Shearing of the plywood panel is rare.
|Common Failure Mechanisms in Plywood - Pullout/Pull-through|
|The 2x framing is visuble to the right of the hand
at upper left. The dark
line is not a split...its a guideline for nailing the paneling at the centerline.
Note the nails in both the 2x and the arching plywood.
|Common Failure Mechanisms in Plywood - At Lumber Framing|
|The highlighted area is a split in the 2x lumber framing.
The nails have acted
as wedges to initiate a long split in the member. Splitting of lumber has
always been a concern in designing plywood shear walls and diaphragms as
can be seen from Code requirements for 3x members at some panel edges.
As noted in the building code and other publications, nails should be placed an equal distance between the edge of stud and the edge of panel, preferably with 3/8" edge distance on the panel. This can be accomplished by driving (or shooting) the nail in at a slight angle to the face of plywood.
|Edge Nailing into a 2x|
|Note that the nails are also staggered slightly vertically
not to concentrate wood fiber rupture in one location.
|Edge Nailing into a 3x|
|Not only are nails staggered vertically in adjacent
sheets of plywood,
they are staggered in the same sheet. This is intended to minimize the
possibility of framing lumber rupture as was seen previously.
When properly set the nail head is flush with the face
of sheathing but does not rupture the face ply. Overdriving (with rupturing
of the face ply) can significantly affect the shear strength due to the loss
of bearing area at the nail shaft. Full head nails should always be used
as a precaution against plywood buckling.
|Properly Set Nail|
|The nail head should either sit firmly on
the face of plywood or it may be set
flush with the face of sheathing causing
|Indications of overdriving are the ruptured
face ply or a recess where the nail head
passed through the outer plies. Half head
nails are/were particularly susceptible to
overdriving due to the reduced nail head
|Nailing at Panel Edges|
|The nails are driven (or shot) home at
a slight angle to the face of plywood.