Things you need to know about plywood

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The trickiest and most important part is the plywood

The most important part factors determine plywood’s ability and a shear walls performance to resist earthquakes and each of these factors must be considered.  The type of nails or staples used, their size and length, plywood thickness, the species of wood, the manufacturing of the plywood, and the wall framing all play a part in the plywood installation.

The table below is from APA Research Report 154.  Below are some instructions on how to read it.

The black and red arrows point at numbers that represent the pounds of earthquake force each linear foot of plywood can resist if nailed in a manner consistent with the table.  For example, if we go the bottom row of the table that starts with 15/32 and follow it left to right we see that:

154 3 Arrows(1) The Panel Grade is Structural 1 plywood.

(2) The plywood’s thickness is 15/32″

(3) The penetration into the framing is 1 1/2 inches.

(4) The nail size is 8d  (8d or 10d is simply a way of describing nails a certain length and diameter)

(5) The plywood is nailed on the edges 6″, 4″,3″ or 2″ apart.

Plywood Strength Makes All the Difference

In this case we get different strengths, for example a shear wall will have 430 pounds of resistance per linear foot if the “Nail Spacing at Edges” is 4″ apart with 8d nails and 870 pounds of resistance when nailed with 10d nails 2″ apart.  What kind of nailing would you want for your house?

These higher capacity shear walls are  not always feasible, and only the person who determines the shape, size, and condition of your existing house can make that determination.

In this case we get different strengths, for example a shear wall will have 430 pounds of resistance per linear foot if the “Nail Spacing at Edges” is 4″ apart with 8d nails and 870 pounds of resistance when nailed with 10d nails 2″ apart.  What kind of nailing would you want for your house?

The Science of Plywood Nailing

The best plywood in a seismic retrofits is called Structual 1.  It has been specially formulated to brace walls against earthquakes, including cripple walls.  The closer the nails are on the edges of the plywood when nailed to the 2 by 4 wall framing, the stronger the plywood will be.   All retrofit guidelines  require nailing that is 4″ apart, even though plywood nailed at 2″ apart makes the earthquake resistance of the plywood 58% stronger.  Depending on how your retrofit was designed, your proposal may mention that the plywood will be nailed 2″ apart.

It is very clear that these guidelines only allow this low strength plywood nailing.  These guidelines recommend the use of the Nailed Blocking Method.  Because these blocks split so readily it was decided that no more than 4 nails would be allowed for each block.  Each nail can resist 117# of force.  Multiply that by 4 and you get 471# of resistance.  There is no point in nailing the plywood such that it exceeds the strength of these blocks.  It was therefore decided that 8d nails would be spaced 4″ apart which provides 484# of resistance per linear foot.  An almost exact match.

If the Nailed Blocking method is not used closer nailing is not a problem as born out by the hundreds of tests done by the American Plywood Association.  The APA’s allowance of this spacing is quite conservative meaning it has no problem in condoning this nail spacing in the nationally adopted International Building Code.  Retrofit has not seen any problems with nail spacing even as close as 1″ apart, let alone 2″ apart as approved

2 by 4 block with plywood

Aspect Ratio-This is very technical

An aspect ration is the ratio between the height and the width.   For example, a shear wall that is 8 feet long and 4 feet wide has an aspect ration of 2h/1w (the height is twice as long as the width).

To use the values listed in the shear wall tables above, a shear wall must have a 2/1 aspect ratio or less.  If the aspect ratio is greater 2/1 but less than 3.5/1, the per linear foot of earthquake resistance of the shear wall must be reduced by what is called a reduction factor.

This maximum 3.5/1 aspect ratio translates into an 8 ft. shear wall 27.5″ wide.  Any narrower than this and you have a post.

The way you figure out the aspect ratio of a shear wall is to divide both the height and the width by the width.  For example: if a shear wall is 64″ high/18″ wide, we divide both the height and the width by 18 to get a ration of 3.5/1.  If the wall is 96″ tall then 96″/18″ = 5.3/1.  This is a post and not a shear wall.

Once we determine the aspect ratio, assuming it is less than 3.5/1, we use a reduction factor of twice the width 2w/h (height).  So if we have a shear wall that is 96″ tall and 32″ wide the reduction factor is twice the w/h = 2 x 32/96 = 0.666.  The allowable shear in the table is multiplied by this factor to get the reduced shear capacity of the narrow wall.