American Plywood Association Research Report 138 addresses horizontal diaphragms.  For an engineer, a diaphragm is a piece of plywood, several pieces of plywood connected together, or a floor that transmits lateral forces to its edges.  When a floor rocks back and forth that lateral movement is transferred to the bolts on the edges of the floor.  This floor is functioning as a diaphragm.

A shear wall is simply a diaphragm that is vertical rather than horizontal.  I asked the APA if there was any reason why the information found in Research Report 138 and its testing of horizontal diaphragms could not be applied to vertical diaphragms (shear walls), and they saw no reason why they couldn’t.

In some circumstances it is desirable to build shear walls that exceed the maximum capacities found in the California Building Code.  When this is the case the information in Research Report 138 is invaluable.

Researchers performed a series of experiments to test the limits of diaphragm strength and found that if the size of framing and number of fasteners were increased it was possible to build shear walls that were almost twice as strong as those listed in the California Building Code. Their findings can be found in APA Research Report 138.

Let’s see what this table tells us about 14 gauge staples:

(1) This table applies to structural 1 plywood.  Either 15/32 or 19/32.

(2) Let’s see what happens when we use 19/32 structural 1 plywood.

(3) “Minimal Nominal Width of the Framing (the studs the plywood will be stapled to) is 3″ wide or (2) 2 x 4s sistered together.

(4)  The 14 gauge staples must penetrate the framing 2”.

(5)  There are two rows of staples (lines of fasteners) that go into the framing on all the edges.

It does not show it hear, but at non-edge staples will be spaced 4″ apart into intermediate framing memebers.

This shear wall can resist 1,200 lbs of earthquake force.

APA Research Report 138 shows that very high strength shear walls can be produced by using multiple rows of nails or staples in wood framing that is wider than the normal 1-1/2-inch-wide framing used in new construction.   As you can see, a shear wall built in this way can resist 1900 pounds of earthquake force and represents the strongest shear wall ever tested.  Even though it has never been tested, a two-sided shear wall of this type could have an enormous ability to resist earthquakes.