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The method employed to combine capacity of materials of the same broad type (gypsum, WSP, lumber sheathing, fiberboard), but with different parameters such as fastener size, fastener spacing, or sheathing thickness is given in SDPWS 4.3.3.3.1. Using this method, you multiply the minimum ratio of shear capacity to apparent shear stiffness Ga from either side by the sum of the apparent shear stiffnesses:
v = min ( v1 / Ga1 , v2 / Ga2 ) ( Ga1 + Ga2 )
For walls with identical materials on both sides, this reduces to the addition of shear capacities.
Factors
Although the SDPWS indicates that nominal (that is, unfactored) shear capacities are to be used, Shearwalls applies the unblocked factor and the aspect ratio adjustment before the calculation is made.
For sheathing nailed to blocking on both sides of the wall, including the blocking factor in all Ga, v1,and v2 is equivalent to applying the blocking factor after the sheathing combination is done, so Shearwalls uses these values factored by Cub .
For sheathing nailed to blocking on just one side, in the absence of specific guidance, the Cub is applied only to the v and Ga on the side with the blocking. Note that it is rare to nail sheathing on just one side of the wall to blocking.
Since both sides have the same material type thus the same aspect ratio factor, applying the aspect ratio to v1,and v2 is equivalent to applying it to the resulting v, so the Shearwalls methodology is equivalent to the SDPWS procedure.
Wind Design
Although 4.3.3.3.1 specifies that this procedure is for seismic forces only, in the absence of guidance in the SDPWS and on the recommendation communicated to us by AWC, it is also done in Shearwalls for wind. Note that SDWPS does not publish Ga values to use for wind design, but Shearwalls calculates Ga for wind design according to the same principals used to arrive at seismic Ga.