Combination of Sheathing Capacities and Stiffness

Similar Material Systems and Blocking

SDPWS 4.3.5.4 and 4.3.5.4.1 state that for walls with a similar sheathing material system as defined in 4.3.7 on either side of a wall, and the sides of the wall are either both blocked or both unblocked, a formula based on relative apparent shear stiffness is used to determine the combined capacity (see Shear Capacity Combination by Relative Shear Stiffness).
The combined stiffness Ga used for deflection analysis with the 3-term equation is derived from adding the stiffnesses on either side.

The sheathing material systems are

• Gypsum-based panels (GWB, GSB)
• Other gypsum-based materials (plaster and lath)
• Wood structural panels (Structural sheathing, Structural 1, Plywood siding)
• Lumber siding
• Fiberboard

If the sheathing specification on each side is identical, this reduces to adding the capacities on either side of the wall, i.e doubling the capacity of one side.

Dissimilar Material Systems or Blocking

For walls with different material systems on either side, or with blocking on one side but not the other, as per SDPWS 4.3.5.4 and 4.3.5.4.2:

Combining capacity

For all but one exception, the combined capacity is two times the lesser of the two capacities or the greater of the capacities. The Exception is for wind design when there are wood structural panels or fiberboard on one side and gypsum wallboard on the other. Shearwalls applies the Exception to gypsum sheathing board (GSB) as well.

Combining stiffness

The combined effective stiffness Ga is the greater of twice the stiffness on the side with the lesser capacity, or the stiffness on the side with the greater capacity. The Exception is for wind design does not apply to stiffness.

Summary

The cases are summarized in the table below.