Combination of Sheathing Capacities

SDPWS 4.3.3.3 states that for walls with "the same material and construction" on either side , the sheathing capacity from both sides of the wall are added, but 4.3.3.3.1 says that for such walls for seismic design, a formula based on relative apparent shear stiffness is used (see Shear Capacity Combination by Relative Shear Stiffness). C4.3.3.3 and C4.3.3.3.1 clarify this to indicate that 4.3.3.3. refers to the case that all material parameters, i.e sheathing thickness, nail type, and spacing, are identical, and that 4.3.3.3.1 applies to the case that wall has the same material type on each side but different parameters, the types being defined as

• Gypsum-based (GWB, Gypsum sheathing, Plaster and lath)
• Wood structural panels (Structural sheathing, Structural 1, Plywood siding)
• Lumber sheathing
• Fiberboard

As this leaves it unclear what is to be done for wind design for non-identical materials of the same broad type, AWC advised us that we should use the stiffness-based approach from 4.3.3.3.1 for wind design as well. (The program calculates shear stiffness G_a values for wind design, whereas these are not yet published in the SDPWS).

When the material type is not the same on either side, according to 4.3.3.3.2 the capacity is the greater of the stronger of the two sides or twice the weakest of the two sides, except for the case of wind design with wood structural panels or plywood siding on one side and gypsum wallboard on the other, in which case the capacities are added.

The cases are summarized in the table below.