Sheathing Combination Rules

1. Combination of Blocked and Unblocked Sheathing Capacities

   The "twice weakest or strongest" rule for combining sheathing capacities on opposite sides of the wall, from 4.3.5.4.2, that previously applied only to shear walls with different material types, now applies walls sheathed with the same materials on both sides but with blocking on one side of the wall and no blocking on the other side.

   1. Materials Affected

      This applies to wood structural panels and gypsum board materials. Fiberboard must be blocked, and the definition of "blocked" shear walls in 4.3.6 refers to the attachment of panel edges to the blocking, which is not applicable to plaster and lath or lumber siding materials.

   2. Material Selection Changes

      Changes were made to the material selections allowed for blocked and unblocked shear walls. The overly restrictive selections based on a literal interpretation of shear capacity Tables 4.3A and C for Table 4.3.3.4 would otherwise unnecessarily create blocked/unblocked sheathing combinations.

      As these changes did not relate directly to a SDPWS provision relating to these tables and have other beneficial effects, they are described in CREF.

2. Combination of Shear Stiffness Ga

   A new rule from 4.3.5.5.2 for combining effective stiffness G_a for the 3-term deflection equation when there are different material types on either side of the wall, or the same material type blocked on one side and unblocked on the other. The combined G_a(c) is the greater of twice the G_a of the side with the lowest shear capacity, or the G_a of the side with the higher shear capacity.

   1. Deflection Calculation

      When using the 3-term equation to calculate deflection, the program used to use the G_a for the sheathing on each side, then apportion the load in such a way to equalize the shear component of each side. This is equivalent to adding the G_a on each side and calculating the deflection using the total load and combined G_a.

      Now, the program combines the G_a values using the twice weakest- or strongest-capacity rule, and uses it to calculate deflection with the total load on the segment.

      1. 4-term Equation

         This provision is not applied when using the 4-term equation from Eqn. C4.3.2-1 to calculate deflections. The program uses the separate shear and nail slip terms on either side of the wall, and apportions force in such a way to equalize deflections.

   2. Wind Exception

      The Exception allowing the straight addition of shear capacities for wind design for WSP/GWB combinations does not apply to the combination of shear stiffness.

   3. Output

      The Deflection table of the Design Results output for the 3-term deflection equation has been changed to reflect this change as follows

      • The term Both for the surface has been changed to Comb.
      • For identical materials, the combined G_a(c) rather than the G_a for one side is shown.
      • For dissimilar materials, only one side is now shown representing the side for the governing strongest or twice weakest. The total v on the segment is shown. Previously both sides were shown with the G_a and v that were calculated for each side to equalize deflections.
      • An "S" or "W" beside "Int" or Ext" indicates whether it was the strongest or twice weakest rule that governed, with an explanation in the table legend.
3. Sheathing Material Type for Lath and Plaster Materials

   Lath and plaster materials are now considered a different fundamental sheathing material type from gypsum wallboard and gypsum sheathing board. This is due to clarifications to the definition of material type in C4.3.7 and the splitting up of the construction requirements for these materials in 4.3.7.5 to 4.3.7.5 for gypsum board and 4.3.7.6 for lath and plaster. We received advice from AWC confirming this intent.

   1. Applications

      They are considered separate materials for the following purposes:

      • when applying the sheathing combination rules, they now use the twice weakest or strongest rule instead
      • the Exception in 4.3.5.4.1 for combination of gypsum wallboard and wood structural panels for wind design now applies only to gypsum board (GWB and GSB) not lath and plaster.
      • when ensuring that all walls on a shear line are the same material type (4.3.5.5).
      • when applying the proviso in the Commentary C5.4.3.3.1 that wood structural panel or fiberboard walls with gypsum wallboard on the other side are still considered as the same material type when applying Exceptions. The program was already assuming this, but no longer does so for lath and plaster materials
      • The new rule in the note to Table 4.3.3 that allows 3.5:1 aspect ratios for gypsum wallboard on the other side of wood structural panels for wind design