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
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.
Combination of Sheathing Capacities vn and Effective Stiffness Ga |
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|
Reference |
Inferred from 4.3.5.4 and 4.3.5.4.1 |
Situation |
Identical on either side |
Design type |
Seismic or Wind |
Material type 1 |
Any |
Material 2 |
Same as other |
Blocking |
Same blocking on either side |
Capacity Rule |
vn(c) = 2 x vn |
Stiffness Rule |
Ga(c) = 2 x Ga |
|
|
Reference |
4.3.5.4 and 4.3.5.4.1 |
Situation |
Similar on either side |
Design type |
Seismic or Wind |
Material type 1 |
Any |
Material type 2 |
Same as other |
Blocking |
Same on both sides |
Capacity Rule |
vnc = Kmin Gac |
Stiffness Rule |
Gac = Ga1 + Ga2 |
|
|
Reference |
4.3.5.4 and 4.3.5.4.2 |
Situation |
Different material type or blocking on either side |
Design type |
Seismic |
Material type 1 |
Any |
Material type 2 |
Any other, or same with different blocking |
Blocking |
Different blocking with similar materials; for different materials, doesn't matter |
Capacity Rule |
vn(c) = max ( 2 x min ( vn(1) ,vn(2)) , max( vn(1),vn(2) ) ) |
Stiffness Rule |
Ga(c) = max (2 x Ga (min ( vn(1) ,vn(2))) , Ga (max( vn(1),vn(2) ))) |
|
|
Reference |
4.3.5.4 and 4.3.5.4.2 |
Situation |
Different material or blocking on either side |
Design type |
Wind |
Material type 1 |
Any except wood-based panel / gypsum board combination
|
Material type 2 |
|
Blocking |
Different blocking with similar materials; for different materials, doesn't matter |
Rule |
vn(c) = max ( 2 x min ( vn(1) ,vn(2)) , max( vn(1),vn(2) ) ) |
Stiffness Rule |
Ga(c) = max (2 x Ga (min ( vn(1) ,vn(2))) , Ga (max( vn(1),vn(2) ))) |
|
|
Reference |
Exception to 4.3.5.4.2 |
Situation |
Different material type on either side |
Design type |
Wind |
Material type 1 |
structural panels inc. plywood siding, or fiberboard |
Material type 2 |
gypsum wallboard or gypsum sheathing board |
Blocking |
doesn't matter |
Rule |
vn(c) = vn91) + vn(2) |
Stiffness Rule |
Ga(c) = max (2 x Ga (min ( vn(1) ,vn(2))) , Ga (max( vn(1),vn(2) ))) |
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