Previous Topic

Next Topic

Book Contents

Book Index

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

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

 

 

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) )))

 

 

 

 

In This Section

Shear Capacity Combination by Relative Shear Stiffness

See Also

Shear Capacity

Design Factors