Deflection Equation and Components

The program calculates the in-plane deflection of all shear resisting elements. Using a Design Setting, you can choose between according to the 4-term equation from SDPWS Commentary, Equation. C4.3.2-1:

or the 3-term equation given in SDPWS 4.3.2, Equation 4.3-1, in which the terms

are replaced by

where G_a is the effective shear stiffness calculated as described in Combined Shear and Nail Slip Deflection.

The meaning of the variables is given in the sub-topics to this topic.

Meaning of Terms in Equation

The terms in the 4-term equation give the contribution to deflection from the following sources, in order

• Bending: Bending of vertical shear wall chords (wall segment end studs)
• Shear: In-plane shear deformation of sheathing
• Nail slip: Slippage of nails fastening sheathing to top and bottom wall plates
• Hold-down displacement: Slippage of fasteners connecting hold-downs to studs, elongation of hold-downs, wood shrinkage and crushing at hold-down location, and additional displacement due to mis-cuts, gaps, etc.

In the three-term equation, the shear and nail slip values are combined into a single term.

3-Term vs. 4-Term Equation

The three-term equation is a simplification of the four-term equation, using a linear approximation to the non-linear nail slippage effects, essentially by setting any terms that are non-linear in the shear force v to their value at shear wall capacity. This ensures that the equation is conservative in the amount of shear wall deflection for all shear walls that pass shear design, but it also has a significant impact on load and force distribution to wall segments using stiffness-based distribution, and this effect can be conservative or non-conservative.