Gypsum Underlay

The program implements the fastener choices and shear resistances in SDPWS Table 4.3B, for a layer of gypsum wallboard or gypsum sheathing board under wood structural panels and plywood siding.

An input labelled Gypsum underlay in the Wall Input view allows you to choose from None, ½”, 5/8” gypsum underlay. It is available only if the sheathing material selected is Structural I, Structural sheathing or Plywood.

Shear Design

When there is gypsum underlay, the program gets the shear resistance values from Table 4.3B rather than Table 4.3A. These values do not depend on the choice of ½” or 5/8” gypsum.

Table 4.3B Notes

• Note 1 is implemented in that we use ½ the published values for ASD design in Shearwalls.
• Note 2 for specific gravity adjustment is implemented similar to note 3 in Table 4.3B.
• Notes 3 and 4 regarding G_a are not implemented, as this is for the three-term deflection equation in the SDPWS and the program uses the G_vt_v values in the 4-term equation. The effect of moisture on G_a is taken into account in the wet and dry values of nail slip E_n used in the 4 –term equation.
• Note 5 for panels is not echoed under the Materials table when there are wood panels on both sides of the wall. The corresponding note for Table 4.3A has never been output. A similar note for 4.7.3.1.4 is output when the conditions are met, both with and without gypsum underlay.
• Note 6 (Table 4.3A note 7) regarding galvanised nail production procedures is not output in Shearwalls.
• Note 2 in Table 4.3A does not have an equivalent for Table 4.3B and is not implemented.

Component and Cladding Design

Nail Withdrawal Design for Wind C&C Loads

The increased distance that the fastener must pass through the sheathing before it penetrates the framing materials is taken into account when determining withdrawal resistance using NDS Eqn. 11.2.3.

Output

In the Sheathing Materials table, the thickness of gypsum is output in a column headed GU after the thickness of the main panel in the shear wall table. The legend explains that GU means gypsum underlay. A dash appears if there is no underlay.

Notes

The note regarding specific gravity adjustment is not changed to refer to Table 4.3B note 2 instead of Table 4.3A note 1, because it is possible to have a mixture of walls with and without underlay on the same building.

Note 2 from Table 4.2A is not relevant to walls with gypsum underlay as 7/16 and 15/32 sheathing always have the same shear resistance, so is not included in Table 4.3B. It is not output under the Sheathing Materials table for gypsum underlay.