Gypsum Underlay

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.

Material Selections

The program implements the panel sheathing and fastener choices in SDPWS Table 4.3B, for a layer of gypsum wallboard or gypsum sheathing board under wood structural panels. It is a more l

Shear Resistance

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. These are a subset of the values that appear in Table 4.3A, but different combinatons of sheathing and nailing selections yield these capacities.

Table 4.3B Notes

• Note 1 is implemented in that we apply the ASD factors in SDPWS 4.1.4 to all shear capacities
• 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 directly implemented and the program for shearwalls with or without gypsum underlay. The program calculates G_a rather than using the tabulated value, so that effect of plywood plies moisture on G_a is taken into account in the wet and dry values of nail slip E_n and the value of G_vt_v , respectively used in the calculation.
• 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 and 7 (Table 4.3A note 7 and 8) regarding galvanised nail production procedures are not output in Shearwalls.

Table 4.3A Notes not Included in 4.3B

The following Table 4A notes do not have an equivalent in Table 4.3B.

• Notes 2 regarding increasing 3/8 and 7/16 values to those for 15/32. It is not implemented when there is gypsum overlay
• Note 10 regarding the 0.92 factor for 10d nails with hold-downs. It is not implemented when there is gypsum overlay.
• Note 9 regarding the use of smooth-shank nails. It was implemented via the removal of deformed nails from the program and the application of ring-shank nails to non-shearwalls only. This has been done regardless of whether there is gypsum underlay.

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.

Sheathing Materials 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 to Shear Results Table

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.