Gypsum Underlay

Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 9 and may not reflect current behavior.

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

1. Input

   A new combo box labelled Gypsum underlay has been added to the Wall Input view, and the corresponding property is saved with the wall definition in the project file.

   1. Choices

      The choices for this box are None, ½”, 5/8”. You cannot type in a thickness other than the two standard sizes. There is no “unknown” choice” – you have to specify a configuration.

   2. Applicable Materials

      The combo box is enabled only if the sheathing material selected is Structural I, Structural sheathing or Plywood.

   3. Fastener Sizes

      When the gypsum underlay thickness is changed fastener sizes selection is updated to account for the available nail sizes for the increased overall thickness of sheathing.

   4. Fastener Types and Spacing, and Stud Spacing

      The behaviour of the interacting fields regarding nail spacing, stud spacing, etc is identical as to when there is no gypsum underlay, as per SDWS 4.3.7.2 which specifies the same construction limitations.

   5. Standard Walls

      It is incorporated into the definition of standard walls. This is how you would specify a default value.

   6. Multiple Wall Selection

      It can be modified for multiple walls selected together similar to all other inputs.

2. Shearwall Design
   1. 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.

   2. Wall Groups

      This property of a shearwall contributes to the distinction of one wall design group from another, that is, shearwalls with gypsum underlay will be in a separate design group than an identical wall without it.

   3. 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 4.3A and is not implemented.
   4. 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.

3. Deflection Analysis

   The program does not consider gypsum underlay in deflection calculations; it uses the shear deformation and nail slip values for the structural wood panel. Shearwalls uses the four-term equation C4.3.2-1, for which there is no explicit guidance in SDPWS, but this approach is consistent with one used with the three-term equation, in that the G_a values used in the combined shear and nail slip term are the same in Table 4.3A for wood panels as those in Table 4.3B for wood panels with gypsum underlay.

4. Output
   1. Sheathing Materials Table
   2. 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 shearwall table. The legend explains that GU means gypsum underlay. A dash appears if there is no underlay.
   3. Table 4.3B 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.