Aspect Ratio Limits and Factors

1. Maximum Aspect Ratio for WSP/GSB Combinations for WInd Design

   A Note 2 was added to Table 4.3.3 for maximum height-to-width (aspect) ratio for shear wall segments. For wind design only, the maximum ratio for gypsum wallboard when combined with blocked wood structural panels (WSP) or fiberboard on the other side is now permitted to be 3.5:1, the limit for WSP/fiberboard,Previously it was 2:1, the limit for gypsum based materials, and remains 2:1 for seismic design.

   1. Applicable Materials

      Shearwalls has applied this change to all gypsum board materials, as we believe the intention is to include gypsum sheathing board (GSB) as well as GWB. It is not applied to lath and plaster materials, as they are now considered a separate fundamental material type from gypsum board.

   2. Aspect Ratio Factor

      An addition to Commentary 4.3.5.5 says that the factor 2b/h to be applied to narrow segments to allow capacity based distribution is to be applied to segments with gypsum on the other side. From this we took it that the intention is that the aspect ratio factor from 4.3.3.2 and the adjustment for capacity- based design from 4.3.5.5.1 should both be applied to the capacity of both the gypsum side and the wood-based side.

   3. Effect on Shear-resisting Segment Generation

      This change creates circumstances where shear-resisting segments can be created for wind and not for seismic. This occurs for the affected segments when the setting All gypsum-based materials … seismic design is not selected.

      This can create dual outcomes for wind and seismic for the building model and design in the following ways

      • The shading of the drawing in elevation view and fill of wall line in plan view  
      • Hold-downs and compression forces will not be created at the segment for wind but will be for seismic.
      • Distribution of associated dead hold-down forces that must be combined with the wind and seismic locations.
      • The definition of the limits of a perforated wall, including the inclusion of openings adjacent to a narrow end segment.
      • Whether a wall line has at least one full height segment and can this be considered a shear line for distribution of forces and design
      • Whether a direction has at least one shear wall segment and whether torsional analysis is valid.
   4. Ignore Gypsum Contribution Setting

      It is no longer necessary to include gypsum wallboard panels for wind design when applying the setting Ignore contribution of... Gypsum-based materials and lumber sheathing when combined with blocked wood structural panels on walls with narrow segments.

      When this setting is checked, for wind design only, the contribution of gypsum sheathing board or gypsum wallboard on the other side of blocked wood structural panels or fiberboard will no longer be ignored. The segment will still be considered a full-height segment.

2. Setting: Ignore contribution of Gypsum-based materials... when combined with...

   As the exception for wind design described above in for the setting Ignore contribution of... Gypsum-based materials and lumber sheathing when combined with blocked wood structural panels on walls with narrow segments further complicates an already overly complicated setting, and the setting still does not take into account all cases where you might want to ignore the contribution of one side of the wall.

   Rather than changing it to something like

   Lath-and-plaster or lumber sheathing, or for seismic design, gypsum board, when combined with blocked wood structural panels or fiberboard, or else combinations of blocked and unblocked materials of the same type, or else unblocked gypsum-based materials combined with unblocked wood structural panels, in narrow segments in segmented or FTAO walls.

   the setting has been simplified to a general rule, as follows:

   1. New Setting

      The setting has been changed to

      Materials for which a segment has an invalid aspect ratio when the material on the other side is valid.

      Other than excluding GWB/WSP combinations for wind design, the setting has broadly the same functionality as before, with exceptions given below.

   2. Blocked/Unblocked Combinations

      Unblocked wood structural panels in combination with blocked wood structural panels or fiberboard on narrow segments will now be ignored if the setting is checked. Previously they were considered non-shear-resisting segments.

      This also applies to blocked/unblocked gypsum board combinations with ratios between 1.5 and 2.0.

   3. Application to Wall Segments

      The shear capacity contribution of the invalid side is now ignored only on the segment with the invalid aspect ratio, not the entire wall. It was rarely advantageous to apply this setting when the contribution of all segments on the wall are ignored.

      It is now always advantageous to apply this setting. It exists to allow you to decide whether it is legitimate to ignore the contribution to shear design of the materials on just one side of one segment of a wall.

   4. Perforated Walls

      This setting no longer applies to perforated walls, as it was determined that there was no advantage to ignoring GWB on the entire perforated wall.

   5. Design Settings Output

      The Design Settings table in the Design Results output echoing this setting as been revamped. Instead of saying

      Ignore non-wood-panel shear resistance contribution...

      with columns for Wind and Seismic, and choices like,

      Always, Never, When combined with WSP,

      it now says

      Ignore shear resistance contribution of..

      with columns Wall segments and Seismic design, and choices for Wall segments

      Side with invalid aspect ratio

      Don't ignore

      and choices for Seismic Design

      Any gypsum, lumber, fiberboard

      Entirely gypsum or lumber

      Don't ignore

3. Design Setting Output for Maximum Aspect Ratio

In the Design Settings table of the Design Results output, the maximum aspect ratios are shown for each material. The output for wood structural panels shows columns for Wind and Seismic, with the same 3.5 factor.

It now shows columns for Blocked and Unblocked, with factors of 3.5 and 2.0, respectively.

Note that you cannot control these ratios with a Design Setting any more, this output was in he program before design codes were standardized and there was a choice of procedures, and was retained in the output for informational purposes.