Cub Factor for Unblocked Shearwalls

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

The program now implements the new provisions in SDPWS 4.3.3.2 for blocked structural panel walls.

1. Input

   The program now allows you to select Unblocked for a wall sides with the following configuration:

   1. Sheathing Type

      Structural I and Structural sheathing can now be unblocked. Plywood siding materials cannot be unblocked.

   2. Panel Edge Spacing

      If Unblocked is selected, the panel edge spacing is limited to 6”.

   3. Wall Height

      Unblocked is unavailable for walls greater than 16’ in height. A wall changed to that height will cause an unblocked wall to be blocked.

   4. Gypsum Underlay

      Structural sheathing with gypsum underneath cannot be unblocked, as SDWPS 4.3.3.2. refers to Table 4.3A only, not to table 4.3B for gypsum underlay.

   5. Multiple Selection

      Changing the blocking selection for multiple walls is now possible. Previously the program disabled the selection when multiple walls were selected.

2. Shearwall Capacity

   The program applies the C_ub factor from Table 4.3.3.2 based on stud spacing and intermediate (field) nail spacing to the capacity of the sheathing on each side of the shearwall independently.

   1. Stud Spacing

      The 20” stud spacing option is not implemented for this version of Shearwalls. The C_ub factor is applied for the existing choices of 12, 16 and 24”.

   2. Intermediate Nail Spacing

      For values of intermediate nail spacing between 6 and 12”, the factor for 12” spacing is used, in other words, the program does not interpolate Table 4.3.3.2.

3. Deflection Analysis

   SDPWS 4.3.2.2 calls for the unit shear value v in the three-term deflection equation 4.3-1 to be divided by C_ub. Equations 4.3.2.2-1,2 show that this is equivalent to dividing the first two terms in the equation (bending and combined shear/nail slip) by C_ub.

   Shearwalls uses the more accurate 4-term equation from C4.3.2-1, so the program divides the first 3 terms of the equation (bending, shear, and nail slip) by C_ub. This is the equivalent procedure to that prescribed for the 3-term equation.

   Note that the program does not divide v by C_ub in the calculation of e_n for the nail slip term. The calculations of hold-down forces used in the 4th term also do not involve division by C_ub.

4. Output
   1. Shear Results

      For seismic design, the C_ub factor is shared with a column that was originally for the seismic height-to-width factor for structural materials, but is now also shared by the fiberboard height-to-width factor. It is now called HW-Cub.

      Note that because unblocked walls are restricted to a H-W ratio of 2, and because fiberboard does not have a C_ub factor, it is impossible to have a combination of these factors, the factors shown will be one of the three.

      For wind design, the value of C_ub for exterior and interior side of a wall is shown in a similar column that has been added for this and for the fiberboard height-to-width factor. The low-rise load case has been removed from the table to make space for it.

   2. Deflection Table

      The unit shear force v shown in the Deflection Table is v divided by C_ub. The force V_n per fastener has not been divided by C_ub.