Story Drift Calculations

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

For seismic design only, Shearwalls checks the maximum story drift for any shearline against the limits in ASCE 7 12.12.1.

1. C_d Factor Input in Site Dialog

   The program now allows input in the Site Dialog for the Deflection Amplification Factor C_d, given in Table 12.2-1 and used in Eq’n 12.8-15.

   1. Default

      A default value appears for the selected Seismic Force Resisting System – Bearing wall or Building frame, from Table 12.2-1, which can be overridden.

   2. Warning Messages

      The system of warning messages that detects the materials used in the structure and the Ignore Non-wood-panel… design settings, and prompts to change the Response Modification Factor R or the setting, has been modified to include the C_d factor in both the messages and the resulting changes.

2. Storey Drift Calculation

   The program implements ASCE 7 12.8.6 and Equation 12.8-15 for storey drift calculations. This calculation is made on each level, and for each force direction ( E->W, W->E, N->S, S->N).

   1. dxe

      The value of d_xe is the largest deflection on any shearline, calculated as described above. If deflections on a line have been equalised (see ), it is the common deflection of all walls on the line. If not, it is the largest deflection for any segment on the line.

   2. Deflection Amplification Factor C_d

      The values of the deflection amplification factor C_d from the site dialog are used. Unless they were over-ridden, they are the values from ASCE 7 Table 12.2-1.

   3. Importance Factor I

      The importance factor I calculated from the Occupancy category entered in the site dialog is used.

3. Allowable Drift Calculation

   The allowable drift D_a calculated according to ASCE 7 Table 12.12-1 is shown for each level only.

   1. Story height

      The storey height h_sx for each level is the wall height plus the upper floor thickness.

   2. Occupancy Category

      The existing input for Occupancy category from the Site dialog is used.

4. Provisions Not Implemented
   1. Torsional Amplification

      The program does not implement ASCE 7 12.8.4.3 Amplification of Accidental Torsional Moment because of the exception for light-frame structures.

   2. Period for Computing Drift

      The program does not implement 12.8.8.2, which allows the upper limit on the period to be ignored for the purposes of deflection design. This would have only come into effect for those users who wished to use a period significantly greater than the approximate period from Table 12.8-2 for shear design, but not for deflection.

   3. P-Delta Effects

      As Shearwalls does not currently include full gravity loading, it does not include P-Delta effects from ASCE 7 12.8-7. It is the responsibility of the designer to check the stability condition θ. This will be considered for the planned combined lateral-gravity version of WoodWorks.

5. Output

   A new table has been added showing the storey drift calculations for each level and force direction on that level, and indicating success or failure of the storey drift check. Refer to for details.