Inherent Eccentricities for Rigid Diaphragm Wind Design

Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 10.0 and may not reflect current behavior

Previously, the program did not include the inherent torsions due to the moment between the center of wind loading and center of rigidity, as this was not explicitly specified in torsional load cases that are now in ASCE Fig. 27.4-8 for the All Heights method. However Commentary C27.4.6 says that the torsional load to be added for Case 2 is due to non-uniform wind loading, not the eccentricities in the loading due to the geometry of the structure. Discussion with ASCE confirmed that for rigid diaphragms, it is the intention to include inherent torsion due to building geometry to both Case 1 and Case 2.

1. Rigid Diaphragm Torsional Analysis

   For the torsional analysis for rigid diaphragms, using the terminology in the log file where F is total force, T is total torsional force, CL is center of load, CR is centre of rigidity, and B is building width:

   1. Case 1

      Previously for Case 1, Shearwalls calculated only direct forces Fdi (using the terminology in the log file). Now Shearwalls calculates a torsional force Fti based on (CL-CR) * F. Note that this creates a torsional force in just one direction, that is, T+ = T-.

   2. Case 2

      Previously for Case 2, Shearwalls calculated only direct forces plus torsional forces based on +/- 0.15B * F. Now Shearwalls includes the inherent torsion, for a torsional force based on (CL-CR) * F  +/- 0.15B *F .

      This is based on the usual eccentricity of 15% given in Figure 27.4-8, this can now be overridden with another value (SEE CREF).

2. Log File Output
   1. Case 1

      The line giving the building width has been removed as this does not apply to Case 1, which does not have accidental eccentricity. A line has been added saying the accidental eccentricity is zero, giving the reference 27.4-8.

      A line has been added giving the inherent eccentricity, and the calculation for torsions is given in place of the note that said they were zero due to ASCE 7-05 Fig. 6-9.

   2. Case 2

      A line has been added giving the torsional eccentricity, and the equation for the torsions given has been appropriately modified.