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For structures with 3 or more levels, Shearwalls generates both Case 1 and Case 2 loads from Figure 27.3-8 simultaneously, and uses the heaviest of these loads and the minimum load case as the design shear line force. For lower structures that are designed with the all-heights method, only Case 1 and minimum loads are used. ASCE 7 Appendix D exempts buildings 2 stories and less from requiring Case 2 loading.
Case 2 - Torsional Load Case
Case 2 loads in Figure 27.3-8 are at 75% intensity plus 15% of building width as moment. It is possible to change these values in the Site Information dialog, as ASCE 7 Commentary C27.3.5 states that the 15% eccentricity at 75% may not cover all cases, and for certain buildings, 5% eccentricity at full loading is more appropriate.
If you have indicated the building is a flexible structure (dynamic analysis) then an absolute value of eccentricity is entered in the Site Information dialog as per 27.3.5. Note that it is unusual for a wooden structure to require dynamic analysis.
ASCE 7 Commentary C 27.3.5 says that the Case 2 torsional load cases are due to non-uniform wind loading, which is applicable to flexible diaphragm buildings as well as rigid diaphragms, and the Exemptions to the use of Load Case , so the program applies the additional moment to flexible diaphragms as described in Torsional Analysis for Flexible Diaphragms.
For structures with Case 2 loads, if you add a load or a direct shear line force manually, both Case 1 loads and Case 2 loads at (default) 75% intensity are created.
Case 3 and Case 4 Loads
Case 3 loads are at 75% in each direction simultaneously, without additional torsional moments, and case 4 loads are at 56.3% in each direction with a 15% torsional moment. As Shearwalls does not design for components loaded in both directions simultaneously, such as a column transferring hold-down forces from both directions, and it is rare for the torsional effects from loads in the direction perpendicular to the design shear line to be greater than the reduction in design loading, Shearwalls has not implemented these load cases. We have yet to have a user request for the implementation of these cases, but if you are aware of a design case for a wood structure for which these cases govern, please contact WoodWorks Technical Support.