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For seismic design, Shearwalls automatically detects torsional irregularity in the building.
Torsional Irregularities for Accidental Eccentricity
As per ASCE 7 12.8.4.2, accidental eccentricity applied only if the structure is torsionally irregular according to table 12.3-1, i.e. on any one story, the ratio of the largest story drift on extreme perimeter shear lines to the average of those deflections is greater than than 1.2 for Seismic Design Categories C-F and greater than 1.4 for SDC B. For each force direction independently, accidental eccentricity is applied on all levels if any one level is irregular. Story drift is defined as the difference in deflections of the top and bottom of the wall in the level in question.
Amplification factor Ax
As per ASCE 12.8.4.3, for SDC C- F, an amplification factor Ax is applied to the value of the accidental eccentricity . Like the determination of torsional irregularity, the ratio of the largest displacements on extreme perimeter shear lines to the average of those deflections is used in the calculation of Ax,, however in this case the cumulative displacement from the base of the structure to the top of the level being analyzed is used, as per Commentary C12.8.4.3. Ax = ( dmax / 1.2 davg)2 , where d represents displacements on extreme shear lines.
Note that Ax is set to 1.0 when performing torsional analysis to create shear line forces and subsequent deflections for the purpose of determining torsional irregularity or calculating Ax.
Torsional Sensitivity Output
If a torsional irregularity is detected, a line appears below the Seismic Information table indicating that Ax is applied for this reason. The value of Ax on each level and direction appears in the Torsional Analysis Details output.
At the end of the Torsional Analysis Details, in a section called Torsional Sensitivity Calculations, tables appear showing the intermediate data for determining torsional irregularity and Ax. For each level and direction, these tables show the extreme shear line forces, the displacements on these shear lines, the average displacement, the ratio determining torsional irregularity, and the value of Ax