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1. Obtain the scaling for each mode as the response-spectrum modal amplitude through Display > Show Tables > Analysis > Results > Modal Results > Response Spectrum Modal Information:
   
                                                                                     

   Figure 1 - Table: Response Spectrum Modal Information

   
   

2. Select Define > Load Combinations > Add New Combo. Select Modal in drop down list under Load Name and enter Mode number and Scale factor for the specific mode from Response Spectrum Modal information Table as given in item 1. Add each mode in a separate load combination, as shown in Figure 2:
   
                                                                                                                                                                         

   Figure 2 - Load combination data

   
   

3. Obtain joint displacements for each mode per item 2 by selecting Display > Show Tables > Tables> Analysis > Results> Displacements> Joint Displacements.
4. Compute the average story drift at two ends of the building, then compare with the maximum story drift for that specific mode i.e. Dmax/Dav<1.2 or 1.4 for each mode.
5. Combine all modes using the SRSS or ABS method to determine whether Torsional Irregularity or Extreme Torsional Irregularity is applicable to the model. For example: SQRT[SUM(Dmax/Dav)^2]<(1.2 or 1.4)xSQRT(number of modes).

Refer also to related watch and learn video below: