To define the virtual load cases for analyzing roof drift:

• Select Mode > Analysis > Load Cases.
• Select Loads > Load Cases. The Load Cases dialog box will open.
• Type VX in the Label edit box.
• Click the Virtual Work option button.
• Click [Add] and the Virtual Load Case Story Forces dialog box will open.
  
• For the Roof Level Type 100 in the Force column (100 can also be used for SI models).
• Leave the Dir. Angle set to 0.
• Leave the X and Y coordinates at the default. This represents the calculated center of mass for the respective level.
• Leave the forces for the other floors set to 0.
• Click [OK].

  This returns you to the Load Cases dialog box.

• Type VY in the Label edit box.
• Click the Virtual Work option button.
• Click [Add].
• For the Roof Level Type 100 (100) in the Force column.
• Type 90 for Dir. Angle.
• Click [OK].
• Click [OK] to dismiss the Load Cases dialog box.

Next, you need to analyze the new virtual load cases:

• Select Process > Analyze.
• When the Analyze dialog box opens, make sure that all of load cases are selected by clicking [Select All].
• Click [OK].