M. To add a seismic load

To add a seismic load to a primary load case, use the following procedure.

Note: You must have at least one seismic load definition in your model before you can add seismic loads.

Either:

On the Loading ribbon tab, select the Load Items tool in the Loading Specifications group
Tip: This will add the load item to the currently selected load group selected in the program status bar.

or
In the Load & Definition dialog, select a primary load case in the Load Cases Details list and then click Add.

The Add New Load Items dialog opens.
Select the Seismic Loads tab.
Note: If you do not have a seismic load definition created, this tab is disabled.
Select the global Direction in which this seismic load acts.
(Optional) Type the scale Factor to apply to the load.
(Optional) Check the option to use Multiplying factor for Accidental Torsion Moment and type a Factor. This value is the ratio of the total building width and is taken from the center of mass for each level.
(Optional) Check the option to use Multiplying factor for Natural Torsion Moment and type a Factor.
Note: This is only applicable for structures modeled with a rigid diaphragm.
This torsion -referred to as the natural torsion- accounts for the static eccentricity which is the difference between center of mass and center of rigidity of a rigid floor diaphragm
Click Add.