Click the Internal Forcestab in the Detailed analysis (steel member design) dialog to display the following dialog.
To define a new load case, select the Manual option available in the Loads field. You may introduce arbitrary concentrated forces, define ribs in arbitrary locations and determine internal forces, corresponding to the defined load. The values of internal forces is defined in the Internal forcestab.
When in Manual mode, the calculation will be carried out only for the manually defined load case. It is not possible to run calculations for both an automatically-defined load case and a manually-defined one.