Use sketching tools to define the perimeter of a beam system.
Video: Place Beam Systems and Structural Floors
Structure panel
(Beam System).
Beam System panel
(Sketch Beam System).
Line to sketch, or click Draw panel
Pick Lines to select existing lines.
You can also click Draw panel
Line to sketch, or click Draw panel
Pick Supports to select the border of the beam system.

Sketched beam system boundary with lines locked to supports
Mode panel
(Finish Edit Mode).

Floor plan and 3D view of completed structural beam system