From the Type Selector, specify a structural floor type.
On the ribbon, click (Boundary Line).
Click (Pick Walls) and select the bounding walls.
Note: Instead of picking walls you can sketch a structural floor. On the Draw panel of the ribbon, use the sketch tools to form the boundary of the structural floor. The sketch must form a closed loop or boundary condition.
To change the span direction, click Span Direction, and click the desired edge or line.