Use a Plane Strain model.
In Workbench, drag out a Static Structural analysis.
Double click on Geometry to open SpaceClaim.
In SpaceClaim, File > SpaceClaim Options > Units > change the units from mm to m.
Click on the Sketch button and select the XY plane.
Click on the Rectangle tool and sketch a rectangle, you can type 20 in each box.
Click on the 3D mode button, the rectangle will become a surface.
Select the surface and click on the Sketch tool.
Click on the Rectangle tool and draw another rectangle from the top left corner. Make this rectangle 6 m wide and 6 m deep. That will split the surface into two faces and you will have your 6 m edge.
Follow these directions (not step 1 in your case, and step 2 applies to Static Structural in your case).