Beams Behaving Badly
Revit Architecture's built in structural beams can be a nightmare in some situations. The problem is that beams just don't seem to want to butt up against one another correctly, especially at a corner join. The default behaviour has the beams joining center to center and cut back so as to allow for a post/column at the corner.
If you try to schedule these beams, you will notice that the length of the horizontal beam will be taken from the blue dot, regardless of where the triangles (Start and End Extensions) are dragged to. You can try dragging this dot but it always seems to want to join to the adjacent beam and any amount of aligning will leave you blue in the face.
The key to this solution is to create a new beam family, by using the Generic Model line based family template, then change the Category of this family to Structural Framing.
Select File -> New -> Family...
Select Generic Model line based.rft, us crazy Metric users will see Metric Generic Model line based.rft
You will see something like this:
Now start by creating some Reference planes. I prefer to have my beams drawn from the edge so I will place a horizontal Reference Plane below the center point.
Throw a dimension between the original horizontal plane and your new one
Go into your Left Elevation. Create another Reference Plane. If you want your beam to sit below your Work Plane when you draw it, make this new reference plane sit below the "Ref. Level". Again give it a dimension
Now we can start giving parameters to these dimensions.
Select a dimension.
Select the drop down box in your Options bar beside "Label:" and click <Add parameter...>.
Call the first dimension Width and the other Height. Make your Width and Height "Instance" parameters for now and group them under Dimensions.