All you need to do is create (or move) the column and beam such that the endpoints are coincident and Revit will automatically create a join. If you are not familiar with writing code to create or move family instances, you can start with the Revit API developers guide http://help.autodesk.com/view/RVT/2016/ENU/?guid=GUID-F0A122E0-E556-4D0D-9D0F-7E72A9315A42
