Are you drawing them in 2D, or 3D? If you are doing a 2D drawing, I would make a symbol of each standard size post, and then draw dashed line between them to indicate the beams.
Okay, 3D can be done a number of ways - covered polylines with the Z-heights set to the appropriate positions would be one. Vertical and horizontal slabs would be another. Knee braces would probably be best as symbols?
