Start in Drawing Mode
Use lines & Arcs
Make sure Snap (F11) is on
Using Mirror Tool helps if you use a center line
Once shape is created, convert to Solid Polygon
To do this click Edit > Form Solid Polygon
Click on each Line and Arc
Right mouse click in Polygon when done
Give Polygon Height (Depth) as you are in Plan View
Create 3D Model - Assemble floor > Choose Render Mode
Export 3D Model as R14.dwg or 2000.dwg to desired location
Click on Plan Tab
Go to File System >System Library
Locate Symbol Library of choice or create new Library
Use Wizard, say no to on screen Symbol
Follow prompts
Orient 3D Symbol
It will create a 2D symbol for you.
You now have your Post & Beam Truss!