Help with steel pipe truss design -- 2

I want to prepare a basic design for a flat (skillion) steel truss using drill pipe (70mm OD; 5mm wall thickness) for the top and bottom and a smaller diameter pipe for the internal section - you should be an experienced structural engineer - your job is to spec the truss, including to suggest the diamter of the internal pipe (available in Australia) and provide a basic design in CAD or similar that can be fabricated by my welding team.