Peter Cowan/Lexy Cameron wrote:

Thanks for the suggestions. Paul.
What is the max bending moment that these two steel sizes can take without
additional reinforcement?
Peter

I assume you mean without the back side truss.

A simple rough calculation is to consider
the 2 by 4 as an I beam and assume the "flanges" take all the
load. Assume also that the mild steel the 2 x 4 is made
from has a yield stress of 30,000 psi. Could be lower. 

The "flanges" then are two inches wide and .1875 thick so the area is .375 times
30K or 11,000 pounds in tension and compression. Max moment is times four 
inches so the beam might take 44,000 inch pounds or about 3600 foot
pounds. Kind of marginal for this application. 

3/16 is pretty thin and the  tube might buckle. Not even considered 
by the above estimate. Filling it with concrete might help that.

Might work if you use two side by side but I would take no 
chances and use the back side truss. Adds little to the work involved.
One 3 foot tube and one additional length of  cable.

If you are thinking of digging a post hole and embedding the column
in concrete don't. It will take a large chunk of steel reinforced 
concrete indeed unless the ground is always frozen solid up there. :)

You will still need the back 45 degree cable brace to the ground anchor.

Paul