In article s.com,
Jim wrote:

I'm erecting a shade sail over a patio and planning to use 4"x4"x1/4"
steel tubing on the four posts that will carry the fabric. The fabric
will be tensioned so that it barely flaps in anything but a very strong
wind. I'm told there will be between 2000 & 3000 # pull at the connection
points.
The tube steel will be embedded in concrete 3' or more below grade
depending on the height out of the ground. Two of the tubes will be 9'
above grade and opposite from one another. Another will be at 10' above
grade and the opposite one, the highest, at 11' above grade.
Can anyone tell me if A500 B grade 4"x4"x1/4" tubing will do the job?

Weight is easy. You also need to calculate the wind loading effects.
The overturning moments can be quite large, and the footings have to be
big enough not to be uprooted.

Joe Gwinn