I want to put up eight three inch square by .120" thick posts twenty feet
long, about two foot into concrete piers. Four of them would form a 7'
square in the center, the other four would form a larger square of about
15". I want to stand them on end. This is for a hot tub/observation
platform.

Where can I find the compression strength of that tubing? I'm sure this is
going to be way strong enough, as the spa is about 300 gallons. So, that's
a little under 3,000# for spa and water. There would be multiple bracing on
the posts, both to each other, on the horizontal and on the diagonal. I
believe a simple box with an X in the middle would give it sufficient
strength to keep any lateral movement to a minimum. 1/4" would be over
double the price, and overkill, IMHO.

Steve