The base of the center piers is a plastic 55 gal drum cut in half, so 23" diameter x 17.5" tall.
On top of those I have 4 ft of 12" sonotube, then on top of those 6 ft of 8" sonotube. The piers on either end are just 4 ft of 8" sonotube in the ground.
Inside the center concrete piers is a steel 3" I-Beam structure I welded together and poured the concrete around, you can see two cross members in a couple of the pics. I also added rebar vertically in all columns about 1-2 inches from the wall of the sonotube.
