Well, after mulling it this way, then mulling t'other way.. hmmm... 9' centers for the columns. It finally dawned on me that at 6' o.c. there's just too many holes and too much lumber going into the ground. I still have to consider all that weight on 9' spans (top and bottom girts), but I'm cogitating a somewhat simple solution for that. More holes, but not 46" deep ones. Instead, approx. 8" x 10" x 30" holes in between the 9' o.c. columns and poured to grade with reinforced concrete. These "footers" will support the middle portion of the two bottom girts. Here's the latest side view.