I'm building a pergola over a paver patio. The pergola will dimension 11'x 11.5' on center of post with 6x6 posts (4 posts), tied together with sandwiched 2x12 beams. I've laid out footer locations and am about to dig for 8" sonotube forms. The footings will be set 2.5" below finished grade and I'll use Simpson Strong Tie Elevated Post Base anchors (EPB66) so that I can lay pavers on top of the footing, with the pergola post resting on the anchor base at the finished grade of the patio. My question is weather or not an 8" dia. footing is truly sufficient. The Strong Tie spec notes a minimum 4" offset from center of anchor pin to edge of concrete, so it would seem that 8" diameter footings would meet the spec. The footings will be close to the edge of a surrounding retaining wall, so I'd like to avoid a larger diameter that would require augering closer to the wall.
(FYI - This is in Central Ohio, so the footers will be 3' deep, heavy clay soils. Occasional heavy snow in winter. I plan to trellis vining plants up and over).
Note: There was a similar question addressed on this forum a year or so ago and an answer suggested using 12" footers, but my situation may be slightly different due to use of the elevated anchor. How wide does a 6x6 concrete post footing needs to be?