How wide does a 6x6 concrete post footing need to be using an elevated post base anchor?

Question

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?

Answer 1

Well...I had to live and learn. I used 12 inch sonotubes with the same anchor and cheap cement (the fence post kind that was recommended to me) and 6 x6 posts, which turned out to be a bad idea. 2 of the 4 cement piers cracked part way down where the achor ties were. My solution was to build 21 inch squares around the base about 12 inches down and past the broken areas and leave the rest of the existing footing intact with the more expensive extra strong cement surrounding the existing pier that goes 3 ft down. Hopefully this holds up well..but I am a DIY person so there is trial and error on my end. Get quaility cement and do a wide base to limit lateral breakage IMO. Of course I dont think there would have been and issue if I had not used cheap post hole cement. I am just glad I discovered the issue before the structure became wobbly.

Answer 2

The 4" to concrete figure you see is a minimum clearance value, and really has nothing to do with the size of the footing required to support a load or prevent twisting or uplift. The size(volume) of the footing is what your engineer calculates out to have adequate mass to support/hold down your structure.

One thing I noticed on the spec sheet you linked to:

Post bases do not provide adequate resistance to prevent members from rotating about the base and therefore are not recommended for non-top-supported installations (such as fences or unbraced carports).

which seems to describe what you are constructing, but I may be misunderstanding, and it may just be Simpson's way of saying 'if it fall over dont call us'.

Answer 3

The simposon spec about rotation is so that DIYers don't use their post bases for fences. A pergola is supported at the top in both dimensions, so rotation about the base is much less of a concern. For most reinforced concrete, you want at least 2" clearance between the imbedded steel. With the referenced Simpson imbed, an 8" DIA post base would suffice. The size of the post is irrelevant in this case. Increased depth would be beneficial for any tipover concern and for frost heave concern. Doubtful that a pergolla would load a 8" diameter base enough to cause settlement.