anchorconcreteconcrete-blockfootingspost
Need to attach 6x6x8 posts for a free-standing pergola – I'll use Strong-Tie anchors attached to the concrete footing.
The hole for the footing will reach well below the freezing line, but how wide should it be?
EDIT
Found these construction guidelines for decks that recommend 14" diameter holes, but not sure if there are other recommendations for pergolas.
My vote is for option 3. I've never seen angled pipe flanges either, but you could make a couple for this purpose out of 1-ft lengths of pressure-treated 2x6 with a pipe-sized 45-degree hole drilled in each. Then anchor those to the walls vertically. That would give you the added advantage of spreading your anchors out a bit in the cinderblock, rather than having 3 or 4 smallish anchors on a 2" radius or whatever.
I don't like option 1 just because you probably don't accurately know the strength of the concrete ceiling either locally or across its span, and that option's failure mode could be disastrous. Not to mention the likelihood (as you mentioned) of hitting steel and needing to rent a core drill and use it upside down on the ceiling. Not fun at all.
My main concern with option 2 is the tension load the bracket anchors are going to see. These blocks are very strong in compression (and by extension any anchors used in them are strong in shear), but not strong at all in tension (and by extension anchors don't resist pullout forces).
You can actually punch surprising large holes in the middle of a beam without diminishing its load-bearing capacity. This is because that capacity is largely a function of only the top and bottom portions of the beam and how far apart they are.
Engineered floor joists are a good example of this:
It would take a structural engineer to tell you exactly how much the load-bearing capacity will be reduced, but I can tell you that this amount will be at least an order of magnitude less than the margin for error in manufacturing the beam.
Don't worry about it. It's what these things are made for.
Best Answer
Since it is a structure that is not occupied in any way, I believe there are no codes that would govern footings for pergolas.
With that said, in my opinion, any hole worth digging for concrete should be at least 12"X12" or 12" round. Just to make a point, the posts could simply be set in the ground, using the 6X6 bottom as the bearing surface. Even not knowing your soil type, or bearing capacity. I am sure that would hold anything that may grow upon your pergola, BUT you have the right idea about the concrete. With the concrete poured a little above grade and the post clip added to the bottom of the posts, you have a good set up for long lasting posts.
The biggest thing will be is, you will need to provide bracing so the pergola does not rack.