Depth of posts with metal post anchors

[Trebbor]

I'm building an arbor with a 5 foot section of trellis on either side. Because we're on rocky ground, I'm not sure how deep I can dig the holes for the 6 4X4 posts that I need to put in. My plan is to dig a hole about 18 inches deep, use an 8 inch sonotube, fill it with concrete, and then put a metal post anchor into the wet concrete. Once all the concrete has cured, I'm going to install the 4X4 posts, which should be no taller than 6 feet high.

My question is whether or not the concrete in the hole will be deep enough or strong enough to support the posts. We live in an area that doesn't get much frost. Would it be easier to just dig a hole and fill it with concrete, rather than using a sonotube? Are the metal post anchors sturdy enough to support a trellis (6X6 inch square lattice) that's about 5 ft X 6 ft?

[lefty]

There is no need to use the sonotube. Your footing will be better if you don't since the concrete will be in solid soil rather than the backfill around a tube.

Post bases or column bases (Simpson PB44, CB44, etc.) are not designed to be used on a non-top supported structure, like a fence or a trellis. Your best bet would be to use steel posts (1-5/8" or 1-7/8" chain link posts) and embed them in the concrete. You can wrap the posts with wood and nobody but you will ever know that they aren't wood posts.

[Trebbor]

Thanks for the good advice. How do you cover the steel posts? Would you nail together 4 1X4s to create the illusion of a 4X4 post?

[lefty]

Don't need 4 -- just 3. The fence board becomes the 4th side of the "post".