What is the proper foundation for t

[lifeofriley]

What is the proper foundation for the following column construction? Is there a formula?
I am seeking to build a row of 7ft columns with 2x6 redwood beams atop. The structure is to act as a pergola between my house and my neighbor's.
Each column will be constructed with hollow
-8in diameter 1ft high 1 1/2" thick concrete tubes.
I want to pour a foundation to accomodate a 2 3/4" x 7ft high (above ground) metal fence post. Then I want to thread each column over the post and backfill with pea gravel.
The finished design of each column will have an 8" high base (1ft dia at ground tapering to 8in) and crown.

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[MikeDB]

I don't see any load or soil information, but the loads appear to be insignificant. Does your local building code or permit have any requirements to meet? Without knowing any more, I would suggest augering a hole 12" in dia. and 30" deep (or deeper depending on your frost depth).

I don't understand the fence post and pea gravel. Looks like it will rust out, and the pea gravel doesn't do much of anything -- like filling it with marbles. Do the concrete tubes interlock or can they be bonded together?

 

[lifeofriley]

Mike,

Thank you for your reply.

I haven't been to the city yet for a permit because the columns can be repositioned.

I think your calculation of 30" depth /12" dia. (in concrete) would be the probable requirement. As a trial, I stacked a seven foot high column on a post in 12" of soil and it seems very sturdy.

I live in California--no frost issue--just earthquake issue--big issue.

The fence post is actually a chainlink metal post--I think it will withstand elements.

Thanks again--I will check the codes for California.