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Yup, this is from the county website: Residential prescriptive design (non-engineered) criteria Seismic: Design Category D Wind speed: 97 mph Roof snow load: 25 psf below 400 foot site elevation. All other elevations can be determined on Structural Engineers Association...

We're actually in Portland, but it's a mild marine climate (CZ 4C), so fairly mild temps and not a very high snow load (at least in our area; nearby mountains are a different animal of course).

Thank you folks, I appreciate all the responses here! To address a couple of the points made: - Our frost line here is 12" deep - The snow load that was used for the garage is 25 lb/sq ft As many of you suggested, I'll just leave the 24" dia caisson design; it probably won't cost that much...

Just to be clear, this is a local company with local engineers, not a nationwide mail order place. And yes, as I hope I've made clear already, I agree with all the comments here to just go with the recommended caissons. And hope that those purlins are sized correctly :).

In this case this carport is designed by a company that specializes in custom structures, mostly large garages and homes. It's not one of those online carport/garage companies that are very prevalent. In fact they designed our 1000 sq ft garage that we're just about to have them build for us...

We do have snow loads, and funny you should mention the purlins - I did in fact I question their 4"x6" purlins with the 15' span, seemed a bit undersized to me. I was told that it's "fine". I'm sure that this is exactly right. I'm having trouble even finding a structural engineer to respond...

Perhaps my post was poorly worded, I did not mean to ask anyone here to do a calculation for me. I was mostly curious about what drives caisson sizing (weight, lateral, uplift), and was wondering if I could run some back of the envelope numbers as a sanity check. Seeing the responses that 24"...

BTW, here are the caissons at the apartment complex. Mine would be twice as wide.https://files.engineering.com/getfile.aspx?folder=d7fe3259-78dd-4ceb-96d9-2b5c9889e7b4&file=20240726_132650.jpg

Those are fair points, and make me feel better about the proposed 24" diameter. Indeed the apartment carport has concrete slab between the posts, which I won't have. And I have no idea how deep their caissons are. So thank you for the feedback, I will ask the carport company to tell me what...

Hmm, interesting point - the apartment carport caissons had the posts embedded in the concrete, not flange mounted. Could the flange be driving the large diameter of mine? I'm not opposed to changing the design to embedded posts, if that's the only reason for the large caissons.

Thanks for the thoughts! I did ask them to look into smaller caissons, and they said they'll review and let me know. So that may be the easiest path forward, if they give me some calcs (or more likely they'll say "yup, that's the right size", without the calcs). We don't need a permit for this...

I had a steel carport designed by a company that will supply the materials, and I will then assemble the carport (picture attached). I will also be responsible for preparing the foundation. The company that designed the carport recommended using four 24" dia X 4' deep caissons under the posts...