Hi all, new here. I have a bit of a plan, but I have no idea whether it's structurally sound.

Just purchased my first house with a garage, but it's a single-car. To avoid races with the wife, we've decided to build a two-car carport and use the garage as a work space. The only place with enough lateral room for this is directly in front of the garage where the driveway is already located, which is pretty severely sloped; I'm talking an inch of drop for every foot. We could level that area, but then it would look like this: http://i.imgur.com/zD3czGl.png

We decided that building a carport with different-height posts is going to be easier than leveling all that. So here's the plan I've come up with: http://i.imgur.com/D4g89Tj.png

That's a side view of it, so it's only showing half of the plan and no roof. We're going to rely on experts for the roof but save some money by putting in the posts and connecting them ourselves. I'm in Tennessee, and the frost line is only about 8 inches here, but I still plan to dig down 2 feet for the... piers? Pilings? Footings? Concrete forms? Not certain. At any rate, I plan to add 6 inches of gravel to the bottom and use concrete forming tubes to make a concrete form. While the concrete has just been formed, I plan to use post bases set into the concrete to secure the posts later. Here are the bases: Simpson Strong-Tie PBS 6 in. x 6 in. Galvanized Standoff Post Base-PBS66 - The Home Depot

That's mostly because I fully expect someone to run into one of the posts and damage it, and it'll be much easier to replace that post if it isn't cemented into the ground. Additionally, it'll be much MUCH easier to cut the posts to their final length if we can remove them from the bases and cut them on the ground instead of on a ladder on a slope.

We'll be using 6x6's for the posts and 2x10's for the... stringers? Runners? Front-to-back-connection-whatsits? Anyway, now that I've laid all this out, here are my questions:

• Are these post bases under-engineered? Should I consider something like Simpson Strong-Tie 12-Gauge Black Powder-Coated E-Z Base-FPBB44 - The Home Depot instead? Is there a better option I haven't considered?
• Since I'm going with 6x6 posts, will I be okay with an 8-inch form tube, or should I up it to 10-inch tubes?
• Is there anything structurally unsound about my plan that you have seen or read? Do you have any more questions?

Thanks in advance.

Check with your local building permit office for the depth needed for the concrete footings. Dig the hole 6" deeper than that to accommodate the gravel base. Your building code may also specify the minimum width of the footings and how far above ground they must rise.

Regarding the galvanized post bases: I like the first one you linked to, because it's designed to be embedded into the new concrete. The second one looks better because it's powder coated, but relies on screws to hold it to an existing concrete base. In any event I'm glad you're thinking along these lines, as it's generally not a good idea to embed wood into concrete, especially for a structural support, because the acids in cement will eat away the wood over time. Ever notice how wood mailbox posts in concrete always seem to rot right at the top of the concrete? Now you know why. Not too big a problem to dig out and replace a wood mailbox post set in concrete every ten years or so, but you don't want that with your car port.

Regarding the diameter of the concrete cylindrical forms: as noted your building code may specify this, but if not --- 8" will barely fit the 6x6 posts (diagonal length of a 6x6 post is approximately 7.8 inches). It may be OK, but using 10" forms will give a bit more clearance to make sure that the portion of the post base embedded in the concrete is surrounded by a good thickness of concrete.

Agree with ebaines. The galvanized bases work great for that exact purpose. Here is a little trick. After you have everything done, get some PVC 1X6 and PVC trim, I like "base cap", and cover the galvanized part. Use PVC cement to form a box and use galvanized finish nails for the trim. Looks very professional. Check with code dept. for tube size but I would use 10" I would not level the driveway, too steep an approach or departure angle. If you go with a flat roof, slant it so rain and snow can run off. If a gable roof, get pre made trusses, they are very easy to install. Actually, with a 9-10' width, you could make your own trusses from 2X4, plywood cleats and use collar ties.
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