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Flurette Farm correctly points out that your proposed construction method provides NO lateral bracing. The first strong wind on the long side of your barn will blow the structure over like a house of cards. Pole barns normally use embedded posts, instead of posts on footings, because such posts resist loads from the side as well as gravity loads downward. Putting the posts atop concrete footings eliminates this benefit and you have to do something else -- like metal rod X bracing or plywood shear walls to prevent the building from folding up. Assuming a 4 in 12 pitch, the top of your roof would be at 24 feet. The projected area of the long side of your barn is thus 2400 square feet. The total design wind force of an 80 mph gust blowing on the side of the building is figured by the UBC as about 38000 pounds, and that's assuming you aren't in a coastal area and the barn isn't in the middle of a wide-open field. This brings us to another problem. The total side load at the base of one of your proposed 4x4s is 3800 pounds. The Simpson AB44 or AB66 post base you might have been thinking of provides ZERO (that's right, 0) allowable side load resistive capability. So if the building doesn't fold up on the first Nor'easter, it will likely blow sideways. Many jurisdictions exempt agricultural buildings from building codes, and you may be allowed to try whatever you want, but it would be a good idea to develop a feel for the forces involved before sinking this amount of money and effort into the building. I recommend the book "Simplified Building Design for Wind and Earthquake Forces", by Ambrose and Vergun.
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