The underlying foundation was made with pilings, because I thought that would be quicker/cheaper than excavating a regular foundation and filling it in with concrete. As you can see, it didn't end up perfect, which is why Bob's help was even more surprising, but appreciated. I warned him not to get dragged into this mess, but luckily for me, he insisted. :-)

My original spec for the panels to go on top of this base called for leaving the center clear because the panels I used had a maximum length of 24 feet, and I needed more like 26 feet to cover the foundation. The octagon on the outside and the square on the outside are made with 2-by-12s doubled up. All hardware was purchased off-the-shelf. In retrospect, I might have gone with a regular foundation, and even if I weren't, I probably should have gotten some custom-made "post-toppers" to go on top of the 6-by-6s that are on top of each piling. On the other hand, not each corner was the exact angle it should have been. I did nearly all of the work on this lower level myself. I rented a Bobcat with an 1-foot auger to make the holes for the pilings, and I mixed and poured the 6400 pounds of concrete for them myself, by hand.