Fabrication on the bigger ones is quite similar to what I described earlier, except it requires a lot more equipment to manage the bigger components and exponentially more welding (the total linear feet of weld on this piece was something like 1300 ft.). I don't use rolls to create the curved sections, I just wrap them along the flat edge pieces and tack them as I go.
Once I have all three rings together, the hard part starts. Manipulating these big pieces into place and getting proper alignment can be a real bitch. I've gotten pretty adept at working by my self, but this is the stage at which a little assistance would have been prudent. In the end, I was able to make the whole thing without any help, and also without being killed. Pretty neat, huh?
Oh yeah. In that last shot, you can see those two main supports inside the base, which protrude upwards into the main ring when finished. These are actually 1/4 in. plate and essentially hold the whole thing up.