I built this staircase last summer and it was an interesting project.
The staircase is made of pressure treated lumber and is suspended on 2” galvanized pipe imbedded in a 10” by 4’ deep cement filed tube. I needed something that took up as little space as possible. The diameter is approximately 6′.
The treads are a rise of 7” and the spacing around the diameter is 12”. The treads were made using 2x6s and Gorilla glue. The railing was laminated strips of 5/16” pine and lots of Gorilla glue and clamps in successive glue ups – which was probably the hardest part of the whole project. The treads were quite simple and just slide down the pipe and screw together from underneath.