In article ,
says...
After spending too much time trying to find plans for a spiral staircase, or
a kit for a reasonable cost, I've decided to start from scratch and make my
own. What I plan to do is weld 4" angle iron "tread brackets" to a 3" pipe
at 30 degree spacing (all material is 1/8 inch). Each tread is 8 " higher
than the next one - this gives 11 treads for an 88 inch rise.

Don't forget about headroom--if you design too few treads per revolution
you'll have to stoop as you go up. If I'm considering everything
poroperly, you'll want at least 10 treads per rev.

I'll cut a 1
1/2" radius curve into the horizontal part of the angle iron to match the
pipe I'm welding it to. The tread brackets will have welds on both sides of
the vertical and horizontal surfaces. Then I'm going to drill and bolt 2 by
12 wooden tread to each bracket. Brackets will probably be 36 to 40" long
as will wooden treads. I still haven't figured out the balusters, but
probably will use a single rod welded to each tread bracket and somehow
curve a metal railing for a hand rail (the rail will be a problem). This
may sound like a lot of work, but I have all the materials on hand, a Miller
175 MIG welder and a plasma cutter. There are only 11 treads to attach, so
I can get this part cut and done in a day, provided I can layout the degree
spacing without botching it. The big problem I see is the railing, but I do
have a small bender which I could bend short sections of pip in, and weld
them together as I work my way up.

I've seen spiral handrails made up from a bundle of smaller rods. I
don't know whether they were bent bundled together and then tacked to
stiffen up the rail, or if laid in place one at a time and tacked as
required.

I built a wooden spiral staircase for my house (actually it's three
straight steps, seven winders, then 3 more straight) and it was a much
bigger job than I planned. The handrail was by far the hardest part.
Welded steel would have been much easier.

Ned Simmons