Circular stairs usually need a center support, but I wanted to have the center open, so I designed this one to cantilever from the wall, leaving a 3' open center.
To accurately form the wall and steps, I cut a dozen 5' -1/6's from 3/4"
Rotating same boards around the pipe gave us the riser forms to pour the actual steps onto the ramp.
Step 1: Sketchup Plan
Bars to hold the steps had to be placed in this wall as it was built, so I needed to know exactly where to place them so each step would have a rebar in tension off the wall. By drawing this staircase in 3/d (sketchup) , I could place the bars exactly. The ramp/steps
We poured the ramp and steps 6 months after the wall was built.
Step 3: Landing and Circular Support Beam
Half the beam was in the wall, and the half you see supports the floor.
Step 4: Forming the Spiral Ramp
I used a 2" u bolt fastened through the end of 6" wide plywood board to make sure the wall was round.
The same boards stacked and rotated to form the bottom support for a foam/fiberglass form we made by tweaking 1" beadboard to the needed warped shape, then fiberglassing it. We made two sections,glassed both sides with epoxy resin.
Step 5: Pouring Ramp
The ramp was poured in 3 sections, glued at the joints with latex concrete adhesive. This let us reuse the fiberglass and support forms, just by rotating the plywood boards and locking in place.
The concrete was very high strength, mixed on site using super plasticizer to reduce water content.
All the concrete was consolidated with a spud vibrator.
The ramp is 3 inches thick.
You can see the "U" bars sticking out. They give a physical lock to the steps poured next.
Step 6: Rule # 1 Celebrate Victories
With only myself and my son Adam working on this part time, it took many weeks to complete, with many celebrations along the way.
Step 7: Steps
Because the forms stack on each other at the main tube, all I had to do was level them and set them at my marks on the wall that came from the sketchup drawing a year earlier.
Step 8: Finish
Someday we might cap the steps with tile or wood, but they look very sculptural as is.