3mm aluminium sheet.
I fabricated all the treads separately and welded them onto a tube that was machined in such a way that they interlocked into each other and then a single tube inserted into the separate tubes (through the top) the full height of the stairs and then the last at the top has an end cap enclosing the inner support tube.
(hope that makes sense) then the tread ends are screwed into the timber walls.
I also fabricated closing panels between the treads but I don't think I have a photo of the finished job.
and believe me it was solid! it's on a pilot boat that we made in work.
Thanks for posting, like it a lot. I see now,each tread welded to a short tube then threaded onto a central support. A job like that could drive you round the bend.
The reason I say this is getting spiral staircases to work out and get though building control is a big fat pain in the you know what. Far harder than actually making one
Got asked for one a few weeks back ( client chromed as well) passed on that one