Here's how I would do it.
Start with layout sketches that define the axis, size and centerlines of the inner/outer radius and height of the staircase.
Use 'swept boss/base' features with helical paths for the rectangular tubing structure.
Model the bottom or top stair step without merging so it's a separate body.
'Circular pattern' that step 'body' however many times to create all the steps. They will be on the same plane after this step.
'Body move/copy' and translate each step one at a time in the z-axis to its correct location. (tedious, I know, but not insurmountably so). or maybe translate all the remaining steps 18cm each time so the translate dimension would always be 18cm, yeah, that would be a little slicker I think.
That would be about it.
If anyone knows a way to 3d pattern it in one shot, say with a 3d sketch driven pattern, or some other way to automate the process please chime in.
There was a golf ball example recently that was a fun challenge to model, I was able to pattern all the spherical dimples (as solid bodies to be subtracted from the main ball) with a single 3d sketch driven pattern, but when I tried to make the dimples cylindrical instead of spherical, they all maintained the orientation of the parent 'body' and I couldn't figure a way to get them to stay normal to the center of the golf ball. I digress...
but, it would be a nice trick to have tucked away, and it's relevant because if we could maintain a solid body 'normal' to an axis, surface, etc., we would be able to pattern the stair steps in this question in a similar fashion rather than a cumbersome 2-step process. If there were 1000 stairs, I wouldn't want to do it as suggested above.
thank you very much for answering
I think there would be a use for reference points, and a sketch driven pattern here, but It will take some experimenting.
Or, it might help to make this in an assembly file so the steps resulting from a pattern could be subsequently mated to the center post of the stairs. Mates are a lot easier to work with than the move/copy tool (in my opinion).