The subject intrigued me, so I kept experimenting a bit. I now made a staircase more or less to your reference example. The dimensioning is a bit sketchy, so I guessed the inner (1 meter) and outer (5 meter) radius of the spiral.
Creating the steps is actually far less sweat than I expected. I used the command
Fin, which creates a surface from a curve on a base surface, with a given width, perpendicular to that base surface. So, before deleting the extruded spiral surface, I typed
Fin, picked the steps pattern and hit enter, typed the width value at -100 cm, picked the spiral surface and the entire staircase was created in one go. Magic.
Btw, you have to be patient after typing the Squish command, at takes a while for Rhino to calculate it (at least om my ageing Mac (10 years)), but it gets there in the end.
As you can see in my pictures, the shape of the steps changes from near rectangular at the bottom to a triangle shape at the top (because I selected a top radius of 1 m, and the same for the width). When you are doing a real design intended for construction, you will have to look into the building regulations governing stair design. It will most likely give you a maximum step height, the minimum width and also the restrictions to the minimum depth of each step as a dependant of the turning radius.
Good luck with your design,
SpiralStaircase.3dm (9.8 MB)
Edit: I noted that some of my pictures are curtailed, so not all gyrations are shown…
Edit 2: Rhino file attached.
Edit 3: Oops! There is a major flaw to all of the above, it is not cone-shaped! And I was so happy thinking I cracked it…What happened is, as the basis is a flat spiral, every consecutive pass is at an even distance to the previous. This was raised to the outer edge of the spiral staircase, and the same is true there, each consecutive pass of a particular station is further inward by an equal amount. But because of the phenomena I discussed in my first reply above, the vertical distance becomes ever smaller the higher you get, and for it to be a true cone, also that should be constant!