How thick should a steel staircase be?

We cut the stringers of our straight staircases from thick flat plate, not thin folded sheet, which gives a rigid flight and a clean side profile. The folded steel treads have a welded bar at the nosing for stiffness, and our spiral staircases are carried by a thick-walled steel tube on an anchored base plate. We size the exact thicknesses and sections to the width, height and use of the staircase and state them in the specification before production.