An I beam section can be Swept around a helix to form a curved stringer, other sections are swept separately to improve the intersections.

An edge of the tread can be created by a radial array apparently cutting a helix in plan, the section of helix can then be set to a common z height.

A surface can be created between the two edges using the RuleSurf command and the riser with the 3DFace command.