Spiral Stair Stringer
The stringer of a spiral or helical stair: a plate rolled to stand on a cylinder as it climbs, alone or as a pair, with a line for every tread. The stringer of a spiral or helical stair: a plate rolled to stand on a cylinder as it climbs, alone or as an inner and outer pair, with a line for every tread.
How the spiral stair stringer is developed, with a worked example · Rings and helicals