Steel circular stairs (sometimes called “spiral stairs”) commonly incorporate “stringers,” structural members that support the treads, side walls (if any), and handrails. These stair stringers can be made of plate, channel, beam and tube (round, square and rectangular). These steel sections are most often curved into a helix but are sometimes formed into more complex compound bends.
For example, some staircases require an elliptical shape. Others incorporate a narrowing or widening of the treads which changes the geometry of a simple helix. Some circular staircases have landings at the top and/or bottom of the stairs was well as in between. In every case, the accurate fabrication of the stringers is critical for the fit up of the staircase.
Another type of stringer is the box plate stringer. Similar to how a built-up beam girder is comprised of three plates welded together to form the web and two flanges, so too a box plate stringer is comprised of four plates welded together. But here all 4 plates must be rolled into a helix. And these plates have to match up perfectly to help create a staircase that is plumb and otherwise dimensionally accurate.
If we think of the box stringer as it will be installed, we can refer to a top plate, a bottom plate, a plate on the inside radius, and a plate on the outside radius. The top and bottom plates could be called bar rings rolled the “hard way’ helically, i.e. a compound bend of a bar against the strong axis. The inside and outside bars could be called bar rings rolled the “easy way” helically, i.e. a compound bend of a bar against the weak axis.
Whatever these components are called, they have to fit together and work with the rest of the staircase parts.
In the picture below you can see that the 1/2in beveled plates have been tack welded together indicating that they fit up together. To confirm accuracy in construction, there needs to be careful measuring of the radius, pitch, and orientation. In this case, the inside plate has an inside radius of 71.5in with a 27.83 degree pitch; the outside plate has an inside radius of 79in with a 25.85 degree pitch; the top and bottom plates are rolled the “hard way,” and are fit up to make a 16 x 8 box; lastly the orientation is “walk up, turn left.”