Recently, a miscellaneous steel fabricator came to me with a very challenging project for circular stairs. They were looking to make a curved monumental staircase using 20 x 8 x 5/8 rectangular tubes. The stair was elliptical, had a reverse “S”shape curve at the bottom, and, of course, required helical spiral bending.
The curvature at the corners became extremely tight to an almost impossible 1 foot 11.875 inch inside radius. Helical bending of a steel tube in all three axes to form a spiral, while holding the inside section of the tube flat to accept the tread and riser would have been very difficult. Therefore, the stringer was designed as a built up box tube from steel plate.
The plates could be cut, bent, and manipulated to provide the correct shape and could be welded together to form a stringer that satisfied the design requirements. The design change called out for the top and bottom plates to be made out of 5/8 x 6.75 and the side plates to be made out of 5/8 x 20. The stair required careful fabrication and erection especially because it would be architecturally exposed and any flaws would be visible.