Elliptical forms are very interesting to build. If you use any sort or steel profile (rectangular tube) for the stringer, keep in mind that rolling equipment for curving the stringer works only on a fixed radius curve. The work-around is to draw the ellipse in AutoCad, take one typical quarter of it and divide it into a series of blended radii. You may need only three or four to complete the quarter ellipse. Sometimes the divide command is useful because it will break the ellipse into equal segments - and the curves can then be formed of, for example, one, then three, then five segments, or whatever works. Of course the hard part is moving from the flat into three dimensions - which on second thought would recommend a flat plate stinger, which would be developed out of the elliptical "helix". The cut out shape would be fed into the rollers at an angle, so that the axis if the stair and radius of curvature would be parallel to the floor. Each segment of curve would be rolled separately, then welded, if rollers are used. The other possibility is brake forming the curves by bumping. The advantage of the former is that each curve can be rolled quickly and uniformly, but requires 18" to 24" of extra material at each end. Brake forming is slower but requires little extra material at the segment end. Also in brake forming, depending on the size of the piece and radius of the curve, sometimes two segents can be blended in one work piece by changing the depth of the stroke to change the radius, but handling large shapes in a press brake can also be very awkward. The fabricator may know all of this, but it helps to get these things on the air in the design phase.
