Bending Corten Steel to Fabricate a Rectangular, Helical, Stair Stringer

A state park committee in Logan, Ohio, had a vision of a monumental tube-steel, circular stair way with an intermediate platform for viewing one of their many spectacular waterfalls. They wanted to use self-weathering steel for its aesthetic and sustainable advantages over conventional A500 tubing.

Unfortunately, a significant design oversight occurred when it was revealed during the bid process that mills do not produce a 20x4x1/2 structural tube in weathering A588 Corten material. Rather than reverting “back to the drawing board” and dragging the design team over the coals, the committee found a local, ornamental steel fabricator who alone was willing to take on the challenge.  The fabricator’s solution was to pair up with a skilled bender/roller to fabricate from Corten plate four-piece, built-up rectangular tubes curved helically to serve as the stair stringers.

With the top and bottom, 4in flat section members identical, rolling these mill bars the hard-way (against the strong axis) helically to match the 19in side walls at a right angle was not easy. Highly skilled and trained machine operators took frequent measurements to insure a precise radius-to-pitch fit.

To complete the next (2) members of this built-up tube and limited to 20 foot lengths, (8) 19in side walls were rolled to 4 individual, inside and outside radii, with (8) varying, rise/run pitch degrees.  Separated by 3 inches, the inner wall will have a different radius and pitch from the outer wall to maintain a parallel rise.

Side Walls for Helical Stair Stringer

With a jig-saw puzzle of loose, rolled, identified arc lengths and “miles” of welding and grinding ahead, the fabricator cut a series of steel spacers secured to the inner surfaces to maintain a consistent 19 x 3 cavity dimension.

Corten Plate Rolled and Identified
Steel Spacers Helped with the Fabrication
Close Up of Stair Stringer of Built-Up, Helical Tube
The Steel Fabricator Knew There Would be a Lot of Welding and Grinding

Progress photos indicate the roller/bender supplier exercised their engineering talents to transform (4) plates into a single helical, steel stair stringer.

The superior corrosive resistance of Corten over regular carbon steel will allow this self-weathering, curved staircase to naturally conform to the surrounding environment. Furthermore, maintenance costs will be significantly reduced because the material does not need to be painted.

Teaming up with the right fabricator and bender/roller can make an enormous difference when you’re building a challenging monumental, curved staircase.

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