A Different Option

by Don Friedman on November 18, 2016


If you work on a lot of big apartment projects, as we do, you work on a lot of spiral* stairs. We’ve probably design twenty-five or thirty spiral stairs over the years. There are two design problems: (1) making a curved hole in the floor and (2) making the curved stair itself.

The hole is easy in steel-framed building and less easy in concrete buildings. The stair can be anywhere from moderately difficult to amazingly annoying, depending on its geometry and where we have access to provide supports. The most difficult stairs are the ones that are free-standing and curve more than 180 degrees in plan.

The stair above is one of our oddest because the building’s rules did not allow the stair to be fastened to the floor at the bottom. That was a huge problem for our normal design** because we count on the connections at top and bottom to be restrained not just against movement but also against bending moment in both axes perpendicular to the floor and torsionally along the stair stringer. The only good news was that the stair was to have partitions surrounding it rather than being free-standing.

The final design has each tread hung from the floor above By using L-shaped hangers that come down and then turn under the tread, we eliminated the need for an inside-edge stringer, at the cost of having more complicated hanger design. The hangers will eventually be hidden in the surrounding partition and the stair soffit. It’s a strange way to get the desired aesthetic, but it works. Shaquana spent a lot of time modeling that stair to be sure that it is strong enough and stiff enough. The latter issue is sometimes neglected in stair design, but we’ve found that people become very, very unhappy when they feel their stairs bounce.

* Technically, these stairs are helical, not spiral, but the pedant army lost that battle long ago.

** Our preferred design is a stringer designed for bi-axial moment and torsion, or in other words a member that has resistance against bending in every possible direction.

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