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Same Word, Two Meanings

Yesterday’s Curbed article on the sale of a luxury apartment in the old Police Headquarters on Centre Street had, by accident, a nice illustration of the difference between the way architects think and the way engineers think. The apartment in question is at the top of the north end of the building, and features a shallow dome. (Click through on the link above to see the pictures of the apartment. It’s very nice. Like me, you’ll want it and not be able to even pretend to be able to afford it.) In the 1940s tax photo above, you can see the dome in question left of center, just above the broken triangular pediment. The much taller central dome is in a different apartment.

As the Curbed photos make clear, the dome is not, structurally, a dome. Architecturally, a dome is a three-dimensional curved space, usually a piece of a sphere or ellipsoid, almost always with a curve that is continuously in the same direction (i.e., always convex on the outside, always concave on the inside). This dome is that. Structurally, a dome is a self-supported shell structure almost always built in masonry or concrete. This, very clearly, is not that. The dome structure is a series of quarter-ellipsoid steel ribs that span from the perimeter of the elliptical room to a support in the room’s center. That support is a steel columns (cruciform section, built up of four angles) supporting a ring beam through four knee braces. As long as you don’t mind the column in the dead center of the space, this is a relatively simple structure.

The building was constructed 1905 to 1909, at a time when steel fabrication was developing rapidly. My guess is that the structural design was driven by several practical issues. First, bending a steel beam through a 90-degree sweep was doable, if annoying, while the technology of the time would have been more hard-pressed to bend a beam through a 180-degree sweep. Second, without welding, connecting multiple ribs to one another at the top center would have been much more difficult. Third, using the ring beam without the column would have meant (a) that the ring beam would have to be designed for torsion, which was not yet practical at that time and (b) that the ribs would have had to have moment connections to the ring beam, which was doable then, but would have involved large and bulky brackets. Had the designers and builders been willing to deal with all these issues, they could have had some form of continuity of the rubs through the center, and the column would not have been needed.

So, it’s a dome but not a dome. Simple, right?