Hanging

The last of my “what is that” pictures from Providence is from inside City Hall:

The building, completed in 1878, is big and quite ornate in the public spaces I saw. It’s a rectangle in plan with a big atrium in the middle; I took the photo from the first floor (half a flight of stairs up from grade) looking up through the atrium to the skylight above. Note that the atrium looks like it’s three stories high but one of those stories is two stories of office space on either side. Any way…how about that stair?

The stair from the first floor to the second, and the stair from the second floor to the “third” both have the same configuration: a straight run from the lower floor up, parallel to the long axis of the skylight, then a landing and stairs up at right angles to the left and right to meet the upper floor. That layout would be quite easy to build if the platform were supported from the general floor structure, but the platforms are not supported from the floors. They appear to hang in mid-air, in a manner that must have wowed everyone in the 1870s.

There are two ways to make this stair structure work. One is to have the stringers of the lower run of stair continuous with the beams that carry their lines across the landing platform. The far ends of the beams would be hung from the floor above, and a moment connection would be required where the stringers “bend” from sloped to horizontal. The other is a similar idea, only the bent beams would be the stringers on the two upper runs of stair combined with beams running across the landing left to right. In that scenario, each bent beam would require two moment connections, one at each side of the platform.

There are beams on all four sides of the platform, so that doesn’t tell us which way the stair is supported. And it looks like each stringer is part of a cast-iron assembly with the handrail above, which would make the moment connections into pretty simple three-foot-high rows of bolts where the stringer/handrail assembly meets the platform newel post.

Since it’s city hall, I’d expect that there are drawings – original and/or alteration – in a cubbyhole somewhere. Short of that, you’d need a very close look at the underside of the platform to be sure.