The picture above shows the underside of a stair landing in an Old-Law tenement, with the structure exposed because leaks from above have caused the plaster to fall. The landing in the stairwell is fireproof because the stair had to be fireproof, and HEY! it’s my old enemy, the Rapp Floor. The link provides some history I wrote two years ago about this floor system. At the time I wrote that blog post, I had seen Rapp in four buildings; it’s now five.
There’s a real engineering argument for this system being as bad as I say it is. It’s composed of three main structural elements: (1) common bricks, which span between (2) inverted Ts of light-gage steel, which span between (3) ordinary steel I beams. Then a layer of fill was placed over the whole thing to provide support for the finish floor, and plaster was put on the bottom for fireproofing and as finish. In the photo, you can see the bricks clearly, the Ts are the brown double stripes running let-right, and the beams are hidden inside the box on the left ad at the edge of the landing on the right. Each brick spans individually between two Ts, each T spans individually between two beams. Neither the bricks nor the Ts are fastened at their supports. So there is no continuity of any kind.
As I described in the earlier blog post, there are some obvious potential failures: shifting of the bricks and Ts at any holes, including plumbing penetrations and other intentional holes; failure of the brittle bricks in bending from any minor defect in production; and corrosion of the thin light-gage Ts. These failure mechanisms also interact. For example, failure of a single brick in bending can allow the neighboring bricks to shift, which can allow one or two of the Ts to shift. Partial failure of a T from rusting can allow movement the same way.
In forensic-engineering language, every single brick and every single T is a “single point of failure,” whose loss can cause the loss of the entire floor by triggering the next failure. Not bad for a simple fireproof floor.