Historic Structural…What? The Metropolitan Floor

by Don Friedman on March 10, 2016

Continuing on the theme of hidden details, let’s talk about how structures end. Not how they fail, how they end. What happens when you get to the edge of a building, or the top, or the edge of a floor at an opening? Part of the basic logic of structural analysis is that forces have to be resolved, which is to say they have to go somewhere. In some cases it’s easy: the axial loads in columns end up (one way or another) pressing against the ground. In some cases, it’s not so easy.

I have somehow become an expert on the Metropolitan Floor System, an 1890s method of creating a fireproof floor using individual wires as catenaries embedded in a plaster-of-paris matrix. It’s a strange and fairly rare floor system, and learning all about it is one of the oddest things I’ve ever spent time and effort on. The floor carries load entirely through the wires acting in tension, with the plaster slab serving only as fireproofing for the wires and to provide a floor somewhat less transparent and somewhat more stable than a bunch of wires. Which leads to the question, what happens when the floor ends? How is the tension in the wires resolved?

Elegantly:

Figure 3

Bolt a steel rod to the structural frame, wrap the wires around it, twist the wrapping a few times, and you’re done! (Side note: there’s nothing wrong with the color on your screen. When I was using a film camera, I often used black and white film to reduce noise in bad lighting.) The tension in the wires is converted to a sideways pull on the rod, to a sideways pull on the beam top, which bears horizontally against the slab. One of the most efficient ways to end a force is to send it back against itself, which is what this detail does. A clever idea, back in 1896.

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