The photo above, showing the interior of the Chesapeake & Potomac Telephone Company Warehouse in Washington, DC, was listed in the HABS/HAER index under “mushroom columns.” That’s fine, as that is the term for the columns in this early form of two-way concrete construction. However – and as I type this you have to imagine steam coming out of my ears – the name is not about the flared column capital or the round cross-section of the columns, which are the visible identifiers of the system. The name – as Claude Turner, the originator of the system, makes clear on, for example, pages 37 and 38 of the 1919 version of Concrete-Steel Construction – refers to the steel reinforcement around the column. Here’s a picture of the reinforcement from one of Turner’s patents:
The rebar around the column is supported on a shallow skeletal dome of steel rods. Figure 3 shows that dome in plan and it’s seen in side elevation in figure 1. In modern terms, the dome is a chair, providing per-concrete support for the main reinforcing bars and pushing them up, at the column, near the top of the slab to handle the negative moments there. It’s a good design for the era and it’s justifiable even today. That shallow dome looks somewhat like a mushroom cap and that’s where the name comes from. Not the round columns, not the visible concrete column capitals, but from the invisible reinforcing.
This may seem like a trivial point, because it is. No one is building this system any more and its popularity peaked over 100 years ago. No one is reading Turner’s book as a design guide today, only as a historical artifact and to understand the old systems. But there is a larger point, luckily for me. Engineering, like any field, is full of “terms of art.” These can be technical descriptions, abbreviations of technical descriptions, or nicknames, among other things. Often they begin as the first, change into the second, and then change into the third. They are usually incomprehensible to outsiders but the second and third groups can be incomprehensible even to people in the profession if they don’t know the specific abbreviation or nickname.
The first conclusion is that good communication is important. The second, and the point I’ve been trying to get to, is that reading old drawings or descriptions of structure is fraught because we think we know the language but we don’t. Words, even terms of art, change meaning over time. I try to focus as much as possible on the pictures in old documents, because they are more trustworthy.