Classification Is For Us Only

Or, more directly: physical objects don’t care how we classify them. When I talk about structures here (or elsewhere) I try to classify them in a way that would be useful to a reader: a building has a frame or it has bearing walls, a bridge has a Pratt truss or it has a Howe truss, etc. But for a lot of structures, quite possibly a majority, the classifications are fuzzy. The actual physical object doesn’t perfectly fit into one class or another, but the whole point of classification is to make everything fit into our classes, so we make it fit.

The picture above (courtesy of Magicpiano) shows Babb’s Bridge between Windham and Gorham, Maine, over the Presumpscot River. (See below for a little history on the bridge.) It’s listed in the HABS/HAER index as a queen post bridge, but that’s a little hard for me to see. Here’s a longitudinal section which conveniently doubles as a truss elevation:

The problem with calling this a queen-post truss is that the “queen posts” are wrought-iron rods clearly intended to work in tension. If I simply looked at the drawings without knowing anything else, I’d call this a Howe truss, where the web diagonals work in compression and the web verticals work in tension (the reverse of the more common Pratt truss). There’s nothing wrong with calling it a queen-post truss, and a lot of covered bridges use queen-post trusses as their main structure. To make things more complicated, William Howe only patented his truss in 1840, the same year this bridge was built. So either he had a hand in it, which is not known, or his patent reflected an idea that was circulating generally, part of the zeitgeist. But do the trusses really work that way? Here’s a view of the side:

Vertical boards and battens sounds an awful lot like a way of making a wood diaphragm. If it weren’t for the possibility of something going very wrong, I’d love to try, as an experiment, disconnecting some of the truss web connections to see if the sheathing is capable of acting as a substitute.

So, is this a queen-post truss (based on similar bridges, the typology of wood covered bridges, and geometry), a Howe truss (based on the truss force diagram), or a wood plate-girder bridge (based on continuity of the wood sheathing, and me playing devil’s advocate)? Here’s a better question: does it matter? I can’t answer the first question, but I can answer the second: it may matter to us, but it doesn’t matter to the bridge.

So, the bridge in the photo isn’t the 1840 covered bridge at this spot. It’s a replica, built in the 1970s using traditional wood-framing techniques, to replace the original which was burned “by vandals” in 1973. The HABS drawings date from 1937 (HAER hadn’t been invented yet) and were presumably used in the reconstruction. The HABS website and the 1974 data card from HABS refer to the road crossing the bridge as “Harry Cane Road” while the state of Maine calls it Hurricane Road. New England accents can be confusing.

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