Overlapping Developments

Everyone seems to love covered bridges, but profile photos of them tend to be very boring: you’re looking at a plank wall. For most bridges, that’s one of the best angles. In any case, that’s the Bunker Hill Bridge over Lyle Creek, in Catawba County, North Carolina. And the inside view shows why I’m writing about it:

Most covered bridges have dense lattice trusses or Burr trusses (combination of trusses and arches) as their wood structure. This one has a very regular and more formal truss. (For those of you reading the graffiti, I hope Linda got the message.) We have ganged top and bottom chords, single verticals, and mostly double diagonals, with mostly wood pegs at the junctures. My first reaction is that this is a double-intersection Howe truss. “Double intersection” means that the diagonals cross two of the panels defined by the verticals; a Howe truss is one where the verticals are in tension and the diagonals are in compression. Howe trusses are inherently less efficient than Pratt trusses (where the forces are the other way around) but were semi-popular in the mid-1800s. Courtesy of HAER, here’s a good drawing of the truss:

HAER, and others, describe this as a Haupt truss. I’ve mentioned Herman Haupt a number of times, but this was the first I learned that in 1839 he had patented a truss. The problem, as discussed at the Bridgehunter link in the last sentence, is that it’s not entirely clear what he patented. Here’s the text of his patent:

It’s clear that he is reacting against lattice trusses. He doesn’t define “braces” and “counterbraces” although the context and the diagrams make it clear he’s talking about the diagonals. He’s substituting vertical “ties” (implying tension) for the counterbraces. Lines 66 through 72 seem to include any truss profile and any angle of brace, which is casting a wide net. Certainly it includes the Bunker Hill bridge. Here are his diagrams, with the page turned for better legibility of the trusses:

Version A looks a lot like Bunker Hill except for the long braces “d” and the fact that it’s triple-intersection instead of double-intersection. But if that’s a Haupt truss, what did William Howe patent the following year? The answer is: not what we call Howe trusses, but rather a truss form that’s either a lattice modified by adding verticals or four overlaid Warren trusses:

The point of similarity between “Howe trusses” and Howe’s truss is the rod verticals, meant for tension only. It seems that Haupt got there first but more vaguely. It’s important to note that I am not talking about Howe stealing an idea from Haupt: American engineers were struggling at that time to improve their understanding of trusses, to start the switch from timber to iron, and to build the bridges desperately needed in a large country with a low population density and a lot of rivers. I’m sure that there were other engineers thinking about how to improve on lattice trusses – like James Warren, who patented his truss in 1846, and Thomas and Caleb Pratt who patented their truss in 1844. Even at a time of rapid change in engineering like the mid-1800s, there are only so many possible structural forms, so overlap was inevitable.

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