What Matters Changes

While trawling through the HABS/HAER index, I came across “Partridge trusses.” I know a little bit about trusses and I’d never heard that term, so I followed up on it. Reuben Partridge was a carpenter based in Marysville, Ohio, near Columbus, with a career spanning from the 1830s to his death in 1900. He got a patent on a truss in 1872 and it was apparently used mostly for wood covered bridges. Wikipedia has him responsible for 90 percent of the bridges in Union County (where Marysville is located) in 1882, with 125 by 1883. His career as a bridge builder was mostly close to home and specialized on one type, which makes sense as long that type is needed and used in that location. The picture above is a nice three-quarter profile of the Pottersburg Bridge over Big Darby Creek in North Lewisburg, Union County, Ohio. He designed it in 1872 and it has his eponymous truss.

So far, a success story: local boy makes good, builds a lot of bridges with a truss he invented. The problem is that I can’t figure out what a Partridge truss is supposed to be. Or rather, I can, since I read his patent description, but I can’t figure out why he thought it was any different or better than other existing truss forms. Rather then quote the stilted patent language, here’s a summary: the top and bottom chords are double timbers, with a set of posts connected between them. The posts, rather than being vertical as we would expect from the later language of truss bridges, are inclined at 60 degrees from the chords, with the bottom ends of the posts closer to the center of the truss span. Then a set of paired braces – in the same planes as the paired top and bottom chords – were used, at 45 degrees to the chords, with the top ends of the braces closer to the center of the truss span. That’s it. Here’s the patent illustration:

Here’s a HAER drawing of the Pottersburg bridge, with the interior longitudinal section A-A showing one of the trusses in elevation:

And here’s a HAER photo showing the trusses:

Note that those tension rods came later, as reinforcement.

It’s hard not to see the other truss forms this resembles: it’s a very open lattice, or a double-diagnal Warren missing two diagonals near the center. The use of the words “post” and “brace” obviously meant something to Partridge, who got to bridgework from a more general carpentry background. Perhaps he pictured each post as taking compression from the top chord down to the bottom, and the braces’ job was to keep the posts from rotating toward the center of the span as the bridge defected. The Wikipedia entry for Partridge trusses is notable in that it lacks the kind of physical explanation that most of the other truss types have. I think I’m not the only person wondering why this type exists and what makes it special.

It’s worth noting that a lot of the truss-bridge patents issued in the mid-1800s are somewhat vague and often focused on geometry. With a very limited selection of analysis tools in general use in the US, and many bridges designed and built by people like Partridge without formal training in engineering, the designs were often driven by geometry, rules of thumb, and a reliance on past designs that worked. As numerical analysis took hold, the logic of specifying minutia, like the angle of a brace, as part of a definition of the truss type gradually faded.

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