That’s the 1871 Ponakin Road Bridge over the Nashua River near Lancaster, Massachusetts. It’s not unique but it’s close enough that I’ve never seen this particular form before. The HAER description calls it a “Post truss,” named after Simeon Post. Supposedly these were common in the 1860s and 70s, which means most of the bridges that used this form would likely have been demolished by 1900 or so. The drawings give a good sense fo the profile of the truss, along with some typical connection details:
If the compression members in the web were vertical, instead of being tilted toward the center (imagine cutting the top chord at node U5 and pulling each side until the two halves of that node were directly over nodes L5 and L6, and then putting in an extra piece of top chord between the two halves of U5) this would be a double-intsection Pratt truss. That’s a Pratt truss where the diagonals span two panels, which is not an overly common design, but they exist. Try as I might, I can’t come up with any way in which tilting the compression verticals of a Pratt improves the design; it makes the analysis more annoying because it introduces a horizontal force component in those members that has to be accounted for. Any design change that makes me use more trig during analysis is bad.
Here’s the center panel:
People were experimenting with truss designs in the mid-1800s, and this is far from the strangest truss design that was invented then and far from the least useful. It just has nothing much to recommend it over the more common Pratt truss, which was easier to build and analyze.
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