I’ve shown some heavily-built truss bridges here, and some lightly-built ones. The bridge above, the Turtleville Iron Bridge over Turtle Creek in Rock County, Wisconsin, is one the very light end of the spectrum. (It also has the name most likely to be stolen from Dr. Seuss of any bridge I’ve discussed.) It’s an 1887 wrought-iron truss, and was built right near the end of wrought iron’s use. Had it been built ten years later, it definitely would have been steel; five years later and it would very likely have been steel.
The heavy/light split largely follows the use of the various bridges: the heavy bridges are mostly for trains, with the concentrated load of a locomotive and the potential line loading of a series of full freight cars being the governing factor. The light bridges are pretty much all road-only, and generally built before cars and trucks, when loads were limited by what a team of horses could pull. This is a road bridge and, amazingly, still in use, albeit with load restrictions. A look at a map shows two heavier and more modern ordinary road bridges nearby, along with an interstate highway. In other words, the bridge has survived by not being heavily used.
In form, it’s the familiar Pratt truss with built-up compression members and eyebar tension members, although the eyebars are exceptionally thin. Here’s a close-up view showing those spidery eyebars, as well as making the river look especially Seussish:
Here’s the connection at the first bottom panel point in from the abutment:
Nothing special when compared to other late-nineteenth-century truss bridges, but well done. I like the double inverted-U bolts carrying the deck girder from the pin through the eyebolts.