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A Trick Of Scale

That’s the North Platte River Bowstring Truss Bridge, AKA the Fort Laramie Bridge, in Wyoming. It’s a good-looking bridge in general and that photo, probably taken near sunset, makes it look especially nice. I’ve had a browser tab open to this bridge for a long time while I’ve written about other topics, and the tab showed one of the HAER measured drawings:

Measured drawing of bridge elevation

It’s three bowstring arches with double diagonals as bracing between each hanger. Given the relative size of the arch and the bracing, this is definitely an arch bridge, not a truss. The top chords are built-up boxes and the bottom chords are double chains of eyebars:

Drawing of details of iron bridge construction.

Both drawings have scales clearly labelled, but for some reason the profile view that I’ve been glancing at for months feels much bigger than it actually is. 125 feet is a very short span for an iron bridge and a fairly short span even for a timber one. In any case, the 1875 construction date, which is still early days for iron bridges in the US, shows up in ways other than the span. The use of multiple specialty castings to make the connections work, for example, is something that died out by the 1890s. Cast iron’s brittleness and the need to make specific castings for each bridge as opposed to using the adaptable nature of generic rolled shapes doomed that kind of detail. The other detail that made sense at that location at that time but looks weird now is having the braced iron piers sitting on timber cribbing. This was access built by the army for low levels of traffic: it’s hard to believe that the people who designed and built it (at the King Bridge & Manufacturing Company) thought that it would still be in existence almost 150 years later.

Only one span is across the river proper, with the other two extending over the portion of the river valley that might flood. From the deck, that distinction is not clear:

Bowstring trusses, like all arch bridges, are inherently pony trusses, since there’s no way to include full top-chord bracing that doesn’t interfere with travel. The top chord bracing at the middle of the spans has, from this angle, a weird telegraph-wire feel to it. I also suspect that the wood handrail came later, as our notions of safety really didn’t exist at that time. The juncture between the handrail and iron is quite awkward…

…although my hat’s off to the carpenter who carved out the end of that wood beam to fit around the large nut that secures the eyeball chain to the cast-iron bearing shoe. The riveted box upper chord is the most modern-looking part of the assembly, seemingly from the distant future of 1910.