Skip links

Anthropomorphism and a View

That’s the Kuskulana River Bridge near Chitina, Alaska, and that is some setting. The forest and mountains go on forever, but here are a few stats: there are three main spans, the longest of which is 230 feet and the bridge is “238 feet” above the gorge. (The quotes are because those heights are always variable depending on the state of the river and exactly where you measure.) It’s a deck truss, so you’re traveling on the top, and it’s less than 15 feet wide. I’m fine with all that, but for anyone even remotely susceptible to vertigo, that’s a narrow path very high in the air with nothing around. The bridge was built for a railroad in 1910, informally converted to road use by having the rails removed in the 1960s, and rehabbed and formally converted to road use in 1988 – including the installation of proper guard rails – so traveling over it more than 34 years ago must have been quite the experience. Here’s the deck in 1982, from HAER:

Note the wood trusswork at the end. I didn’t find an explanation, but my guess is that there was damage to the short end span, from the end support of the end main truss span to the abutment, and it was simply removed. Here’s a better view of the wood:

Here’s a view of one of the main end spans. Note the double verticals at the steel-framed pier on the right: the three main spans are not continuous. Also note the intact short end span, apparently just steel girders, on the far left.

I was about to write “this an oddball truss configuration” but who knows? Maybe it’s one that’s known but not by me. There are six panels in the span, and the two in the middle are arranged as if it’s a Pratt truss and the two on either end are arranged as if it’s a Warren truss. If this hybrid has a name, I don’t know it or why anyone would use it. Maybe to deliver the load in the and panel diagonals directly to the supports? The heavy wind bracing of the top chords is visible here, as well as the location of the deck girders above the plane of the wind bracing. The center span is a more straightforward Pratt truss, with eyebars as the bottom chord at the center panel instead of the built-up boxes used elsewhere.

Nice gorge!

The other piece of the design that caught my eye, and the second half of this post’s title, is the way the piers between the main spans were built. This is a twentieth-century bridge, so it’s no surprise that the piers are steel, but they are notably tapered:

That shape – like the truss horizontal-plane bracing – suggests that wind design was taken very seriously at this exposed and remote location. The splay also feels like it’s a symbol of strength as much as it’s actually about strength, to the point that I’m tempted to use the phrase “man-spreading.” I try not to anthropomorphize structures, but that pier just feels like it’s daring me to try to push it over.