Uniformity

The spidery-thin bridge structures I’ve highlighted here have mostly been trusses of various types. Here’s a spidery suspension bridge: not only are the cables thin (it’s a shortish span) but so are the stiffening truss members and, notably, the truss-work towers. It’s the old bridge over the Little Colorado River at Cameron, Arizona. Unfortunately, HAER only took the picture above, which shows both the old bridge (the suspension bridge in the foreground) and the new bridge (the deck truss behind it) at an angle which makes them blend together.

The bridge was constructed in 1911 and closed to traffic in 1959 when the new bridge opened. It has had a second life, carrying a natural gas pipeline across the river gorge. The pipeline is affixed to the wind bracing between the top chords of the stiffening trusses and there is still a deck, but the bridge is closed to all pedestrians except, presumably, pipeline company workers. But it’s there, and it’s working, and it’s visible from the new bridge.

The bridge length and main span are pretty much the same – 660 feet – since the towers are mounted right at the edge of the gorge on each side. There are no side spans, so the cable back-spans run straight from the towers to the anchorages. The stiffening truss is a Pratt through truss; the truss depth is relatively large for the span because of the need for a minimum height for vehicles, so the bridge is reasonably well-braced for wind despite being made of such thin members. My favorite detail is that the deck-and-truss width entirely fits within the towers, so that the suspenders run diagonally in from the main cables to lower chord of the trusses. Taste is an odd thing, and for some reason purely vertical suspenders look wrong to me. I prefer them to be in a tilted plane (as is true here and at the Brooklyn Bridge) or to run diagonally along the length of the bridge (as at the Humber Bridge).

A lot of suspension bridges, probably most, have towers that are visually heavier than the rest of the structure. If we assume that the bridges were properly designed, having the towers look heavier makes sense in that they are primarily in compression versus the tension in the main cables and the (overall) bending in the stiffening girders or trusses. But that’s a philosophical position: it’s possible for the towers to be constructed of open truss-work and be strong enough to carry their loads. The Cameron Bridge gives a sense, at a small scale, of what a suspension bridge with a uniform appearance looks like.

Interesting tidbits from the National Register nomination (here) and fantastic and thorough photo documentation from the Bridgemeister (here).