I feel some pressure to find steadily more extreme examples of structure. Now that I’ve transitioned from stocky trusses to slender ones, what can come next but the spindly example seen above? (Off the main topic, but note the two men standing on the lower chord of the truss and waving, below the second and third cars back of the engines.)
This is the Union Pacific Rairoad’s Dale Creek Crossing in Wyoming, the longest bridge on the line, a mile and half above sea level, and between two tunnels cut into mountaintops. (The steep grades explain the double engine configuration, of course.) This section of line was constructed in 1868 (for the first transcontinental route) as a timber trestle; the iron bridge in the photo was built in 1876 by the American Bridge Company of Chicago. I don’t associate American Bridge with pushing the envelope on design but here’s proof that they sometimes did. The bridge was reinforced less than ten years later to carry heavier trains.
Note that the spans, which are fairly short, are inverted queen-post trusses. This is not a form of structure that is going to get you a long span, but it does allow for very light lower chords, which are pure tension members. In small bridges of this type, the lower chords are usually rods – they may be flat bars here for the heavy loads, but the picture makes it clear how slender they were.
If you look closely, you can see some of the guy wires used to provide lateral stability to the high and narrow bridge: thin white lines running diagonally upper left to lower right below the first three cars back of the engines. This is not cheating – another famous example of guys used in this manner is John Roebling’s Niagara Bridge, where they were placed below the deck to keep it from moving upwards – but it’s generally not as good a solution as having the bridge stiff enough and strong enough to work without them. The guys can relax over time and, since they are anchored down near the river elevation, are vulnerable to flooding and ice dams. They are a good solution if the site is difficult to reach (true in this case), if the logistics of construction are constrained (true), and if the lifespan of the bridge is known to be limited (maybe true). The entire history of the Dale Creek Crossing is 33 years, from the construction of the wood trestle to the abandonment of the reinforced iron bridge in 1901, when the UP changed the alignment of this entire section of line to shorten it and reduce the grades. Did the railroad know that they would abandon this bridge even as they built it? Maybe. Or maybe the assumption was that a newer and bigger bridge would inevitably be built as traffic and train weight increased. The history of railroad bridges in the US suggests that anything built before 1900 was temporary, whether or not the railroads and the design engineers knew it at the time. At some point, this obviously became general knowledge, the question is when.
But that is one skinny bridge.
