It Takes A System To Be Ordinary

The Wilson Shute Bridge over French Creek, south of Meadville, Pennsylvania is similar to a lot of the truss bridges I’ve discussed here. It was a steel Pratt through-truss, constructed in 1889, listed on the National Register and surveyed by HAER in 1988, and demolished in 1997. As the pictures below show, it made heavy use of built-up members and specifically lattices of small bars. In my opinion, it was better-looking than the average, but that’s as subjective as an opinion can be.

Here’s a general view:

Note that there are secondary diagonals that blur the Pratt configuration, present to help address unbalanced loads on the deck. Here’s the end view, showing the lattice portal frame at the end, the lattice cross-deck bracing girders at the top, and the double-diagonal wind bracing:

Here’s a view of the deck girders, stringers, wood deck, and double-diagonal wind bracing below the deck:

And finally, a lower-chord pin connection, with the deck girder hung by a U-bolt:

The mistake is to think of this as a bunch of unrelated pieces: eyebars, lattices, U-bolts, even the wood deck. This is a complete structural system, which is why this type of bridge was so popular for so long. Engineers and builders like to make things work, even bad ideas, so almost anything can get built once or twice. People will find a way to make it work. But for a design to be built hundreds or thousands of times, it has to make sense – more on this below – without being jimmied to fit. Spidery-thin Pratt trusses will work if secondary bending stresses in the web verticals and diagonals are minimized, which means pin connections. There’s no obvious way to fasten a pin to a large built-up plate girder parallel to it, so it makes sense to hang the girder below the pin. In the era before real reinforced-concrete, it made sense to use wood plank for the deck, but a plank deck has very little stiffness as a diaphragm, so you need wind bracing at (or just below) the deck the same way you need it at the top. And so on…there’s a specific design logic to every piece of it.

So these bridge were complete systems, but much of the logic is no longer true. A horse-drawn vehicle in 1889 was unlikely to have enough speed or weight to damage the trusses in an accident, unlike a modern car. The custom-made eyebars, and U-bolts are now specialty items and far more expensive relative to steel in general than they were then. Wood is no longer used for roadbeds and sidewalks. As parts of the system age into “the past”, the system gets harder to repair and, perhaps more important, people begin to see it as obsolete and not worth saving.

When I said that a design has to make sense to become a popular system, I meant that it has to make sense on its own terms, which include the context of the time. What technology is available, what are the relative costs of labor and materials, what lifespan is expected or desired for a structure? Since the context changes over time, we may not see the logic of an old system at first. A large part of engineering investigation is getting into the mindset of the people who designed and built the old building, bridge, dam, whatever, so that you can see what they were aiming for.

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