Today’s blog post started out as one topic and became something very different. The starting point: I associate lattice trusses with wood covered bridges and short-span wrought-iron trusses. So seeing the bridge above, the Gateway Bridge built by the Union Pacific Railroad over the Weber River near Ogden, Utah, surprised me. That’s a big steel lattice truss for a railroad bridge, particularly a deck bridge, where the steel work is entirely below the tracks. So I was going to talk about how structural forms are linked to materials. Some are tightly linked, like ductile metal and catenaries, some are loosely linked, like arches and masonry, and some are not really linked at all, like trusses. But never mind that: when I started going through the pictures of this bridge, things got more interesting. Here’s a close up of one end of the main span:
My first reaction is that’s a rat’s nest of steel. (That tangle is made worse by the fact that there’s a second similar bridge beyond.) My second reaction is that I’ve been misled: the truss nearest us is a lattice, and I think I see a lattice on the far side, but there’s quite clearly a warren truss in the middle. There’s also a lot of cross-deck bracing making it harder to read the trusses. Here’s an end view of the main span:
The vertical latticed members nearest us – the sunlit prominent vertical in the center and its companion on the far left, support the plate-girder end span on the left, and are not part of the main span. They are, however, in the same plane as the two lattice trusses. (One lattice is on the right side of the bridge and can be seen in a lot of detail; the other is in shadow and behind a lot of bracing on the left.) The warren truss is halfway between the two lattices: its near-end vertical can be seen directly behind the crossing of the wind bracing on the left end of the main span.
I’m not going to say that no one ever designed a deck bridge with three trusses, but it’s weird. The fact that the center truss has a different form than the side trusses is weirder. As is so often the case, Bridgehunter provided the answer. The last photo there for this bridge is a postcard that was apparently fairly popular, since I was able to find a bunch of copies:
The plate-girder approach span is on the right, so we’re looking at the opposite side of the main span than the photos above. And there it is: the lattice trusses are present but the warren truss and the cross-deck wind bracing are not. So my off-the-cuff reading of the history is that the bridge opened in 1899 with the lattice trusses and was reinforced (most likely for heavier engines) later on. (Since the second bridge is not in the postcard picture, it was likely built for a double-tracking; since the second bridge seems to have warren trusses, it may well have been built as part of the same work campaign that reinforced the original bridge.) The warren truss was either put together between the lattices or maybe slid in place from below in one or two big pieces. The cross bracing in this scenario serves two purposes: it acts as wind bracing, which the lattices were a little light on, and it ties the three trusses together. Triangulated bracing connecting the three trusses allows them to share load by forcing them to deflect more or less in lock-step, which forces the load from the lattices to the warren truss. Neat.
The ending point: you could upgrade the old truss bridges if you really wanted to.
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