Very Temporary

I mentioned recently that the designers and builders of some bridges may know in advance that their work is temporary. The bridge above is an example of one that was meant to last only a couple of years. “U.S.M.” means the U.S. Military Railroad, which ran portions of captured railroads for the Union army during the American Civil War. This road bridge was built in 1864 by the army and was the first crossing of the Tennessee River at Chattanooga. It stood for two years after the war ended, but was destroyed by a flood in 1867.

In general form, the bridge greatly resembles the historic masonry arch bridges of Europe, but there’s one obvious difference: it’s entirely wood. The deck arches are assemblies of logs and the river piers are timber grillages, as can be seen more clearly in this view:

It’s got a slightly weird look to it, being a log structure in what is usually a masonry form. Since it was built of green lumber, the deck, the arches, and the upper portions of the piers would inevitably loosen as the wood dried and shrank, so there is no way it could have survived without extensive rebuilding. It was almost certainly not designed for flood loading – I’m not sure it was designed for gravity loading as opposed to assembled using rules of thumb – so it was inevitably going to be damaged in a flood.

It may not have functioned as an arch bridge. Depending on how the logs were fastened – most likely lashed and spiked together – the diagonals between the deck and the bottom arch may have enabled the logs to act as pairs of cantilevers springing from each pier. Again, I have my doubts that there was any numerical analysis and design of the bridge.

What makes a successful design? This bridge worked as well as was needed for as long as was intended. It was built quickly and inexpensively and did little damage to the surroundings. All structures should have such a good record.

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