A Curious Misconception

The title of this 1890s photo is “The Levee Below The Bridge” and it shows the main channel of the Mississippi River at Winona, Minnesota. We’ve got a couple of riverboats tied up (the one on the left is registered in La Crosse Wisconsin), what appears to be Tom Sawyer fishing, and two bridges, one low and one high. The low bridge was a rail swing bridge constructed in 1871 and, amazingly, in operation until 1977. It was demolished in 1980. The high bridge was a road bridge, as you might have guesses by how light its truss work is. Here’s a postcard view of it that, by virtue of its view angle, makes the low bridge look like it’s hanging off the high bridge’s undercarriage:

The high bridge was a cantilever truss, constructed in 1892 in steel and wrought iron, and replaced by the extant high bridge in 1941. The reason that the old bridge was considered obsolete was not its load capacity, but rather the two right angles in its approach path, clearly visible in the postcard. Horses pulling carriages can make sharp turns that cars driving at faster speeds cannot. There were inevitably accidents as the traffic on the bridge switched over to cars in the twentieth century.

The reason the old photo at the top made an impression on me is the undulating curve of the upper chord. Most cantilever trusses in the US have had less smooth curves: the replacement bridge at Winona has a pair of sharp peaks where the top chord meets the towers, for example. That smooth curve may well have been less structurally efficient that a less-aesthetic shape – although it can’t have been too bad, seeing how light the truss members are – but I don’t care because I like the look.

Anyway, back to the title of this post: a lot of people, and specifically a lot of architects, seem to believe that steel structures lend themselves to straight lines and concrete structures lend themselves to curves. It’s simply not true. Curves in concrete are made by using curved formwork – or more often segmental formwork with the segments small enough to approximate a curve – and if you can make steel and wood forms follow a curve, you can make steel members follow a curve. You can also bend steel in the shop to actually be curved, although that’s less common. The beautiful curve of the top chord at Winona, for example, is a bunch of straight-line segments.

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