I’ve talked about Phoenix columns several time – built-up circular or polygonal sections of wrought iron or steel – but this bridge is a spectacular use of them. The bridge actually has at least three separate claims to fame, which is impressive. It’s the 1873 Kern Truss Bridge over the Le Sueur River near Skyline, Minnesota.
To start with some physical features, it’s a through bowstring truss, which it say that it’s a tied arch, with the tension lower chords also serving to support the deck beams. There are bracing connections between the center nine (out of thirteen) top-chord panel points, so the top chords are well braced against lateral load and compression buckling. As is true for nearly every truss bridge I’ve discussed, the bigger members are built up by riveting; in this case the compression top chord is a phoenix section (hollow, polygonal, built-up) or at least phoenix-like. The hangers from the top chord to the deck are wide built-up lattices; half of the lateral braces at the top are impressively deep built-up lattices.
The list of reasons this little bridge is famous… First, at 188 feet between abutments, it is apparently the longest bowstring through truss in the US. That’s not so surprising, as the tied-arch form has little to recommend it over either an ordinary truss or an arch with full bearing for thrust on its abutments. This bridge is a hybrid between arches and trusses, and those two forms went their separate ways.
Second is the use of phoenix sections for the arch. This was an inherently good idea given the state of wrought-iron technology in the 1870s and would still be a good idea today.
Third, the designer was Squire Whipple. In one sense, this is not surprising as Whipple was a proponent of bowstring trusses, and had a patent for one type. I first encountered him when I was looking at the history of structural analysis in the US. That topic is quite different from the history of structural analysis: engineers and mathematicians in Europe were far ahead of their home-grown US counterparts through most of the nineteenth century. Whipple wrote the first book by an American and published in the US that provided an accurate method to analyze trusses. He did not invent that method, but the publication of his book was the beginning of the end of the practice of all engineering textbooks used in US schools being imported. As it happens, his book was unclear – he was a better engineer than writer – and the second book with a useful method of truss analysis, by Herman Haupt, became more popular.
That’s a lot of history to dump on one bridge. It is in limbo now: at the time of the HAER survey in 2017 it was closed to all traffic; in 2020 it was disassembled and stored, waiting for restoration and a new location within Minnesota. The history of disassembled structures is not encouraging, with some properly rebuilt, others abandoned, and at least one stolen. Hopefully this bridge gets a new home.
Permalink