Rare With Some Reason

The bridge in the photo above is the Austin Peay Bridge over the Cumberland River at Gainesboro, Tennessee. It was a three-span through-truss bridge constructed between 1926 and 1928, and the reason it’s worth talking about (the reason that HAER spent the time to document it) is that the larger center span was a K-truss.

There’s a certain progression of truss form. The Pratt Truss, at least an idea in the US, goes back to 1844 when Caleb and Thomas Pratt patented their truss. Here’s the basic idea, from the handy HAER Truss Identification Poster:

The dashed diagonals were sometimes added to railroad bridges because off-center loading could cause compression in the regular diagonals. The Pratt is the ancestor of the Parker truss, which partially equalized the chord stresses by increasing the depth at midspan:

And the Parker is the ancestor of the K truss, which changed the bracing pattern to shorten the unbraced length of the compression verticals:

The K truss had diagonals and verticals half the length of an equivalent Parker truss, but at the cost of more connections and more members in compression. So the trade-off was smaller web members in exchange for more connections. Depending on the relative costs of steel material and of fabricating the connections, the K could be cheaper than the Parker, but it generally was not. Except in the very early days of steel in the 1860s and 70s, material has been cheaper than fabrication. The K was not a bad idea, just a more or less uneconomical one. It was better and cheaper for long spans than a Pratt or Parker truss, but less good than cantilever trusses, leaving it nowhere to be popular.