Whether You Want To Or Not

Some days, the new thing I learn that day is something that is annoying to the point where I regret wasting the 30 seconds it took to learn it. Today, I learned what a “half-hip Pratt truss” and I’m going to inflict it on anyone reading this.

The two most common configurations in US bridges of the nineteenth and early twentieth century are Pratt and Warren trusses. Warren trusses have a very pleasing symmetry about them: the web members connecting the top and bottom chords are all diagonals, alternating slope direction so that the truss is series of triangles. (The upper right diagram in the illustration above, from Milo Ketchum’s The Design of Highway Bridges, is a Warren truss.) A Warren truss is most efficient if the diagonals are all at 60 degrees, so that the triangles are equilateral, but having a slightly different angle doesn’t matter that much. There’s a popular variant where there are verticals added to the web at every panel point, and there’s a slightly less popular variant where there are verticals at half of the panel points, which I’ll get to in a parenthetical below. The diagonals in a Warren truss have to be designed to carry both compression and tension, with the actual forces depending on how the load is applied at any given moment.

Pratt trusses have alternating verticals and diagonals, with the diagonals set so that they will be in tension. (Under uniform and most point loads, that is. There may be some loadings where they get some level of compression.) The lower right diagram in the illustration shows a Pratt truss.

Another important aspect of truss bridges is where the deck is relative to the truss. The most common arrangement in the century-or-more-old era I’m talking about was a through truss, where the deck is at or near the bottom chords of the two side trusses, and there’s some kind of wind bracing connecting the trusses at the top chords, so that the deck is inside a lattice tunnel. Or the deck can be at or near the top chords, which is a deck truss. Or the deck can be at the bottom chords but the trusses are not connected at the top (usually because they’re too short to be connected without interfering with vehicles on the deck), which is a pony truss. Pony trusses are common with short spans, where deep trusses are not needed.

(If you have a Warren through truss, you might add verticals that start at the upper-chord panels points, as they will cut the lower-chord spans in half and help transfer load from the deck to the truss. If you have a Warren deck truss, you might add the verticals that start at the lower-chord panels points, as they will cut the upper-chord spans in half. Or you can add both sets of verticals.)

A half-hip Pratt truss is a configuration that only remotely makes sense for a pony truss, so it’s a “half-hip Pratt pony truss” which is starting to sound like an order at a fancy coffee shop. It’s distinguishable from a Pratt pony truss because the last vertical at each end is missing, usually accompanied by a slightly shorter panel. The middle-right diagram in the illustration is a half-hip Pratt pony truss.

The reason for using that odd variation is to save the expense of the verticals at the end. That’s it: just cheeseparing. It only works for small trusses because the necessary increase in strength in the last diagonal would outweigh the benefit in a large truss. There’s nothing wrong with it, other than looking weird to anyone who knows trusses, but there’s nothing much right with it either. Here’s an example, the Sprague Bridge over the Yellow River in Juneau County, Wisconsin:

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