That is the Hamden Bridge over the Raritan River in southish, western New Jersey. It was built in 1858 and is, unfortunately, gone since 1978. By modern standards it was quite small and to our eyes looks almost cute, but it represented a turning point in structural engineering in the US. Rational analysis of trusses was still new in this country in the 1850s, and the use of wrought iron for tension members and cast iron for compression members shows a level of sophistication missing a few years earlier. Finally, the Fink truss type was an invention, meant to be designed for the specific loading and span, rather than a tradition guided by rules of thumb.
If you look at a drawing of the truss in elevation (as in the upper picture below) there is little difference between the actual physical layout and the force diagram.
The thin lines represent wrought-iron rods carrying tension and the thick lines represent cast-iron shapes carrying compression. The diagonals that span two and four panels are in-between in graphic weight and are in-between in realty: they are rectangular-section wrought-iron. The biggest problem with this diagram is that it represents an unbuildable idea: for the truss to work properly, the pieces have to join at the nodes (where the verticals meet the top and bottom chords) but not where two diagonals cross, and the two-dimensional picture doesn’t allow that to happen. Here’s a photo of the upper connection at the end vertical (the portal):
The vertical is…vertical. The piece running off to the upper right, with the name of the builder (the Trenton Locomotive and Machine Manufacturing Company) cast into it is the top of the portal, connecting the two trusses across the roadbed. The heavy piece running diagonal to the lower part of the right edge of the picture is the cast-iron top chord of the truss. The thin rod running to the right edge is bracing in the plane of the top chords, running between the two trusses. And the fire diagonals running down to the bottom are three separate truss diagonals: the small rod is a diagonal within the first panel, the two steeper-angles bars to its right are diagonals that cross the first two panels, and the other two bars cross the first four panels to the mid-point of the truss. The three diagonals (first panel, first two panels, and first four panels) can be seen in the truss elevation diagram above.
Since the diagonals can’t physically be in the same place or they won’t be able to cross, there are three locations: the first-panel rod is at the centerline of the vertical and top chord, the two-panel bars are symmetrically around that rod, and the four-panel bars are symmetrically around the first pair of bars. There are five separate planes in which the diagonals run to prevent interference. The plan and elevation details at the upper left of the drawing below show this:
A lot more practical thought and cleverness went into the design of those details than were required to analyze the truss. As I’ve said before, math is the easy part; making the thing work in reality is the hard part. We spend far more time working on details than we do on analysis.
Finally, a couple of bonus pictures of an intermediate top-chord connection and the bridge as a whole:
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