Interesting Because It Is Ordinary

Today’s post and a few more to follow are going to focus on the Nevius Street Bridge in Raritan, New Jersey. It’s simultaneously historic and interesting, and utterly common and boring. It was built in 1886 during an era when thousands of similar truss bridges were constructed across the United States. There is nothing in particular about its origin that is unique. However, it has survived its obsolescence and the effects of weathering, and so has made it onto the National Register as a historic structure. I was introduced to the bridge by chance, but had the opportunity to look at it a bit.

The formal description of it makes perfect sense if you know your bridge lingo: it’s a “two-span, 10-panel, double-intersection Pratt, pin-connected, through-truss bridge.” Each one of those adjectives has a specific meaning. There are three piers that support the bridge, one at each bank of the river and one mid-stream, so the bridge has two spans, from the first bank pier to the middle, and from the middle to the far bank. (Note that the Wikipedia description is wrong. It says that the bridge has a 150 foot length, when it’s actually 300 feet, with each span being 150 feet.) Each span is divided by vertical members into ten pieces, or panels. A Pratt truss has its members arranged so that the verticals are in compression and the diagonals in tension; since the diagonals are longer, this is more efficient since the tension members are not subject to buckling. A double-intersection Pratt truss has diagonals that span two panels (crossing a vertical) rather than simply running across one panel. Pin-connected members are not riveted to one another, but rather have eyeholes that circle around cylindrical pins, allowing (in theory, any way) the connections to rotate under load in exactly the way the analytic model says they will. And a through-truss is one where the roadbed is at the level of the bottom chords and the top top chords are connected overhead, making the bridge into a latticework tunnel. All of these descriptors represent design choices by the engineer (F. A. Dunham) and could have been different, creating a bridge with a different appearance and different performance characteristics.

It is incredibly light even though it was originally designed for vehicular use. (As the benches show, it’s now restricted to pedestrians.) This is part of the rise of the structural engineering profession in the nineteenth century, with the use of analytic design allowing for the construction of structures that were both stronger and lighter than their predecessors, like the wood bridge that used to be here.

Tomorrow: member design and construction.