The swing bridge that carries the west side rail line over the Harlem River is currently being repaired, so long-distance trains heading north have returned to their original home at Grand Central Terminal from Penn Station, where they’ve been since 1991. My two recent trips upstate gave me the opportunity to ride this route, which I haven’t been on in a while. My favorite spot is around 181st Street, when we pass under the Washington Bridge.*
In technical terms, the Washington is a two-span steel two-hinged deck arch with masonry abutments. There are six parallel arches in each span. One span passes over the Harlem River and the low riverbank on the Manhattan side; the other passes over the low riverbank on the Bronx side. The Bronx bank includes the railroad, built decades before the bridge, and a mid-1900s highway; and the Manhattan bank includes the road seen above, built in the 1880s as a pleasure drive called The Speedway and now used as a highway called the Harlem River Drive.
The spandrel panels – the area above the arches and below the deck – are what make the bridge special. Their structural function is limited to carrying gravity load** from the deck down to the arches, and therefore they could have been utilitarian and boring. Instead, they are utilitarian and fascinating, consisting of a dense three-dimensional grid of identical and thin built-up members. When seen head on, as on the left in the 1901 picture above, they fade almost away. When seen from an angle, as in the HAER survey photos…Piranesi couldn’t have done it better.
***
In my opinion, Cartesian coordinates have never looked so good. That’s personal taste, and that’s my point: the spandrels didn’t have to be designed this way. Had a different engineer worked on it, it would look different. Part of engineering design is making choices based on personal preference and then making the design work with the result.
* Not the George Washington Bridge over the Hudson, the Washington Bridge over the Harlem. Very, very different and the one I’m talking about is some fifty years older.
** And, maybe, some small amount of wind load applied on the deck’s narrow edge.
*** Note the gap between the steel and stone. The bridge flexes significantly more under load than the abutment does so the steel grid can’t be tied to the stone. If you look closely you can see the expansion joint in the roadway near the edge of the stone.
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