While looking through the HABS/HAER index, the item “parabolic arches” jumped out at me. There are only five surveys with that keyword: the bridges over the New York State Barge Canal (the renamed and expanded Erie Canal) in the Genesee Valley Park in Rochester; the Sixteenth Street Bridge over the Piney Branch Parkway in Washington DC; the Fountain Island Bridge in Fond du Lac, Wisconsin; the Bayonne Bridge over Kill Van Kull between Staten Island, New York, and Bayonne, New Jersey; and the Henry Hudson Parkway in Manhattan and the Bronx. That’s the Bayonne Bridge above.
The Genesee Valley bridges, the Sixteenth Street Bridge, and the overpasses on the Henry Hudson are all concrete arches. The Fountain Island Bridge is a cute Pratt-bowstring pony truss in wrought iron:
The Bayonne Bridge is a huge steel arch, with the upper chord and trussing acting solely as stiffening for the load-bearing lower chord. If you look at the near abutment on the top picture, you can see how the top chord simply ends in that odd rectangular prism of steel framework at the end of the arch. That steel was supposed to be clad in granite, to give an impression of being a monumental abutment. The granite was eliminated during construction as a cost-savings measure, leaving the result more honest but a bit gawky.
A parabola is a very efficient shape for an arch that is, overall, subjected to uniform loading. Efficiency matters with unreinforced concrete or masonry arches, where bad geometry can lead to local points of tension in an arch. On the other hand, the Fountain Island Bridge would be more efficient if the upper chord were straight between panel points. This is also true of Bayonne, but I’m not sure that it’s not straight between panel points.
In any case, there’s really very little need for the parabolic geometry with a steel or wrought-iron arch. First, for a low-rise arch, the difference between a circular arc, a parabolic arc, and a catenary arc is small. Second, a steel or wrought iron arch can handle some bending stress in addition to the arch compression. Too much local bending will cause a failure from combined stress, but small amounts, that would cause worrying cracks in masonry, are fine. Whatever slight gain there is in efficiency also had to be weighed, in the pre-computer days, against the pain of creating parabolic geometry versus a circular arc.
This whole discussion is for the engineers. If two Bayonne Bridges were built side by side and one was a parabolic arch and the other a circular arch, most people would not be able to see the difference. People sensitive to geometry – architects, for example – would be able to see the difference but, I’d guess, would be split as to whether one looked better than the other.
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