That’s the side of Pier A, at the foot of the Hudson River, which I wrote about a couple of years ago. It’s a steel-frame structure sitting on wood piles and clad with sheet metal, but because of it was built in the 1880s it has architectural aspirations beyond being a box. What caught my eye when I took this picture was the presence of arches in the door openings along the side.

A while ago, I spent a lot of time discussing “arching action” which is the physical distribution of forces that makes arches work. The phrase “arching action” is most commonly used when an opening in masonry wall functions as an arch even though the masonry is not shaped like an arch. The short version of that discussion: you can have “an arch” even when the geometry is not arch-like. This is no surprise, of course, to anyone who has ever seen a flat-arch lintel.

Perhaps because of my early exposure to beaux-arts architecture, I have an urge toward symmetry. We have arching action without arches, and Pier A is a good example of an arch without arching action. The force in arching action is compression, and there is no compression worth talking about in the sheet-metal skin of Pier A. That arch carries no load, but rather is a decorative scrim hung from a steel frame. The short version is that we don’t have an arch even though the geometry is arch-like.

Like so many other words, “arch” has both a common definition and a professional one. The common definition is based on geometry, which is why people outside the field sometimes are confused when we refer to “flat arches”: an arch is a curved thing, so how can it be flat? The professional definition is a building element that carries load in compression. A plaster arch separating two rooms is an arch by the first definition only, while a flat-top unfinished opening in a brick wall is an arch by the second definition only. Most curved-top openings in old masonry walls meet both definitions, while Pier A is one of the old NY examples that I know of arched exterior-wall openings that are not structural arches.

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