Arching Action, Too

by Don Friedman on January 31, 2017


One of the odd administrative aspects of running a blog is that I get statistics on how people arrive here. A blog post from last spring, “What Is Arching Action?” is now our third-most popular post, with a few people arriving here almost every day after searching for those words. (As of today, that blog post from us is the second hit on a Google search for those words, which frankly astonished me.) Given that the blog post was only partially about true arching action, I feel like I should elaborate. Also, for the people who get here looking for “arcing action,” I’m afraid that I don’t know enough about that topic to be of much help.

The freestanding curved structure in the picture above, the top of a folly at the foot of Battery Park City, near Battery Park, is not an arch in structural terms. The best structural definition I can come up with for the word “arch” is that it is a structural mechanism for spanning horizontally and carrying vertical load that uses only compression. In the same manner, a catenary is a structural mechanism for spanning horizontally and carrying vertical load that uses only tension, and a beam is a structural mechanism for spanning horizontally and carrying vertical load that uses bending and shear. Note that these definitions do not discuss form: they are definitions that might work best with some geometry but are tied to force not geometry and material.

Why isn’t that big brick arched thing at the top an arch? Because it can’t carry load in compression. A close up of one end makes it clear, particularly if you click on it to enlarge it:


Not only are there expansion joints running across the “arch” width, which would relieve any possible compression that might build up, there are expansion joints surrounding the end where the thrust of a real arch would have to transfer to an abutment, What’s actually going on here? My guess us that there’s a curved reinforced-concrete beam hidden behind the curved brick, so the brick is just a hung veneer.

In the words of every huckster ever: Wait! There’s more! There’s a second “arch” in the top photo, at the ground level. Is that an arch? In other words, is the lower curved brick element acting structurally as an arch? Yes and no. I’m fairly certain that the brick is not carrying the load of the wall and roof above as an arch. My guess, again, is that there’s a concrete or concrete-block structural wall hidden behind the brick.* That would make the brick just a veneer. But, since there is no visible lintel for the brick at the lower opening, it’s quite possible that the visible brick “arch” is an arch supporting just the weight of the veneer.

To recap, there are two distinct entities that are often confused but are worth considering separately:

There are “arches” which are building elements that form some kind of regular curve with their ends nearer the ground than their middles. They are often used for architectural effect but have no particular structural form.

And there are arches, which have an “arch” geometric form and which carry gravity load in compression. They require abutments, adjacent arches, or some other structure at their ends to counteract the horizontal thrust that they inherently develop; their geometry is subject to constraints related to their span and the loads that they carry; and they must have solid continuous load paths from end to end.

Prior to the introduction of iron framing in the nineteenth century, the vast majority of “arches” were also arches. Today, in the U.S., very few “arches” are arches.

 


* The thing to keep in mind about guesses is that they are often wrong. I don’t know what the actual structure of the folly is and so I’m trying to figure it out.

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