That’s my artistic photo of two windows in an 1880s building in upstate New York with jack-arch heads. The term “jack arch” is variously defined but usually means a flat or low-curvature segmental arch. There’s a fanciful story that the name comes from the resemblance of the arch to the hats that the jacks in a deck of cards are wearing, but it’s more likely from a nearly obsolete usage of “jack” meaning short, as in “jack rafter.”
Once again, we have to put ourselves in the mindset of a nineteenth-century builder. Steel was expensive, labor was cheap, the basics of fire-protection were reasonably well known. Using wood to support the masonry above a window opening was (and is) a terrible idea, vulnerable to both fire and weathering; using steel angles was expensive. The jack arch was a variation on an idea that goes back to at least the ancient Romans’ construction techniques.
Jack arches like the ones above shouldn’t work. They are built with common bricks, so the arch shape is created through the use of tapered mortar joints, which are by necessity not particularly narrow. An arch where mortar is something like 1/6 of the material in the line of thrust is going to be weak and too flexible. And yet, jack arches have traditionally performed just fine most of the time.
Perhaps the question we should be asking is what does the jack arch actually support? If we design a steel-angle lintel as the window head, we usually use in analysis a triangular load that crudely represents the masonry below the a theoretical arching-action curve. Whether we say so or not in our calcs, the use of that triangular load means we are relying on arching action in the wall above helping us out. If we use a jack arch instead of a steel lintel, that arching action does not go away. The jack arch is still supporting only the masonry below the theoretical arching-action curve; since the jack-arch is relatively flexible, it won’t pick up any additional load from the stiffer hidden arch above.
As a matter of tact, the curve of the jack arch eliminates some of the brick below the arching-action curve that might be vulnerable to dropping out. The higher the curve of our jack arch, the closer it approximates the arching-action curve and the less likely we are to have any difficulties. In short, I could easily have recycled the title from Monday’s blog post and called this one “Arching Action Visible” as well.
More on this topic: here.
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