Friday, October 11, 2013

Arch # 7

The Catenary Arch


A 'catenary arch' is the curve a hanging flexible wire or chain assumes when supported at its ends and acted upon by a uniform gravitational force. 


It is very close to a parabolic curve. Apparently it is the strongest of arch shapes.

Hanging a chain is a great way to figure out the shape of this kind of curve if your building a dry stone arch as long as you are planning to build it upside down.

The word catenary is derived from the Latin word for "chain." The curve is also called the alysoid, funicular and chainette. My problem is that making a catenary curve using a chain gives you many different shapes depending on how sort the length of chain is. It looks like a narrow rounded gothic when the chain is a long loop. As it gets shorter it takes the form of different segmented arches and if it gets really short it becomes almost a straight line! Can anyone explain this?

Does this mean that a jack arch can also be a catenary? It can't be.



Is there any arc besides a pointed one that can't be amongst the catenaries?



2 comments:

  1. An arch works by resisting gravity and deflecting gravity, which are kind of two separate things. How does an arch know which is which? The deal with the catenary chain theory is that the chain, when hung between two points, hangs in such a way that gravity pulls it so that it musters in the middle, since it is heaviest in that place. Let's call it spot X. In other words, the chain has reacted to gravity the most severely at point X and the entire chain has accommodated this action naturally, so that any spot along the chain is pulling back slightly more than a point above it. Now if you to flash freeze the chain and invert it such that it forms an arch, the very place that has naturally accepted gravity is now at the height of the arch, where it is now in the perfect shape to resist gravity, and the entire chain is in the best, natural shape to help transmit this force around the window and into the ground. So, back to the hanging chain, even if you pull it tighter, gravity will still pull the centre differently, so technically it's not really a segmental curve. Even a true flat top jack arch is stronger with a slight camber in its intrados (underside). Hey, I just came in for a beer... what's with all the questions?

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  2. A really good answer John Scott. Very easy to follow.
    Thank you. You should teach...
    JFS

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