Friday, October 25, 2013

Arch # 17


A keystone-less arch.

People who explain arches to other people often like to point out how the keystone is the most important part of an arch. It's isn't . Actually it isn't any more important than any of the other voussoirs that make up an arch.  It's just that its often the last one to go in. 
Removing it is no more likely to have the arch fall down than removing any of the other stones.
There are many arches that don't even have keystones.


2 comments:

  1. This one has a nice big tree to keep the arch from falling...

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  2. It's hazy, but I recall this being a three-beer debate in an alehouse in North Carolina with you. You are right, it will stay up, but you have to look at the bigger picture. Masonry performs best when it's in compression, balanced and non-breached.. and there are a bunch of things that help with this, such as bonding, craftsmanship, etc. However, an opening in a wall, such as a window, causes all kinds of unrest... sort of like when you're enjoying a peaceful moment at home and suddenly your in-laws drop in. Now, an arch helps by ensuring the above loads are convinced that it's better to turn around the window than collapse into it, and the shape of the arch, position of the voussoirs, and the amount of mass waiting to absorb it, all work together to make it effortless for the wall... as if the window was never there (deception is the foundation of all good stories). However, the cost of turning compression sideways is "tension", and masonry hates tension. The place where tension occurs the most is at the very top of the inside of the arch. Since there is no keystone, you end up with long vertical joint smack dab in the middle where tension will occur... and since long vertical joints are considered a weak spot, you are essentially giving the arch a free pass to eventual failure. In other words, you are minimizing compression (which strengthens masonry) and maximizing tension (which weakens masonry). On the other hand, a keystone, which is fatter at the top and tapered, maximizes the upper compression (which masonry loves) and minimizes the tension (which masonry hates), converting this weak spot into a pretty strong solution. Further, the addition of a keystone requires an odd number of voussoirs (ie the odd one is in the centre with the same number of stones on either side) , such that the key shoots the force EQUALLY left and right. So a keystone actually helps an unwelcomed breach by maximizing compression, symmetry and balance while minimizing tension. Much the same as alcohol does when the in-laws drop in. (in-law jokes aside, family - especially extended family - is the strength that provides the strong side support required to bridge any gap! (We've both been blessed with good ones!))

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