Architecture of the Keystone Arch

[vivian maxine]

Yesterday, we were looking at one of my favorite sculptures (right word?) at our art museum: Andy Galsworthy's Stone Sea. Stone Sea is a collection of keystone arches constructed with Missouri stone.

http://www.architecturaldigest.com/stor ... -stone-sea

We were talking about the amazement of a single stone at the top supporting an entire arch. A friend with us was wondering if the builders have a support underneath while building the arches. I have found that modern keystone arch builders do indeed use some forms of support and one of the pictures on that page seems to show a support form which is not there now on the finished sculpture.

What about when Romans were building these arches? Did they use supports until they got the keystone set? Can a keystone arch ever be built using no form of support? Our science museum provides foam blocks for anyone to try to build without any support underneath. I've seen many try but not seen anyone succeed.

Can it be done?

By the way, there is no cement of any kind between those stones. Fortunately, the entire sculpture is in a walled-in courtyard where no one can climb on it.

[TheVat]

Supports are needed for any keystone arch, Roman or Gothic, it being made of separate masonry elements. The Roman arch had a problem, which is that it is twice as subject to horizontal thrust -- which can bring down a wall -- and so it needs more buttressing against side stresses. When the medievals started building tall cathedrals, they found the Gothic arch was better - had only half the side stress, and you could build a taller arch (that's the obvious visual component you notice) for a given width. Gothics still needed buttresses - commonly called flying buttresses because of how visible they were jutting out from the side of the arch.

So, basically, when you say a keystone is supporting the entire arch, you really mean just the vertical thrust, i.e. the weight pressing down from above, can't be properly distributed over the whole arch until the keystone is placed.

[vivian maxine]

Supports are needed for any keystone arch, Roman or Gothic, it being made of separate masonry elements. The Roman arch had a problem, which is that it is twice as subject to horizontal thrust -- which can bring down a wall -- and so it needs more buttressing against side stresses. When the medievals started building tall cathedrals, they found the Gothic arch was better - had only half the side stress, and you could build a taller arch (that's the obvious visual component you notice) for a given width. Gothics still needed buttresses - commonly called flying buttresses because of how visible they were jutting out from the side of the arch.

So, basically, when you say a keystone is supporting the entire arch, you really mean just the vertical thrust, i.e. the weight pressing down from above, can't be properly distributed over the whole arch until the keystone is placed.

I think you are saying that some of those arches need a permanent support. Makes sense to me. Then some do not. Our Stone Sea has none now that it is up. But then it also isn't supporting anything. Each arch is free-standing. I guess that makes the keystone enough?

According to his book, Andy Galsworthy never uses any cement in his walls - at least not willingly. Somewhere in New Englahd, they told him he had to cement those stones in place for safety because people would walk on them.. He is alleged to have told them "In England we do not walk on our walls." I wonder.

Of course, there is a great difference between a wall and an arch. Yes?

[TheVat]

I think you are saying that some of those arches need a permanent support. Makes sense to me. Then some do not. Our Stone Sea has none now that it is up. But then it also isn't supporting anything. Each arch is free-standing. I guess that makes the keystone enough?

Been away for a day or so, sorry so slow.

Yes, an arch by itself can do okay without buttressing. But in a lot of architecture (I started out, eons ago, studying architecture and structural engineering) an arch is part of a wall, which is part of a building, and so you have to deal with lateral stresses. The arch stones will start slipping and grinding against each other when those lateral thrusts occur. The Roman arch will go first, but that wasn't so much of a problem because the Romans built sturdy and didn't build high (like say, Chartres cathedral).

[vivian maxine]

I think you are saying that some of those arches need a permanent support. Makes sense to me. Then some do not. Our Stone Sea has none now that it is up. But then it also isn't supporting anything. Each arch is free-standing. I guess that makes the keystone enough?

Been away for a day or so, sorry so slow.

Yes, an arch by itself can do okay without buttressing. But in a lot of architecture (I started out, eons ago, studying architecture and structural engineering) an arch is part of a wall, which is part of a building, and so you have to deal with lateral stresses. The arch stones will start slipping and grinding against each other when those lateral thrusts occur. The Roman arch will go first, but that wasn't so much of a problem because the Romans built sturdy and didn't build high (like say, Chartres cathedral).

Surprising in a way. You'd expect the building to help support the arch but not, I guess. I love those flying buttresses.

Ours at the museum looks so grand. I always feel a bit sad to see it squeezed into that tight spot but has to be. Wouldn't it be beautiful standing out front for all to see as they go by? Wouldn't do. We know what would happen. So we put trees out there and that's good, too.

Hope you had a good mini vacation.