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How Do Arches Work?

Norman arches  StewkleyThe two nearest arches support the weight of the tower. There's another arch over the window, and above it you can just see the ribs of the roof, which is in the form of two intersecting barrel vaults. A barrel vault is a long arch, like the tunnel at Gayhurst in last week's Monday photo. This is the Norman church at Stewkley.

The Monday Photo

I see a lot of arches travelling round for the North Bucks Wanderer. The last five posts have all had something to do with arches, even if you couldn’t see them in the photos.

So how do they work, and why don’t they fall down, even if they are hundreds of years old? It’s all due to the way stone, or brick, is strong.

Stone and brick are enormously strong in compression; when they are being squeezed. Arches take advantage of this, by directing the load or weight of whatever is above the arch sideways.

This is how it works. The bricks or stones around the arch are wedged shaped. The weight above them makes each wedge shape (they are called Voussoirs) try to drive itself deeper between its neighbours.

They all squeeze each other and the weight is directed around the arch and into the abutments on each side.

How arches work2

As long as the abutments (see the diagram) are strong enough to resist the weight, the arch doesn’t have to be a complete semi-circle, and some arches are very shallow indeed. This is very handy for things like railway bridges; a train won’t get over a hump backed bridge.

The red arrows show the direction of the forces involved. And yes, they do show the abutments pushing back at the arch. They have to, weird as it may sound, or they would be crushed. With arches like the ones in the photo, there are more voussoirs and they just direct the forces (the thrust lines) vertically down.

The Voussoir at the crown of the arch is sometimes bigger than the rest and called the keystone. That’s just for decoration; it’s no more important than any of the others.

Arches are very stable. Even if the foundations shift, it is likely the arch will stay up. With a simple wall, if the thrust lines stray outside the wall, it will hinge or crack around that spot and fall down. The wedge shape of the voussoirs in an arch means they will just settle into a different position.

You need at least four hinge points before an arch will fall down. You can see cracks above the big arches in the photo. Arches are great for spanning large gaps. That’s unlike stone slabs, which will just crack and fall if the gap is too big.

The first known arches were in Egypt and Mesopotamia, 5,600 years ago and were made of brick. Stone ones came later.

If you want to see a wide selection of arches here on the North Bucks Wanderer, look for churches or bridges in the categories in the sidebar.

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