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What is a Segmental Arch?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 01 December 2016
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A segmental arch is a semi-circular arch which is comprised of a circle segment, not a complete half circle. This type of arch is utilized in architectural styles from all over the world and throughout history, and serves functional as well as aesthetic functions. People can usually identify a segmental arch because it may look slightly shallow or flattened as a result of the fact that it is not a whole semicircle. Once could also imagine creating a flipped mirror of the arch directly below, in which case the two segments would create an almond shape, rather than a circle, illustrating that they are only segments of a circle.

When a segmental arch is designed properly, it can be structurally very important. The arch is under compression, rather than tension, distributing and bearing weight evenly to support a larger structure. Such arches can be used in a grouping to distribute the weight of a large structure, or they may be established as standalone features in a larger structure. They may be arranged in a number of different ways to meet structural needs, especially in large buildings.

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Using a segmental arch allows people to create openings for doors, windows, colonnades, and other features. These bring light and air into a structure, making the space more pleasant to be in. Practical design can also be important for the climate, allowing air to circulate through buildings in hot areas, for example, or allowing light in to save on energy bills by reducing the need for artificial light.

Segmental arches can be made from a number of different materials, with the arch being assembled in different ways. When such arches are designed and engineered, care is taken to make sure that they will function as anticipated. If the segmental arch is not designed and installed properly, it can create a weak point by isolating tension in the arch, potentially laying the groundwork for a catastrophic collapse.

Any arch which contains a circle fragment rather than a full semicircle could be considered a segmental arch. The arch may be open, or filled, depending on the taste of the designer and the needs of the structure. In some arches, a center keystone may be left visible, while in others, the appearance of the arch may be smoothed to create an even, uniform look. Decorative features such as molding and tiles can also be affixed to a segmental arch to make it more visually interesting.

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Bertie68
Post 4

When I was visiting Rome, I saw some of the segmental arches. The architects of the day must have gone through a lot of trial and error to finally come up with something that worked. I think they knew what they wanted and just had to keep trying after many failures.

In a children's museum, I saw blocks that were precisely made so that kids could work with it to put the arch together so it could stand alone. It's really a pretty amazing thing.

croydon
Post 3

@umbra21 - In fact there is a very ancient example of a segmental bridge that you can see in Rome. More than 2000 years old, which is quite impressive considering it would have been cutting edge technology when they built it.

The Ponte San Lorenzo is also considered quite extraordinary because its piers are very slender for the time it was constructed.

Unfortunately, because of the town growing up around it, it's quite difficult to see now. It's kind of buried in the buildings, and the public is only allowed to see it at certain times.

umbra21
Post 2

@Mor - I think quite a few people find it interesting. They might not go to Rome just to see segmental arches, but they go to visit the birthplace of a lot of different architectural principles.

Although I think the segmental arch isn't quite as interesting as some of the other innovations they came up with. Often it seems to be used quite conservatively from what I've seen, in doorways and windows for example, rather than in larger arches with larger loads to bear.

Of course, there are examples of Ancient Roman segmental bridges too, which I think are more impressive.

Mor
Post 1

Apparently it was the Ancient Romans who first realized that you can make an arch without a perfect semi-circle. To be honest, I have trouble understanding how it works now, even though I can see that it obviously does.

It seems counter intuitive to me though, so you have to have respect for the people who first realized that it was possible.

I've never been to Rome, but I've seen pictures of ancient doorway arches and bridges there which use a segmental arch.

I'd love to go there and see them in person. It doesn't seem very exciting, I know, but the fact that someone just came up with a whole new concept in architecture is interesting to me.

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