Segmental Arches

"Modified Sharp Arch" Stone Arch Bridge

Segmental arches are arches with an arc less than a 180-degree Roman arch. They seem to have been popular primarily in the 1700s and 1800s, although they were occasionally built in medieval times — rarely even by the Romans.

What is a Segmental Arch Bridge?

The term “segmental arch” is rather a loose; technically anything less than a 180-degree segment of a circle is a segmental arch. However, historically many bridges have been built that while technically segmental arches are so close to Roman arches in rise, they tend to be lumped together and treated like Roman arch bridges (for example, a 160-degree segment of circle as opposed to a 180-degree segment).

A skewback is a triangular stone that starts a segmental arch off at the correct angle to achieve a 180-degree total arc. So, for example, a 160-degree arch will have two skewbacks, one for each end of the arch, with an angle of ten degrees — (10+10+160 = 180). However, segmental arch bridges that are as rounded as in the example above are frequently not started off as skewbacks. Instead they are often just laid flat down like a Roman arch, with excessive angling in the first few arch stones making up for the lack of skewback.

"Modified Sharp Arch" Stone Arch Bridge
The skewbacks on this DIY stone arch bridge are located right at the ground level of the arch. They are triangular shaped stones which have an angle of roughly 30 degrees each in this case. The arch is a 120 degree segment of a circle.

Segmental Arch Bridge Challenges

The primary challenge of a segmental arch is its tendency to slide horizontally. The flatter the arch, the greater the sliding tendency. For an arch with a 90-degree or flatter arc, this horizontal thrust becomes quite significant.

Another factor not as often realized is that flatter segmental arches have less “give” than more rounded arches. A round arch is capable of reshaping itself while still remaining stable to a degree that is nothing short of surprising. A very flat arch, on the other hand, is already so close to buckling that a slight shifting of the foundations can cause serious weakening of the arch. This means that a segmental arch will need better foundations than a Roman arch. Also, because of the sliding tendencies of a flatter arc, it should also have wider abutments.

Segmental Arch Advantages

The primary advantage of the segmental arch is the fact that it is capable of spanning a gap without too much of a “hump.”

Also, whereas multiple Roman arches may be needed to cross a stream, a single segmental arch can cross the same stream, eliminating the use of a pier. These advantages are huge and are why the segmental arch historically was commonly used.

Segmental Arch Disadvantages

The primary disadvantages with the segmental arch are several. The most obvious is the tendency to slide horizontally, necessitating robust abutments.

Also, there is less room for error, as it has less yield than a Roman arch. This means the foundations need to be extra strong, and the masonry in the arch itself must be more precise.

This all equates to the flat arch requiring more care in design and building.