What is a segmental arch?
A segmental arch is an arch whose shape is a part (segment) of a circle, smaller than a semicircle. The centre of the circle therefore lies below the line where the arch starts (the springing line). This makes a segmental arch flatter than a round arch, which is exactly a semicircle.
Key terms:
- Span: the horizontal distance between the two springings.
- Rise: the height of the arch, measured from the springing line to the inside of the crown.
- Springing: the point where the arch rests on the wall or pier. In a segmental arch the springing is usually finished at an angle (a skewback), so that the arch bricks bear properly on it.
Forces: every arch pushes not only downwards but also sideways on its supports. The flatter the arch, the greater this horizontal thrust. A segmental arch therefore pushes harder sideways than a round arch. There must be enough solid masonry beside the arch to resist this force, or a tie rod or continuous beam.
| Segmental arch | Part of a circle smaller than a semicircle, with the centre below the springing line. Flatter, so it pushes harder sideways on its supports. |
|---|---|
| Round arch | Exactly a semicircle. Less flat, so it pushes less hard sideways than a segmental arch. |
Application
Segmental arches are very common above windows and doors in brick facades, especially in the nineteenth and early twentieth centuries. You also find them as relieving arches above a lintel, in cellars, in bridges and in vaulted floors (jack arches between steel beams).
Tips for building or repairing a segmental arch:
- Use a centring: a temporary timber former in the shape of the arch, on which you lay the bricks. Only remove the centring once the mortar is strong enough.
- Aim the joints at the centre: all joints must point towards the centre of the circle. A string line from that point helps.
- Odd number of bricks: make sure there is a keystone in the middle, so that the arch is symmetrical.
- Wedge-shaped joints or cut bricks: with ordinary bricks, the joints get wider towards the top. For neat work, use bricks sawn to a wedge shape or specially shaped arch bricks.
- Cracks in an existing arch: a sagging arch or cracks at the springing often indicate masonry spreading sideways or settlement. Have that assessed before simply repointing.
Related Terms
- Round arch
- Basket-handle arch
- Centring
- Relieving arch
- Lintel
- Springing of an arch
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