A voussoir is a wedge-shaped block — typically of stone, brick, or concrete — that is one of the individual units making up an arch or vault. The tapered shape of each voussoir ensures that the joints between adjacent stones radiate toward the center of the arch, allowing the structure to carry loads through compression.
Application
Voussoirs have been fundamental to arch construction since antiquity. Understanding their geometry and placement is essential to masonry and structural engineering:
- Keystone: The central voussoir at the crown of the arch is called the keystone. It is the last stone placed during construction and locks all the other voussoirs into position.
- Springer: The lowest voussoirs, resting directly on the impost (the supporting ledge or capital), are called springers. They mark the point where the arch begins to curve away from the vertical support.
- Geometry: Each voussoir is cut so that its two radial faces (beds) converge toward the center point of the arch. The outer curved face is the extrados; the inner curved face is the intrados.
- Material: Traditionally cut from natural stone (limestone, sandstone, granite), voussoirs are also formed in brick (using tapered bricks or tapered mortar joints) and precast concrete for modern applications.
- Centering: During construction, voussoirs are laid over a temporary timber framework called centering, which supports the arch until the keystone is placed and the arch becomes self-supporting.
- Decorative treatment: In classical and Renaissance architecture, voussoirs are often given rusticated or alternating decorative treatments to emphasize the arch visually. Oversized keystones with carved faces or scrollwork are common on monumental buildings.
The structural integrity of an arch depends entirely on the precise cutting and fitting of its voussoirs, making stonemasonry one of the most skilled trades in traditional construction.
Related terms
- Keystone
- Arch bridge
- Centering
- Impost
- Extrados and intrados
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