What is extrados?
Extrados is the upper or outer surface of an arch or vault — the visible top side. Its counterpart is the intrados, the inner or under side of the arch. Both terms come from French and Italian construction language and are widely used in masonry and bridge engineering.
Anatomy of an arch
An arch consists of several parts, each with its own name:
| Part | Description |
|---|---|
| Extrados | Outer surface of the arch |
| Intrados | Inner / under surface |
| Keystone | Central stone at the top |
| Springing | Where the arch starts |
| Spandrel | Area between arch and surrounding masonry |
| Backing | Stonework above the arch |
Application
The distinction extrados/intrados is relevant in many contexts:
In masonry
- Brick sturck arch — top edge is the extrados
- Rollshift / rowlock course — top side is the extrados
- Relieving arch — the extrados lies under the brickwork above it
In vaults
In vaults the form differs between upper and lower surface:
- Romanesque barrel vault — extrados curves uniformly
- Gothic rib vault — extrados irregular with ribs
- Dome — extrados is the outer dome, intrados the inner
In bridge engineering
For stone and concrete bridges, distinctions are made:
- Flat extrados — straight top above the arch
- Concentric extrados — top follows the arch curve
- Filled arch construction — space above extrados filled in
Force distribution
The extrados experiences compression under load, while the intrados is in relative tension. This is why arches of stone or concrete work so well — stone is strong in compression.
For compression-stressed materials (stone, cast iron, concrete) the arch is the logical choice. For tension-stressed materials (steel, timber), beams or trusses are more common.
Examples
- Roman aqueducts — flat extrados, semicircular intrados
- Gothic cathedrals — complex extrados in rib vaults
- Stone bridges — concentric extrados following the watercourse
Related terms
- Intrados
- Arch
- Keystone
- Springing
- Vault
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