What is the intrados?
The intrados is the inner or underside of an arch or vault. It is the curved soffit you see when you look up at an arch or vault from below. The intrados is the opposite of the extrados, the outer or upper side of the same arch.
Application
The intrados is a basic concept in designing and building arches and vaults:
- Shape — the curve of the intrados determines the type of arch, for example a round arch, pointed arch or segmental arch.
- Formwork — when building an arch, it is turned on a timber form (a centre or centring) that matches the shape of the intrados exactly.
- Flow of forces — the stones (voussoirs) stand at right angles to the curve; the load runs via the intrados and extrados to the springing and the supports.
Relation to other arch terms
An arch involves several fixed terms: the springing (where the arch begins), the keystone (at the top) and the spandrel (the fill above). The intrados and extrados together bound the thickness of the arch. A crisp, even intrados is decisive for the look of a masonry or stone arch and for how the forces are carried down cleanly.
Related terms
- Arch
- Extrados
- Vault
- Keystone
- Springing
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