What is a barrel vault?
A barrel vault (also called a tunnel vault) is a vault with an arched cross-section that continues along its length, so that it looks like a barrel or half-cylinder cut lengthwise. It rests along its full length on two parallel walls. It is the simplest form of vault and was used on a large scale by the Romans.
Forms
- Semicircular barrel vault: with a round-arched section, typical of Roman and Romanesque architecture.
- Pointed barrel vault: with a pointed-arch section. The pointed shape produces less outward thrust than a semicircular arch.
- Segmental barrel vault: with a low, flat arch, widely used for cellars and bridges.
- Barrel vault with transverse arches: arches across the underside that divide the vault into bays and stiffen it.
How does it work?
Like an arch, a barrel vault carries loads through compression. At the crown the stones push against each other, and lower down horizontal thrust develops: forces that push the supporting walls outwards. That is why the walls under a barrel vault are thick, or are supported by buttresses, tie rods or adjoining vaults.
A drawback is that the vault needs support along its entire length, which makes large windows in the supporting walls difficult. This limitation led to the groin vault, which channels the forces to four corner points.
Uses
- Roman baths and cellars.
- Romanesque churches and monasteries.
- Vaulted cellars beneath old town houses and farmhouses.
- Modern applications: tunnels, culverts and concrete shell structures.
Tips for old barrel vaults
- Never simply break an opening into a vault or the walls supporting it. Have a structural engineer check it first.
- Cracks running lengthwise along the crown may indicate that the walls are spreading.
- A masonry vault needs sound, breathable joints. Repair it with a lime mortar that matches the original masonry.
Related terms
- Groin vault
- Cloister vault
- Transverse arch
- Horizontal thrust
- Buttress
- Roman thermae
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