A dome is a vault with a curved surface over a circular, polygonal or square plan, which carries its load not in one direction but radially in all directions to the support. In section a dome is an arch; in reality it is that arch rotated about a vertical axis, which introduces forces in the hoop direction as well.
That is both its strength and its problem. Hoop stress is compressive near the crown but tensile lower down: the dome wants to spread at its base. Historic domes take that thrust with thick walls, with a ring of supporting structures, or with a tension ring or chain around the springing.
Forms
- Hemispherical dome — the classic form
- Shallow or saucer dome — flatter than a hemisphere, producing more thrust
- Onion dome — with a pinched base, known from Russian and Islamic architecture
- Ribbed dome — with load-bearing ribs and lighter infill panels
- Shell dome in concrete, steel or timber — the modern thin-walled version
Meeting the plan below
A dome is round, a room is often square. The transition is made with:
- Pendentives — curved triangular spandrels leading from square to circle
- Squinches — small arches or niches cutting across the corners
- A drum — a cylindrical wall under the dome that lifts it and allows windows
Where you find it
- Churches, mosques, mausolea and baptisteries
- State rooms and palaces
- Station halls, exhibition buildings and sports halls in steel and concrete
- Rooflight domes — the small modern plastic version on a flat roof
Practical tips
- Cracks running vertically up from the springing are typical of a spreading base — a structural signal, not a cosmetic one; have it assessed
- Check the tension ring and ties first when a historic dome shows damage; the cause is almost never in the shell itself
- Ventilate the space beneath a dome — warm air collects at the crown and condenses on the coldest surface
- On a rooflight dome, the upstand is the weak point, not the dome: give it enough height and detail the membrane properly
Related terms
- Vault
- Dome drum
- Pendentive
- Cloister vault
- Oculus
- Rooflight dome
- Calotte
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