Rules · Domes · Structure · Vaults
Build a round chamber of horizontal courses, each ring projecting a little beyond the one below, dress the inner face to a smooth curve, close the top with a capstone, and heap earth over the whole.
The Treasury of Atreus (c. 1250 BC) is 14.5 m across and 13.2 m high, built of 33 courses of conglomerate ashlar. It was the widest unsupported span in the world for well over a thousand years, until the concrete domes of the Romans. A corbelled dome is more stable than a corbelled vault because each ring is a closed circle: its stones are wedged against their neighbours and resist sliding inward by hoop compression, as the stones of a well lining do. The earth mound, built up course by course as the dome rose, both served as a working platform and added weight to the stones’ outer ends.
Pausanias was shown such buildings at Mycenae as the underground treasuries of Atreus and his sons, and the name has stuck.
In a corbelled vault each course may project only a little beyond the one below, and each overhanging half must have its centre of gravity behind the inner face of its wall; load the tails of the stones with heavy hearting until the capstone is set.
stable if Σ W·x (behind the springing edge) > Σ W·x (in front)
A beam is squeezed along its top and stretched along its bottom; stone resists squeezing well but stretching badly, so a stone lintel must be deep in proportion to its span and cannot be long. Doubling the span quadruples the stress from its own weight.
σ ≈ ¾ ρ g L² ⁄ d (self-weight) · + ≈ 0.43 ρ g L³ ⁄ d² (a wall above)
In a bonded masonry wall the courses above an opening corbel over it of their own accord, so the lintel carries roughly the triangle of wall with sides rising at 60° from its ends; everything above that triangle arches onto the jambs.
load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g
Weight for weight, timber is many times stronger in bending than stone; long spans in trabeated buildings are therefore roofed with timber beams resting on stone walls and columns.
bending strength ÷ density: oak ≈ 70 · limestone ≈ 3 (kPa per kg ⁄ m³)
At Stonehenge each lintel is pegged to its uprights by a mortise-and-tenon joint and locked to its neighbours by tongue-and-groove joints, and each is curved to follow the circle: the joints of a timber frame reproduced in dressed sarsen.
In a megalithic chamber the room can be no wider than the largest slab the builders can raise over it; the upright stones are set to suit the capstone, not the other way round.
Cite this page Chris Paton, “The corbelled dome: rings stepping inward”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/corbelled-dome. Text, drawings and models licensed under CC BY-NC 4.0.