To make a small temple dominate the skyline, build a tall crest of masonry on its roof, set over the heavy rear wall and pierced with openings to lighten it; model it in painted stucco with the figure of the ruler or god.
Keep each half of a corbel vault over its wall
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 corbel vault is not an arch. Its stones stay horizontal and press straight down; nothing pushes sideways, so no buttress is needed, but nothing holds the overhang up either except the weight behind it. Each half is a cantilever that would tip into the room about the inner edge of its springing if its centre of gravity moved in front of that edge. The Maya solved this with walls of a rubble-and-mortar core, faced with thin cut stones and carried up well above the springing, so that the hearting outweighs the overhang.
Turn off the hearting in the model and the courses become single overhanging stones: short stones and a wide span make the stack overturn. With the hearting in place the same vault stands with a comfortable margin.
Related rules
Keep the span of a corbelled room to about three metres or less and make it long instead of wide; to enlarge a building, add parallel galleries rather than widening them.
Maya room spans ≈ 1.5–3 m; vault rise ≈ 1–1½ × span
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.
Cite this page Chris Paton, “Keep each half of a corbel vault over its wall”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/corbel-vault-stability. Text, drawings and models licensed under CC BY-NC 4.0.