Post and lintel
Two posts and a beam: how the bending stress in a stone lintel grows with the square of its span, and why stone spans stay short.
The rules it illustrates
A stone lintel’s span is limited by its tension. 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.
A lintel carries only a triangle of the wall above it. 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.
Timber spans where stone cannot. 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.
Where it appears in the book
In the styles Prehistoric and Megalithic.
In the chapters Post and Lintel.