Span

Rules that use the word

Corbelled rooms are narrow. 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.

Make every opening a trapezoid. Give doors, windows and niches sides that lean inward, so that each opening is narrower at the top than at the bottom, and span it with a single stone lintel.

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.

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.

The flatter the arch, the harder it pushes. An arch pushes outward on its supports as well as down. For the same load and span the horizontal push grows as the rise falls: halve the rise and you roughly double the thrust.

Pointed arches reach any height on any span. A round arch must rise half its span, so arches of different spans on one springing line have crowns at different heights. A pointed arch can be given any rise at least half its span, so arches of different widths can all be made to reach the same height.

Where it appears in the book

In the chapters What Is Architecture?, Post and Lintel, Walls and Openings, The Arch, Vaults, Domes, The Roof, Frame and Skin.

A feature of Ancient Egyptian, Gothic architecture.

Nearby in the glossary

Solar · Solar altitude · Sōrin · Space frame · Spandrel · Spandrel panel · Specus · Spolia