Post and Lintel

Every building has to do two things before it can do anything else. It has to stand, carrying its own weight and whatever is put on it down to the ground, and it has to span, bridging the empty space that is the reason for building at all. The simplest way to do both is to set up two supports and lay something across them. A child does it with building blocks; the first farmers of Europe did it with stones weighing thirty tonnes; the Greeks refined it into the orders. This is the system of post-and-lintel, also called trabeated construction, from the Latin trabs, a beam.

In a post-and-lintel frame the two kinds of member do two quite different kinds of work. The posts (walls, piers, columns) are compressed: the weight above squeezes them, and almost any solid material is good at resisting being squeezed. The lintel is bent: it sags between its supports, its upper part squeezed and its lower part stretched. Most of what follows in this chapter comes from that single difference.

Why the stone beam stays short

Stone is magnificent in compression. A column of good limestone could in principle rise more than a kilometre before the stone at its foot crushed under its own weight. But stone is weak and unreliable in tension: pulled apart, it fails at perhaps a tenth or a twentieth of its crushing strength, and it fails suddenly, from whichever hidden flaw or bedding plane happens to lie where the stress is greatest. A lintel is a beam in which the bottom edge is pulled. Its span is therefore governed not by what stone is good at, but by what it is bad at.

Worse, the lintel is loaded by its own weight, and that weight grows with its length. For a beam of span L and depth d the bending stress from self-weight grows with L²⁄d. Double the span, and the lintel must be twice as deep just to be as safe, and then it is twice as heavy again. Put a wall on top and the load grows with the span too, so the stress climbs with something like the cube of the span. The model below computes an indicative stress for a lintel of the stone and dimensions you choose, and compares it with a typical bending strength divided by a factor of safety of three. Lengthen the span and watch the verdict change; lengthen it further and the lintel cracks at mid-span, where the tension is greatest.

℟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.

σ ≈ ¾ ρ g L² ⁄ d (self-weight) · + ≈ 0.43 ρ g L³ ⁄ d² (a wall above)

The numbers explain a great deal of architectural history. Clear spans between stone columns in Egypt and Greece are rarely more than three or four metres; the few wider ones were achieved with exceptionally deep blocks, careful selection, and occasionally hidden iron. Vitruvius, writing at the end of the first century BC, knew the danger well. Of the widely spaced diastyle temple he warns:

This arrangement involves the danger that the architraves may break on account of the great width of the intervals.

His own rules of spacing, the five intercolumniations of the classical temple, are at bottom rules about how far a stone architrave can safely reach. For the widest spacing of all he gives up on stone entirely, as we shall see.

The first frames: megaliths

The oldest surviving post-and-lintel structures are the megalithic monuments of Neolithic Europe (Prehistoric and Megalithic). The simplest is the dolmen: a few upright slabs (orthostats) carrying a single capstone, originally buried under a mound of earth or stones. The chamber can be no wider than the largest slab the builders can raise over it; the plan follows the stone, not the other way round.

Stonehenge, raised in its sarsen form around 2500 BC, is post and lintel as architecture. Thirty uprights of sarsen sandstone, about four metres high, carry a continuous ring of thirty lintels, each gently curved to the circle; inside stands a horseshoe of five trilithons, rising towards the Great Trilithon in the south-west, whose uprights stand more than seven metres above the ground. The gaps between the uprights of a trilithon are only a few tens of centimetres, and the lintels over them are a metre deep: the builders were nowhere near the limits of their material. What they were doing was making a frame, and they made it the way carpenters make one.

℟Joints of carpentry worked in stone

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.

The lintels of Stonehenge are held on their uprights by mortise and tenon joints and locked to one another end to end; the whole ring reads, from inside, as a single horizontal band floating on its supports. The axis of the horseshoe runs out to the north-east, towards the midsummer sunrise, so that the frame is also an instrument for watching the sky (Set the monument on the solstice axis). The idea that a lintel can frame something (a view, a door, the sun) is as old as the lintel itself.

The wall above the opening

Most lintels do not stand free. They span a door or a window in a wall, and the wall goes on up above them. It might seem that the lintel must then carry the whole height of wall over the opening, in which case no stone lintel could span more than a metre or two. Fortunately masonry does not behave like that. In a wall laid with broken joints, each stone overlaps the joint below it and can reach a little way out over a gap, as a stack of overlapping books can reach out beyond the edge of a table. The courses above an opening therefore corbel over it of their own accord, and pass most of their weight sideways to the jambs. Only a roughly triangular patch of wall (traditionally taken as an equilateral triangle on the span) actually bears on the lintel.

℟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.

load ≈ equilateral triangle on the span: W ≈ 0.43 · L² · t · ρ g

The builders of Bronze Age Greece (the Aegean) took this principle and made it visible. Over the great lintels of their gates and tombs they left the triangle empty: the courses on either side step inward to meet at the top, and the lintel carries nothing but its own weight and the thin slab that closes the opening. At the Lion Gate of Mycenae (c. 1250 BC) the slab is carved with two lions flanking a column, the oldest monumental relief in Europe. The relieving triangle is post and lintel admitting its own weakness, and turning the admission into ornament.

℟Relieve the lintel with a triangle

Over a large lintel leave a triangular opening by corbelling the courses on either side, so that the wall’s weight passes to the jambs; close the triangle with a thin slab.

Egypt: the forest of columns

Egypt had excellent stone (limestone, sandstone and granite) and very little good timber. Its builders therefore did something almost no one else attempted: they roofed large halls entirely in stone. Stone slabs rested on stone architraves, and the architraves on stone columns. Because all of these members are stone beams, each of them had to be short, and the columns had to stand close together and be stout, typically five to seven diameters high. The result is the hypostyle hall, a room that is less an open space than a forest of columns.

The Great Hypostyle Hall at Karnak (c. 1290–1224 BC) holds 134 columns in sixteen rows. The builders made a virtue of necessity. The twelve columns of the central aisle, carrying the widest architraves, were made about 21 metres tall, with broad open papyrus capitals that give the beams a generous bearing; the side columns are much lower, with closed buds. The height difference was filled with stone grilles, a clerestory, so that daylight fell into the middle of the hall from above. It is post and lintel scaled up until the frame itself becomes a landscape.

℟Raise the central aisle and light it from above

In a great columned hall make the columns of the central aisle taller than the rest, and fill the difference in height with stone grilles, so that light enters high up over the side aisles.

The Egyptians also gave the post-and-lintel frame its first explicit symbolism. Their columns are plants (papyrus, lotus, palm) tied at the neck with bands; their roofs were painted as the night sky; their floors, rising towards the sanctuary, were the mound of creation. The support is not merely a prop but an image, and this, too, the Greeks would inherit. Even the fluted Greek shaft has Egyptian forerunners in the rock-cut tombs of Beni Hasan (The fluted shaft before the Greeks).

Persia: the timber beam

Weight for weight, timber is many times stronger in bending than stone, and far lighter. Wherever long spans were needed and timber could be had, builders set stone or brick supports under wooden beams. The Persian kings of the sixth and fifth centuries BC (Achaemenid Persia) could draw on cedar from Lebanon, and at Persepolis they built halls of a kind no stone-roofed architecture could achieve. The Apadana, begun by Darius I around 515 BC, is a square about sixty metres across, filled with thirty-six columns on a grid nearly nine metres apart.

Because the columns carried timber rather than stone, they could be astonishingly slender (about twelve diameters, roughly twice as slender as an Egyptian column) and widely spaced. At their tops, pairs of kneeling bulls set back to back hold the main beams in the saddle between their necks: the capital here is visibly a bracket, the place where post meets beam.

Vitruvius makes the same distinction when he reaches the widest spacing of columns, the araeostyle:

In araeostyles we cannot employ stone or marble for the architraves, but must have a series of wooden beams laid upon the columns.

Greece: the frame perfected

The Greek temple is the most refined post-and-lintel building ever made. It is also, in origin, a timber building. The Mycenaean megaron (a porch with two columns between the side walls, and a hall with a hearth ringed by four timber columns) survives in its plan; and the Doric frieze, with its triglyphs and mutules, is generally read as the memory of beam-ends and rafters translated into stone, as Vitruvius himself explains in his fourth book. When the Greeks rebuilt their temples in stone in the seventh and sixth centuries BC, they kept the arrangement of the timber frame but had to live with the limits of the stone lintel.

That is why the clear space between the columns of a Greek Doric temple is commonly only one to one and a half diameters, and less at the corners; why the architrave is so deep; and why the cella, the room inside, is narrow unless it is subdivided by internal colonnades. The roof over the cella was of timber, resting on the stone walls. Within those constraints the Greeks refined the proportions of every member to one another until the frame became an order: a system in which the size of every part can be derived from one module. That story belongs to the Greeks and to the chapters on the orders.

℟The five intercolumniations

Columns may be spaced 1½ diameters apart (pycnostyle), 2 (systyle), 2¼ (eustyle), 3 (diastyle) or more (araeostyle). The eustyle, with a wider central bay of 3 diameters, is the best for use and beauty.

clear space = 1½ · 2 · 2¼ · 3 · >3 D

Beyond the lintel

There are only two ways of spanning a gap with pieces of stone that are individually too short. One is to corbel: to step each course a little further out than the one below until the two sides meet, as in the tholos tombs of Mycenae, the chambers of Newgrange and the vaults of the Maya (The corbelled dome: rings stepping inward, Keep each half of a corbel vault over its wall). Corbelling never puts the stone into tension, but it needs a great weight behind each course to stop it tipping, and it spans little. The other is the arch, in which wedge-shaped stones lean on one another so that every joint is in compression and the load is turned sideways into the supports. The Mesopotamians used brick arches for drains and gateways very early; the Romans made them the basis of a new architecture.

Yet the post and lintel never went away. The Romans who built vaults of concrete still dressed their walls with columns and entablatures; Renaissance architects made the orders the grammar of their buildings; and when iron, steel and reinforced concrete at last gave beams the tensile strength that stone had always lacked, the frame of posts and beams returned as the skeleton of the modern building. The lesson of the lintel, that a structure is limited by the weakest way its material is asked to work, is the first lesson of structure, and it is written most plainly in stone.