Building: materials, vaults, timber frames, earthquakes

A wall stands up on its own. The difficulty is the hole you make in it.

As soon as you want a door, a window or a room, you have to span a gap without bringing down what is above it. The whole history of building comes down to this question.

The problem of the hole in the wall

The simplest solution is a stone laid across, above the opening: the lintel.

The method works, but only over a short span. A stone laid flat breaks in the middle as soon as it is too long. For a door, that is enough. For a room ten metres wide, you would need a stone that nobody could cut or carry.

So something else had to be found. Several societies managed it, each by its own route.

Stacking while advancing

The first idea: build two walls facing each other, and make each course of stones overhang the one below slightly. Course after course, the two walls draw closer, until they meet at the top.¹¹

This is corbelling. Its great advantage: no scaffolding is needed, since each course rests on the one before. Its drawback: it cannot span a very wide space, and it requires thick supporting walls to carry all that weight.

Maya builders roofed entire rooms in this way. But the technique is not theirs alone: it is found in Neolithic European tombs, in the temples of Angkor, and in the dry-stone huts of the French countryside. It appears wherever there is stone but no timber for framing.

The arch, which stands because it pushes

The second idea is radically different. Blocks are cut slightly wedge-shaped and arranged in a semicircle. Each block pushes against its neighbours. None falls, precisely because they all push at the same time.

This is the arch. Repeated in depth, it gives the vault. Closed in on itself, it gives the dome.

The arch allows far wider openings than the lintel. In return, it pushes outwards, and it needs walls thick enough to absorb this thrust. During construction, it must be held up by a temporary timber support until the last stone at the top — the keystone — is set in place.

The most telling difference between the two systems is this. In a corbelled vault, the slab at the top can be removed: nothing falls. In an arch, if the keystone disappears, everything collapses. These are not two levels of success; they are two opposite ways of standing up.

The Romans added a third idea: concrete. With a mortar made of lime and a volcanic powder, builders were no longer limited by the size of the stones they could cut and carry. The mixture was poured, it hardened, and the vault became a shell in a single piece. This is how they roofed immense halls — the dome of the Pantheon, in Rome, spans more than forty metres. To lighten it, they stacked lighter and lighter materials the higher it rose, and hollowed out coffers in its thickness. This concrete also stands up remarkably well to seawater, which allowed them to build harbours.

Setting a circle on a square

A dome is round. A room is most often square. So the circle does not sit on the four corners, and the difference has to be made up.

The first solution: throw a small arch across each corner. The square becomes an octagon, the octagon becomes almost a circle, and the dome can rest on it. This corner arch is called a squinch. The oldest still standing is in Iran, in the palace of Ardashir, built in the third century (the years 200 to 299).

The second solution: slip into each corner a curved triangle, bulging like a fragment of a sphere. This is the pendentive. Roman builders experimented with it; the architects of Constantinople mastered it. Hagia Sophia, completed in 537, rests on four pendentives, and its dome seems to float.

Seven centuries after the palace of Ardashir, builders in Iran and Iraq stacked squinches one on top of another, smaller and smaller. The transition then became a honeycomb decoration, cascading down beneath the dome: the muqarnas. Some genuinely bear the weight. Others are merely hung from underneath, in plaster or in wood.

Building flexibly

In regions where the earth shakes, a rigid building is a bad building. It resists, and resists, then breaks all at once.

In Japan, wooden pagodas use a remarkable device. A great mast runs through the tower from bottom to top: the shinbashira. Depending on the building, it rests on a base of stone or it does not even reach the ground, hanging freely in the middle of the building.²²

The most surprising fact is that this mast does not carry the storeys. The weight of the pagoda is borne entirely by pillars, on which it rests. The central pillar is not a column: it is a damper.³³

Stranger still. Because the tower narrows at each level, the load-bearing columns of one storey are not in line with those below. In a five-storey pagoda, each storey is independent: no column ties them together.⁴⁴

The storeys are not joined to one another: they are simply set one on top of the other, with joints loose enough to let them slide.

When the ground shakes, each storey sways in its own way and damps the motion of its neighbours — this is called the snake dance. The central mast, for its part, stops each one from going too far: the storey strikes it, and the mast holds it back like a bolt.⁵⁵

One qualification, however, and it comes from the researchers themselves. The mast does not explain everything. The very deep eaves also contribute, playing the part of the arms of a balancing toy: the further the mass is from the axis, the easier the whole is to balance. A simulation of the oldest pagoda still standing demonstrates this clearly. The researchers tested seven mechanisms in it — including the sliding of the joints, their friction, the snake dance and the balancing effect of the eaves.⁶⁶

Taken one by one, none is enough: in each case the tower would exceed the deformation at which it collapses. Combined, they bring the displacement down to 56% of what a rigid structure would undergo, and the tower holds.⁷⁷

The resistance therefore comes not from a single trick but from a combination.

The authors add an unexpected remark for an engineering paper: the most effective factor may be the care of the faithful who have maintained the building for thirteen centuries. The same idea is found at Djenné and at Dujiangyan.⁸⁸

The result, at any rate, is on record: in thirteen centuries, no document reports a five-storey pagoda being toppled by an earthquake.⁹⁹

In the Andes, the solution is different: blocks of stone cut to fit together, laid without any mortar, with joints so tight that a blade cannot be slipped into them. The walls lean slightly inwards, and the doorways are narrower at the top than at the bottom. Without mortar, the stones can shift a little during a tremor instead of cracking.¹⁰¹⁰

A story that is often repeated calls for caution here. These walls are said to be “earthquake-proof”. That is not accurate.

Engineers have used computer models to study how an Inca wall behaves during an earthquake: beyond a certain intensity, the wall is damaged and may become unstable. Researchers have found, at several Inca sites, the deformations left by past earthquakes.¹¹¹¹

The reality is more interesting than the legend. These walls are not indestructible. They behave better than rigid walls — and they have suffered all the same.

The essential point lies elsewhere: building in wood or fitting stones without mortar is no less skilled than building massively. Where the ground moves, it is often more intelligent. But no technique makes a building invincible, and being wary of stories that are too good to be true is part of the craft.

Building in earth

The most widely used material in the whole history of building is neither stone nor wood. It is earth.

Mud brick dried in the sun, earth rammed between boards, a mixture of earth and straw: people have built this way on every continent. Earth is available everywhere, costs almost nothing, and keeps cool in summer just as it keeps warm in winter.

Its weak point is water. An earth wall must be protected and maintained.

In Djenné, in Mali, stands the Great Mosque. UNESCO describes it as the largest mud-brick building in the world. Every year, the mosque is replastered during a great festival that brings the inhabitants together.¹²¹²

A surprise awaits anyone who looks into how long it has been there.

There has indeed been a mosque on this site since the thirteenth century (the 1200s): a ruler of the city had it built in place of his palace. But that one no longer exists. Around 1834, a leader who considered it too sumptuous let it fall into ruin.

The building admired today dates from 1907. It was rebuilt at the request of a local notable, organised by the French colonial administration, which then occupied the country, and built by the masons of Djenné under the direction of a local master mason, Ismaïla Traoré. Its three great towers, the part everyone recognises, are probably an invention of that building campaign.¹³¹³

Does this take anything away from the building? Quite the opposite.

An earth wall does not stand up on its own. What lasts in Djenné is not the material: it is the act of remaking it. A mosque from 1907, replastered every year since, illustrates the point of this page even better. What lasts, lasts because it is remade.

The material is called banco. It is earth and water, to which rice bran, baobab powder and shea butter are added. It has to be stirred at length before use — and children are invited to play in it, which ensures the mixing while letting them take part.

The work is directed by the city’s guild of masons. Men, women and children take part, and the elders, who did it before them, watch and advise. It is not an emergency repair: it is the normal functioning of the building, repeated every year for more than a century.

A monument, here, is not a thing whose construction is finished. It is a thing that is remade together, every year. This is also how the skills are passed on to the young: through practice.

The page on water tells the same story in China: the Dujiangyan works have divided a river for two thousand two hundred years, and they hold because they are dredged every winter.

Who built?

Two final remarks.

First, what we see of the past is misleading. What remains is above all the buildings made to last: temples, palaces, tombs. Of ordinary houses, the walls almost never survive — but the floor, the post holes and the broken pottery often do. Archaeologists excavate these traces, and one site has even preserved them whole: at Joya de Cerén, in El Salvador, a Maya farming village was buried under the ash of a volcano around the year 600. The earthen houses are there, the tools too, and even the remains of meals.¹⁴¹⁴

Second, these buildings were built by someone. The answer changes from one building site to another — which is exactly what makes the question interesting.

The cathedrals? Paid workers, at least at Durham, in England. It is often believed that they were built by volunteer worshippers; there, the building accounts say otherwise. They record, week after week, who worked how many days and for how much.¹⁵¹⁵

The pyramids of Giza? Neither slaves nor volunteers. Probably peasants who owed a tax in labour, and who came in rotation. Their village has been excavated: they ate beef, their fractures were treated, and they had their tombs near the building site. Their skeletons also show work that wore bodies down.¹⁶¹⁶

The United States Capitol, in Washington? Enslaved men. From 1792, they were hired from their owners to clear the ground, quarry the stone, raise the walls. The payrolls still exist: the money went to the owner, who signed in place of the man who had done the work.

One of them is known by name. Philip Reid took part in casting the great bronze statue placed at the top of the dome. It is called Freedom. He received his Sunday pay; on the other six days, the money went to the man who owned him.¹⁷¹⁷

Spanning a gap: five solutions