How an arch holds up
Start with what an arch replaces. A stone beam laid across two posts, a lintel, bends under weight: its lower edge is stretched, and stone breaks easily when stretched. An arch avoids the problem by changing the shape. Its blocks, called voussoirs, are cut as wedges, wider at the top than at the bottom, and set side by side along a curve. A load on top pushes each wedge against its neighbours, and the whole ring ends up squeezed, which is what stone does best. The central block at the top is the keystone; the point where the curve starts to rise from its supports is the springing.
Two consequences follow. An arch can span a much wider opening than a lintel, and it can be built from small blocks that a few workers can lift, instead of one huge stone. But an arch also pushes sideways, at its feet, as if it were trying to open. That push, the thrust, has to be stopped by a thick pier, a heavy wall or another arch next to it. A row of arches works like a line of people leaning shoulder to shoulder: each one holds up its neighbour, and only the two at the ends need a wall to lean on.
A Roman engineer’s warning
Roman builders knew all this. Vitruvius, the architect who wrote ten books on architecture under Augustus, advised placing “arches consisting of wedges concentrically arranged” over doors and windows, so the wall above would not crush the beam below. He also warned that in a row of arches the outer piers must be wider than the others, because the wedges “press towards the centre, and have a tendency to thrust out the abutments”. That is the whole theory of the arch in two sentences, written two thousand years ago.
Building one required a temporary support. The voussoirs were laid on a wooden frame, the centring, cut to the shape of the curve, and the frame could come out only once the keystone had closed the ring.
So who invented it?
Not the Romans. Arches and vaults were built in Egypt and Mesopotamia many centuries before Rome existed. In Italy, the Etruscans built arched city gates, and Rome learned much of its building from its Etruscan neighbours. The difference was one of scale and confidence. Rome turned the arch from an occasional solution into its standard way of crossing a gap.
One shape, many jobs
Once you start looking, the Roman arch is everywhere. It carries roads over rivers in the bridges and water over valleys in the aqueducts. The Aqua Claudia, finished in 52 CE, ran for almost ten kilometres on arches as it approached Rome, and in the Parco degli Acquedotti its arcades still rise nearly 28 metres. Stack arches in rows and you have the façade of a theatre: at the Theatre of Marcellus (13 or 11 BCE), each arch is framed by a half-column, Doric on the lower floor and Ionic above, so the Greek orders dress a structure that no longer needs them.
Push an arch through depth and it becomes a barrel vault, a curved ceiling over a corridor or a hall. Rotate it around its centre and it becomes a dome. Most of what makes Roman interiors feel so large comes from these two steps.
An arch with nothing to carry
Rome also turned the arch into a monument. A triumphal arch carries no road and no water; it stands across a street as a gateway that leads nowhere in particular, and its job is to be walked through and remembered. The Arch of Titus, built after the emperor’s death in 81 CE, celebrates his victory in the Jewish War. Inside the opening, one relief shows Titus on his chariot crowned by Victory, the other the procession carrying the spoils of the Temple of Jerusalem, the seven-branched candlestick among them. On the ceiling, Titus rises to heaven on an eagle. The Arch of Constantine, dedicated by the Senate in 315 CE after Constantine’s victory over Maxentius at the Milvian Bridge, reuses sculpture taken from earlier monuments of Trajan, Hadrian and Marcus Aurelius.
The Roman arch, then, is an old idea that Rome made its own. Its strength comes from wedges pressed together and from supports strong enough to hold the thrust; its reach comes from the Roman habit of using it for everything.
Where to see it
In Rome: the Theatre of Marcellus near the Tiber, the Arch of Titus on the Via Sacra in the Roman Forum, the Arch of Constantine beside the Colosseum (both inside or next to the Parco archeologico del Colosseo), and the aqueduct arcades in the Parco degli Acquedotti, on the south-eastern edge of the city.
Sources
- Encyclopaedia Britannica, “Arch”; “Vault”.
- Vitruvius, On Architecture 6.8.3-4 (trans. Gwilt, LacusCurtius).
- Frontinus, De aquis 13-14 (trans. Bennett, LacusCurtius).
- Turismo Roma (Roma Capitale), “Parco degli Acquedotti”; “Teatro di Marcello”.
- Parco archeologico del Colosseo, “Arch of Titus” (restoration) and “Arch of Constantine”.
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