Arches for strength, piers for everything else
A bridge has two jobs: carry its load across the gap and stand up to the river. For the first, the Romans relied on the Roman arch, almost always a full semicircle. A stone arch can span far more than a stone beam, because the load presses its wedge-shaped blocks together instead of bending them, and it can be built from blocks small enough to carry. Most Roman bridge builders did not even use mortar between the stones: the precision of the cutting was enough.
But every arch also pushes sideways at its feet, and in a bridge of several arches that push lands on the piers in the river. A row of arches can rest on fairly slim supports, because each arch leans against its neighbour and their thrusts cancel out, as long as the two arches at the ends have solid banks to push against. The weak point was the foundation. A pier on rock is easy. A pier on a soft riverbed, in moving water, is not.
How do you build a pier in a river?
The Roman answer was the cofferdam. Workers drove wooden piles into the riverbed in a ring, so close together that they formed a wall, and sealed it, often with clay. Inside that enclosure they could work on the foundation. Then came the material that made the difference: concrete mixed with pozzolana, the volcanic ash that sets under water, poured into the ring of piles. The pier rose on a base that the river could not wash away.
Most Roman bridges that survive stand on rock, which tells you how much the builders preferred it. The Ponte Sant’Angelo in Rome, in front of Castel Sant’Angelo, is the exception that proves the method: its piers still stand on cofferdam foundations laid in the bed of the Tiber more than 1,800 years ago.
Rome’s oldest working bridge
The Pons Fabricius, today Ponte Fabricio, links the left bank of the Tiber to the Isola Tiberina, the island in the middle of the river. It was built in 62 BCE to replace an older wooden bridge, and inscriptions on its two arches record the builder, Lucius Fabricius, who held the post of curator viarum, the official responsible for the city’s roads. Rome’s tourist board calls it the best-preserved Roman bridge in the city and, with the Ponte Milvio, the oldest still in use. You can walk across it today on your way to the island.
Bigger: the Tagus and the Danube
The great Roman bridge of Spain crosses the Tagus at Alcántara, a town whose name is simply the Arabic for “the bridge”. It was built in 105-106 CE in honour of the emperor Trajan, runs 204 metres on six arches with spans of about 29 metres, and some of its voussoirs weigh up to eight tons. The Moors destroyed it in 1214; it was restored in 1543 and many times since, and it is still standing.
Trajan also built the most ambitious crossing of all, over the Danube. The historian Cassius Dio describes twenty piers of squared stone, 150 feet high above their foundations and 60 wide, standing 170 feet apart and joined by arches. In Dio’s own day only the piers were left: Hadrian had removed the upper part of the bridge, afraid that enemies might use it to cross into Roman territory.
Why they last
A Roman bridge survives because its arches work in compression, because its piers sit on rock or on concrete that sets under water, and because, like the roads they served, they stayed useful: people kept crossing them and kept repairing them. The same structural idea carried water as well as traffic. On aqueducts such as the Pont du Gard, the arches hold a water channel instead of a road, but the logic of piers, voussoirs and thrust is the same.
Where to see it
In Rome, cross the Ponte Fabricio to the Isola Tiberina, and the Ponte Sant’Angelo in front of Castel Sant’Angelo; the Ponte Milvio lies further north. In Spain, the bridge of Alcántara is in the province of Cáceres, near the Portuguese border.
Sources
- Encyclopaedia Britannica, “Arch bridge” (Roman arches, cofferdams, pozzolana, Sant’Angelo, Alcántara); “Alcántara”.
- Turismo Roma (Roma Capitale), “Ponte Fabricio”.
- Cassius Dio, Roman History 68.13 (trans. Cary, LacusCurtius).
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