Simple bridges

Even the most primitive human communities must often have created bridges from material lying easily to hand. Hunters and gatherers follow favourite paths; streams need to be crossed. A fallen tree can be dragged into position to serve as a plank. Forest tendrils may be intertwined as an elementary suspension bridge. Or rafts can be tied together in a pontoon.

But bridges over more than a narrow width require an architectural device which arrives relatively late on the scene. This is the Roman arch - an invaluable principle which they make their own. It is with them that the story of bridges begins.

Roman bridges: 1st - 2nd century AD

Bridges are as much part of the Roman architectural achievement as Aqueducts, and they present even greater constructional problems.

Some of the most impressive Roman bridges are over ravines. A fine surviving example, built for Trajan in AD 105, spans the Tagus in Spain, at Alcántara. Its two massive central arches, 110 feet wide and 210 feet above the normal level of the river, are made of uncemented granite. Each wedge-shaped block weighs 8 tons. During construction these blocks are winched into place by a system of pulleys, powered perhaps by slave labour on a treadmill. They are supported on a huge timber structure standing on the rocks below - to be removed when the arch is complete.

An equally remarkable feat of Roman construction is the building of bridges across rivers where no rock or island emerges from the water to carry the piers. An example survives in Rome - the Sant'Angelo bridge, built for Hadrian in AD 134 as an approach to his great circular mausoleum, now the Castel Sant'Angelo.

The building of such bridges is made possible by the Roman perfection of cement and Concrete, and by their invention of the Cofferdam.

Inhabited bridges: 12th - 16th century AD

The greatest contribution of the Middle Ages to bridge building is the attractive notion of bridges with houses on them. This development has two practical origins. In walled cities, where accomodation is strictly limited, any firm foundation for a building is valuable; and with Water mills now a common source of power, a bridge with a mill upon it serves two useful purposes.

Inhabited bridges are built in considerable numbers. France is known to have had as many as thirty-five. In the 16th century in Paris the Ile de la Cité is joined to the banks of the Seine by three such bridges on one side and two on the other.

The most famous bridge with houses is also one of the earliest and the longest lasting. London Bridge is built between 1176 and 1209, with the work apparently entrusted to Peter, chaplain of St Mary Colechurch. His task is formidable. This is the world's first stone bridge to be constructed in a tidal waterway, with a large rise and fall of level every twelve hours.

The stone foundations of the nineteen pointed arches are placed within timber cofferdams, in the technique pioneered by the Romans. The piers and their protective cladding are so thick that the river's width is reduced by 75%. Water surges between them like a mill race - with the useful side effect of keeping the water level artificially high upstream of the bridge.

Old London Bridge, with its tall and picturesque rows of houses and shops, lasts for more than six centuries until finally replaced in 1823.

Of surviving medieval bridges with houses on, the Ponte Vecchio in Florence is probably the best known. When built in 1345 it replaces one on the same site also known as the Ponte Vecchio, so the present bridge is first known as the Nuovo Ponte Vecchio ('new old bridge'). The covered way which forms a top storey above the shops is added in 1565 to enable the Medici to walk from the Uffizi to the Palazzo Pitti on the other side of the river without descending to street level.

Ironbridge: AD 1779

In the space of a few months in 1779 the world's first iron bridge, with a single span of over 100 feet, is erected for Abraham darby (the third of that name) over the Severn just downstream from Coalbrookdale. Work has gone on for some time in building the foundations and casting the huge curving ribs. But in this new technology little time need be spent in assembling the parts - which amount, it is proudly announced, to 378 tons 10 cwt. of metal.

The lightness of the structure strikes all observers. An early visitor comments: 'though it seems like network wrought in iron, it will be uninjured for ages.' It is uninjured still. A great tradition, bringing marvels such as the Crystal palace, begins in this industrial valley.

This History is as yet incomplete.