The earliest improvement on the simple beam bridge may well have been a single pair of stones wedged into an inverted V-shape between the banks of a stream. Stepping on the stones merely presses them more tightly together, providing a secure bridge.
The occasion and date of the historic transition to a wholly man-made arch is, of course, now hidden from us. Certainly, by 1500 BC, we have evidence that 'corbelled' brick arches were being used throughout the Middle East to support the walls above doorways – a solution to the 'bridging' problem of carrying a load across a gap.
In the seventh century BC the Sumerians used five such corbelled arches to take an aqueduct across a 20-metre-wide valley. This was the first recorded arch bridge - rather clumsy by modern standards, but such is all innovation when viewed with hindsight.
© Simon Dakeyne
By the second-century BC the Romans had realised that it was possible to move from vertical to horizontal by means of a smooth curve, and had gone on to invent the familiar rounded arch. The art of bridge-building had really begun.
The basic arch design
The arch is really a beam curved to form a semicircular shape, which is prevented from straightening and spreading sideways by strong 'abutments' at either end.
The traditional shape of the arch is made from a series of blocks carefully cut to fit together perfectly. These 'voussoirs' are wedge-shaped, and gradually take the curve of the arch from the central and vertical 'keystone' down to the outermost and horizontal 'footers'.
As the keystone is pushed downwards by the load above it, its wedge shape means that it pushes outward onto the voussoirs to each side. Thus the forces are spread sideways, rather than downwards, and thence around the arch. Ultimately, the entire load (the weight both of the bridge itself and of any traffic crossing it) is transferred partly down into the ground and partly out to the elements of the bridge to either side of the keystone.
© structurae.de (Jacques Mossot)
This is the key feature by which the arch improves on the simple beam – the partial dissipation of vertical forces horizontally. This does mean, however, that for an arch to work there always needs to be substantial material to the sides, to stop the arch spreading and the central section collapsing inwards. This is what makes early arch bridges so massive in form.
The road deck can be around the curve of the arch itself or it can rest on the arch.
© structurae.de (Per Wåhlin)
The Romans used the concept of the stone arch for a huge range of building applications, from doorways to vaulted roofs and from bridges to sewers. They were certainly the first civilisation to build large-scale bridges, many of which still stand today.
In many ways, the strides the Romans took in bridge-building mirrored their strides in road-building, and were the foundations for the rapid expansion of the Roman Empire. An army and its supplies need to travel efficiently, and crucial to achieving that end is an ability to cross rivers and chasms with heavy loads.
© structurae.de (Nicolas Janberg)
It is important to note that Roman arch bridges often consisted of several small arches supported on intermediate piers. This should remind you of the drawbacks of small spans, mentioned earlier when we were discussing beam bridges.
First, there is the problem of building sturdy foundations halfway across a river. This called for the development of ingenious ways of creating dry spaces mid-river while workers dug down to bedrock and built stone and wooden 'mini-towers' to support the piers. The 'caissons' and 'cofferdams' invented 2000 years ago are still used today, but can now be sunk to depths of 30 metres or more.
Second, there is the problem that any piers built mid-stream obstruct both the flow of the river and any traffic using it.
© structurae.de (Jacques Mossot)
This is still a problem, particularly during times of heavy rainfall when the current through the narrowed gaps beneath the arches can make passage upstream literally impossible. Paris, which sits astride the mighty River Seine, regularly suffers from this problem.
Arch bridges remained the design of choice for heavy traffic bridges for over 2000 years, but bridge engineers were constantly incorporating changing fashions and new materials. As we will see, the arch has gone through some extraordinary and often beautiful reincarnations.
A single arch of the Pont Neuf, Paris.
The central parts of the Alcantara Bridge in Extramadura, Spain, date from Roman times, but it was partially destroyed by the Moors in 1214. "El Kantara" means "bridge" in Arabic.
Pont du Gard Roman aqueduct near Nimes in France.
Three arched Bridge of the Archbishopric (Pont de l'Archevêché), built in record time between April and November 1828 under the supervision of the engineer Plouard. Pont de l'Archevêché straddles the Seine in Paris, France, and suffers access problems between the arches during heavy rainfall.