Bridges

How it is calculated

History and fun facts

Roman stone arch bridges

The Romans built bridges with semicircular arches of wedge-shaped stones. Such an arch was built on a temporary wooden centring; once the keystone in the middle was in place, the arch carried itself and the timber could go. The piers stood on foundations in the river bed. Many of these bridges are still standing after almost two thousand years.

• The Pons Fabricius in Rome dates from 62 BC and is still in use.
• The bridge of Alcántara in Spain was built under the emperor Trajan, around the year 105.
• A round arch pushes mostly down and outwards. That is why the arches of a long bridge lean against each other on narrow piers.

Medieval bridges with cutwaters and refuges

Medieval stone bridges often got pointed cutwaters on the piers. They split the current and hold back ice and driftwood. Above the cutwaters builders made refuges, so pedestrians could step aside for a cart. The middle arch was often the largest, for boats, which gave the deck a hump in the middle.

• Charles Bridge in Prague was begun in 1357 and has 16 arches. The statues were added much later, mostly in the Baroque period.
• The Pont Neuf in Paris was completed in 1607 and, despite its name (new bridge), is the oldest standing bridge across the Seine in Paris. It has half-round refuges above the piers.
• Only four arches of the famous bridge of Avignon are still standing.

Bridges with houses: Ponte Vecchio and old London Bridge

Space on a bridge was valuable: everyone passed by. So people built shops and houses on it. The Ponte Vecchio in Florence was rebuilt after a flood in 1345 with three segmental arches. At first butchers worked there; since the end of the 16th century it has been goldsmiths and jewellers. Old London Bridge was finished around 1209 and stood for about six centuries, full of houses and shops.

• The Vasari Corridor runs over the Ponte Vecchio, a covered passage from 1565 that let the Medici walk dry and safe from palace to palace.
• In 1944 the Ponte Vecchio was the only bridge in Florence that was not blown up.
• The thick piers of old London Bridge narrowed the river so much that rapids formed. Shooting the bridge by boat was dangerous.

Fortified bridges: Pont Valentré

A bridge was also a weak spot in a town's defences. That is why some bridges got gate towers with battlements. The Pont Valentré in Cahors, France, was built in the 14th century, with six pointed arches and three towers. It is one of the best preserved fortified bridges in Europe.

• According to legend, the master builder made a pact with the devil to get the bridge finished.
• During a restoration in the 19th century the middle tower got a small stone devil, as a nod to that legend.
• The tall cutwaters run up to the deck and make the bridge look extra tough from the side.

Packhorse and humpback bridges

In England, mainly in the 17th and 18th centuries, narrow stone bridges crossed streams along the routes of packhorses. They are narrow and have low parapets, so the baskets on the sides of the horse did not hit them. The arch is high, so the water can pass underneath in a flood.

• Because of their steep back they are also called humpback bridges.
• Chinese moon bridges have such a high round arch that the arch and its reflection together look like a circle.
• An even older form is the clapper bridge: flat stone slabs on piles of stones, as on Dartmoor.

Covered wooden bridges: Kapellbrücke

A roof protects the wooden deck and beams from rain, so the bridge lasts much longer. The Kapellbrücke in Lucerne dates from the 14th century and is considered one of the oldest covered wooden bridges in Europe. Next to it stands the octagonal Wasserturm, which served as a prison, an archive and a treasury, among other things.

• Triangular paintings from the 17th century about the history of Lucerne hang under the roof.
• In 1993 a large part of the bridge burned down. It was rebuilt afterwards.
• In the United States covered wooden bridges from the 19th century are also very well known.

Drawbridges and Dutch bascule bridges

A castle with a moat could raise its bridge so attackers could not cross. Often there was also a portcullis behind the leaf. In the Netherlands drawbridges with a portal and balance beams are well known: the heavy beams on top balance the leaf, so one person can open the bridge. The Magere Brug in Amsterdam is a double bascule bridge across the Amstel with two leaves that meet in the middle.

• There are several stories about the name Magere Brug (skinny bridge). A well-known explanation is that the first bridge was very narrow.
• In 1888 Vincent van Gogh painted the Pont de Langlois near Arles, a drawbridge in Dutch style.
• The beams run parallel to the leaf. That is why the rods between them keep hanging straight down.

Trestle bridges

In the 19th century railways and mines in North America quickly and cheaply built long wooden bridges on trestles: rows of posts with cross bracing. Timber was plentiful and the bridges could cross a deep valley in a short time.

• Many of those wooden trestles were later replaced by steel or filled in with earth.
• The cross braces keep the posts stiff, so the bridge does not wobble sideways.

Roman aqueducts: Pont du Gard

An aqueduct carried water from a spring to a town, with a very small fall. Where a valley was in the way, the Romans built a bridge with arches. The Pont du Gard in southern France dates from the 1st century AD and was part of a water channel about 50 kilometres long to Nîmes. It has three rows of arches and is about 49 metres tall.

• Over its whole length the water channel drops only about 12 metres.
• The large stones of the lower rows mostly sit on each other without mortar.
• The Pont du Gard has been on the UNESCO World Heritage List since 1985.

Log bridges

A tree across a stream is probably the oldest bridge there is. People laid one or more logs side by side and sometimes made a railing of rope or branches. In forests and mountains you still see them along footpaths.

• A top that is hewn flat makes a log much less slippery.
• A support in the middle halves the length a log has to carry.