Bridges

The Golden Gate Bridge and San Francisco, CA at sunset taken from the Marin Headlands (Photo credit: Wikipedia)

From early times, people have built all kinds of structures: tunnels and bridges to cross natural barriers; canals for transporting goods and for irrigation. Today, engineers are constantly improving construction methods and materials to produce bridges and canals that link oceans and skyscrapers that are more than a hundred store's high.


The first bridges were made by placing tree trunks across rivers, and laying
flat stones on rocks in the middle of streams. The Romans built bridges using concrete made by mixing 

volcanic soil, lime and water. 

The concrete was so good that six 

out of the original eight bridges 

across the River Tiber near Rome are 

still standing. Today, bridges made 

of concrete and steel, span lakes,

rivers, deep valleys, roads and railway 

lines.

Types of Bridges

Suspension


The deck, or roadway, is suspended from long steel cables attached to tall towers standing near each end of the bridge. The cables are made of thousands of steel wires bound tightly together. The longest suspension bridge is the Humber Bridge, England.

Arch
This type of bridge is supported by an arch, propped up by abutments. The deck is usually straight, and does not follow the curve of the arch. It is sometimes built below, as well as above, the arch. The biggest steel arch bridge in the world is across the New River Gorge in West Virginia, USA. It is 518 m long.

Bascule
This is a type of drawbridge. Its two sections can be raised at an angle to allow ships to pass through.

Beam
Most bridges with a short span are beam bridges.Each end of the bridge rests on the ground or on piers. In modern beam bridges, the beams are often long, hollow boxes made of steel or concrete. This makes them light but very strong. Bridges constructed like this are called box-girder bridges.

Swing
This type of bridge can be swung to one side to allow ships to pass through.

Pontoon
This type of bridge floats on the water. The deck rests on hollow concrete blocks, called pontoons, which lie on the water. Each pontoon is anchored to concrete blocks, which are buried in the bed of the lake.

Cantilever
This is a type of beam bridge. Each half of the bridge balances on a supporting pier that is embedded in the river. The world’s longest cantilever bridge is the Quebec railway bridge across the St. Lawrence river, Canada, built in 1917.

Cable-stayed
The deck is supported by steel cables connected to towers. Early cable-stayed bridges had pairs of towers, but today, single towers are also built in the centre of the deck. Its style is midway between a beam
bridge and a suspension bridge. It requires fewer piers than a beam bridge and does not need the heavy anchorages of a suspension bridge. The Normandy bridge across the Seine estuary in France is 856 m long. It is the longest cablestayed bridge in the world.