BuiltWithNOF
Bridge Types

Bridge design stretches back a long way in time! New bridges are being developed to make use of new materials, constructional methods and design ideas. Perhaps one day we will see a bridge made from plastic or glass!

Bridge types are related to each other and sometimes the features of one type are incorporated into another so that the final bridge is a hybrid. Some bridge types have gradually ‘morphed’ over the years as new techniques and materials are developed. This makes naming the bridge type complicated in some cases!

Beam bridge

A beam is simply a slab of strong material placed across a gap. The beam resists bending caused by the downward force applied when we walk on it. The force applied to it is transmitted through the beam down into the supports which rest on the ground.

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This is a stone slab ‘clapper bridge’. The beam is a slab of stone and it rests on rocks on either side of the river. Clappers like this aren’t very long so they can’t span very big gaps. Clappers are very very heavy so bridges like this are difficult to build!

beam bridge concrete

This is a concrete beam bridge. It is made from several long narrow beams of concrete with a normal tarred road laid on top. The beams are made by pouring the concrete into a mould and allowing it to set hard. Steel wires are set into the concrete to reinforce it and make it better at resisting bending.

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This picture shows two beam bridges. The lower bridge is made from concrete beams and the upper bridge is made from steel beams. The steel beams have a cross sectional shape of the letter ‘I’ as below.

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Arch bridge

This picture shows the 1850 Britannia Bridge in North Wales designed by Robert Stephenson of Newcastle. It is a box girder beam bridge and was the first of its kind. Right is a real piece of the girder used in the original bridge. It is made of flat metal plates riveted together.

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An arch provides a longer span then a simple beam and lots of headroom. Arches were originally built from shaped stone blocks but now they can be cast in concrete or made from steel girders.

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This is one of the lovely small scale bridge seats at the Isambard Kingdom Brunel Museum in London at the entrance to his famous Thames Tunnel. The concrete arch has very little material in the centre but gets its strength from the thickness of the material curving out symmetrically from the supports to the left and right. The force at the centre is transmitted very efficiently to the supports by the arch shape. This is a superior design to the simple beam bridge but looks very similar.

This London Bridge 1880. It is a series of arches made from stone blocks. Each arch rests on piers either side which support it. The piers are really columns which go down deep into the water and into the rock foundations below.

This is the famous bridge which was sold to the Americans and rebuilt in America where it is still in use today.

This is Causey Arch railway bridge in County Durham. It was built in 1725 as a railway bridge and carried horse drawn carts loaded with coal on wooden rails to the River Tyne to unload into ships.

At the time is was the biggest single span stone arch bridge in Britain! This is one of my favourite bridges and you can walk across it and visit the nearby Tanfield Steam Railway.

The Romans were well known for their stone arch bridges. They also used concrete in their bridges and this improved the strength.

As iron and steel became more widely produced they were used in all sorts of bridges including arch bridges. When iron is produced in a blast furnace it is in liquid form and can be poured into moulds to make all sorts of girder and curved frame shapes. A whole bridge structure could now be made from individual parts cast in iron and the parts could be transported to the site where the bridge was to be built. A very important example of this is the iron bridge at Coalbrookdale in Shropshire which was built in 1781.

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Girder ‘Truss’ bridge

Compared to the Causey Arch stone bridge you can see that there is lots of empty space in the iron bridge!

The amount of material in an iron bridge is far less than in a stone bridge.

Iron bridges are easier and quicker to build.

The pieces of an iron bridge fit together like a lego model.

Advances were made in the manufacture of metals which allowed cast iron to be made into ‘steel’ which was much stronger and could resist very larger forces. Very soon it was possible to join steel girders together in all sorts of configurations to make ‘Truss’ bridges.

A Truss is simply a geometric pattern of connected steel girders (or ‘beams’). Here are some Truss bridge designs...

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This is a ‘Bailey’ bridge which was designed for quick assembly in the second world war to replace bombed bridges.

It has the features of some of the designs above but is composed of rectangular sections which bolt together in a repeated sequence. Bailey bridges were transported as a kit of parts on lorries and could be be built from one side of a river to the other starting from on side and finishing on the other! - how did they do that?

Mechanical bridges (moving bridges)

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All sorts of mechanisms have been used in mechanical bridges. The bridges might ‘tilt’, ‘lift’, ‘rotate’ or ‘traverse’.

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The amazing bridge below is the Gateshead Milenium Bridge which links Gateshead and Newcastle across the River Tyne in the North East of England. It is a pedestrian and cycle bridge and is nicknamed the ‘blinking eye’. It tilts to allow ships underneath. It is a moving cable stayed arch bridge.

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Here is the ‘blinking eye’ fully raised. You can see far left the cycle-way and pedestrian way raised up in the air - closed off to the public of course when the bridge is raised!
The arch to the right is the main support arch when the bridge is ‘down’. With the main arch above and the deck below it is the job of the steel wires to support the deck and the weight of the people crossing the bridge. All of this weight is transferred through the arch and down into the foundations at each end of the bridge. Brilliant!

The most famous moving bridge of all is most probably this one...

Tower bridge

Tower Bridge across the River Thames in London!
It has two ‘cantilever bridges’ which meet in the middle but which lift from the horizontal to almost vertical to allow ships to pass.
A cantilever is a beam which is supported at one end only!

Question. What is the connection between London Tower Bridge above and the Swing Bridge in Newcastle (Swing bridge in the foreground below)?

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Answer. Lord William Armstrong of Cragside in Northumberland designed the mechanisms which make both bridges move!
When originally built they both worked by ‘Hydraulics’ or Water Power.
Another connection is that both bridges are ‘cantilevers’. The Swing Bridge is supported centrally on a single pier and the two ends stick out over the river either side. Tower bridge is supported at its ends and the bridge halves stick out and meet in the middle.

In both cases the ends of the bridge are not being supported by a structure - they stick out into free space! The Swing Bridge main structure is a steel arched truss bridge - or ‘bowstring’.

Also pictured above on the River Tyne are the iconic ‘Tyne Bridge’ and the very recent milenium Bridge. The Tyne bridge is a hybrid bridge being a combination of a steel arch made from trusses and a suspended road deck hung from long steel rods called stays.

Cantilever bridge

As mentioned above a cantilever is a beam supported at one end so that the free end sticks out over a gap unsupported. Many bridges are of the cantilever type but it can be tricky to spot because the bridges can be constructed in various ways and may look very different!
The Forth rail bridge in Edinburgh is the classic example of the cantilever bridge.

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It is a combination of 3 double cantilevers. Here they are before the central road decks were added.....

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The double cantilever structure can stand on its own. It is balanced and starts from a central concrete pier. The steel truss structure juts out at either side unsupported.
The gaps between the cantilevers are then bridged by smaller steel truss bridges which hang in the air but are really being supported at each end by a cantilever.

This beautiful modern concrete and steel bridge is in Barcelona Harbour. It has two cantilevers which stick out from large concrete foundation piers.
Each half can stand alone but they neatly meet in the middle to complete the bridge.

 

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Without the cantilever principle we would need an extra pier in the middle to support the free ends of the bridges and this would be a major obstacle in the harbour!

Suspension bridge

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This is the Humber Bridge.
The two very long curved black lines are very thick steel cables made from thousands of twisted steel wires. Hanging from these two main suspension cables are thinner steel cables which attach to the concrete and steel sections of the road deck. The road deck is hung or suspended from the main support cables.
The two support towers are on very deep foundations in the river and on either bank the ends of the two main support cables go deep into concrete foundations to hold them tight.

Tower Bridge also incorporates two suspension bridges - one at either end on the approach to the moving cantilevers.

Other special bridges and combinations of bridge types

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