Suspension Bridge
what is it and how does it work?
The Structure
Modern suspension bridges use a box section roadway supported by high tensile strength cables.
In the early nineteenth century, suspension bridges used iron chains for cables. The high tensile cables used in most modern suspension bridges were introduced in the late nineteenth century.
Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking.
Structure Fail
Some bridges have in the past suffered from structural failure. This may be combination of poor design and severe weather conditions.
When it was opened in 1940, the Tacoma Narrows Bridge was the third longest suspension bridge in the world. It later become known as "Galloping Gertie," due to the fact that it moved not only from side to side but up and down in the wind. Attempts were made to stabilize the structure with cables and hydraulic buffers, but they were unsuccessful.
The Structure
In the early nineteenth century, suspension bridges used iron chains for cables. The high tensile cables used in most modern suspension bridges were introduced in the late nineteenth century.
Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking.
The First Suspension Bridge Built
The first suspension bridge ever built is Brooklyn Bridge. Construction started in January,1870 and of course it was built in Brooklyn. The single greatest wonderment of the Brooklyn Bridge is not its size, beauty, function or even technology, but the fact that it was created by hand. When construction began, neither the light bulb nor the telephone (nor the jackhammer) had been invented. It is truly the Great Pyramid of bridges.
Akashi Kaikyo Bridge
The Akashi Kaikyo Suspension Bridge is the longest suspension bridge in the world and it is probably Japan’s greatest engineering feat.
It took two million workers ten years to construct the bridge, 181,000 tonnes of steel and 1.4million cubic metres of concrete. The steel cable used would circle the world seven times.
Why is it there?
It has six lanes and links the island of Awaji and the mainland city of Kobe, a distance of four miles. The concept of building a bridge across the Akashi Straits became urgent after a disaster in 1955. A ferry carrying over one hundred children sank after colliding with another ferry, in the busy shipping lane. One hundred and sixty eight children and adults died in the disaster. Political pressure for a bridge increased and in 1988 construction began.
Structure
The Akashi Straits is four miles wide at the bridge site with sea depths of one hundred metres and currents averaging fourteen kmph.
The construction of a suspension bridge involves the use of two main cables stretching between two towers. The roadway beneath these is suspended by more cables. To stop the towers, roadway and cables collapsing, they are held at either end by large anchor blocks (the Akashi anchor blocks weigh 350 000 tonnes). In the case of the Akashi-Kaikyo Bridge, suspension bridge technology was pushed to the limit.
The Cable Anchors
The Cables
Cables: If it wasn’t for the huge cables to suspend the bridge, the bridge would sag. They undergo a lot of tension and compressive forces, and for this they need to be extremely strong. The material used in these cables is steel, it is an alloy of iron, manganese and tungsten, which make it extremely strong, durable and hard.
The Tower
Towers: The towers are yet another important component of a suspension bridge. They help keep the cables suspended, and convert the heaviness from the cables into pressure that forces the towers into the ground. They can be made out of iron/steel just like the cables or sometimes stone.
Closure Disadvantages
Suspension bridges can be unstable in extremely turbulent conditions, with extreme cases requiring temporary closure of the bridge.
(Picture above, Mackinac Bridge)
Foundation Disadvantages
When built in soft ground, suspension bridges require extensive and expensive foundation work to combat the effects of the heavy load on foundation towers.
(Picture above, traffic on Anzhaite Long-span Suspension Bridge)
Heavy Loads Disadvantages
Flexibility can be a disadvantage to suspension bridges, which can flex under heavy, concentrated loads. Suspension bridges are not generally used for regional rail crossings that carry maximum weight loads, causing added stress on the bridge.
(Picture above, Traffic on Golden Gate Bridge)