Suspension Bridges

By Om Rami

What they are used for

Suspension bridges are used to cross great distances. These types of bridges are used to cross deep water channels, cannons or gorges where constructions of supporting piers can be difficult. The towers can be placed far eliminating the need for multiple towers and piers. Some suspension bridges have a main span longer than 4000 ft. The longest suspension bridge in the world is in Japan (the Akashi-Kaikyo Bridge) and has a span over a mile long. The largest suspension bridges have cables several feet wide which weigh thousands of pounds per foot.


Design

Suspension Bridges may be the most impressive type of bridge with their long span and beauty, infact the world's longest bridge isn't a suspension bridge.These bridges have a roadway that hangs from steel cables supported by 2 high towers. A suspension bridge has at least 2 main cables. These main cables extend from 1end of the bridge to the other.The other end of the suspender attaches to the roadway.


History

Some of the earliest suspension bridges were made from twisted grass. In the early 19th century, suspension bridges used iron chains for cables. 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 tonne without breaking!


Structural failure

Some bridges have in the past suffered from structural failure. This may be because of poor design and severe weather conditions. When opened in 1940 the Tacoma Narrows Bridge was the 3rd longest suspension bridge in the world. It later became known as the Galloping Gertie due to the fact that not only moved from side to side but up and down in the wind. Attempts were made to stabilize the structures with cables and hydraulic buffers but they were unsuccessful. Eventually on November 7 1940, only 4 months after it was built, the bridge collapsed in a wind of 42mph. The bridge was designed to withstand winds of up to 120 mph. The Tacoma Narrows Bridge was rebuilt in 1949.


Watch video below!

Tacoma Bridge

Towers

When the construction starts, the towers are the first to be put in place. There are usually two of them, each placed roughly one-third of the way in from the ends. A four-walled shell, the size of the support, is put in place in the water, and the water is then pumped out. This allows the workers to set the supports in place and secure them. Once in place, the shell is removed.





Cables

The larger cables are strung from one support to the other, and stretched down to the beginning of the bridge on both sides. They are anchored in place to secure them. Smaller cables are attached to the larger cables and hung down. These will hold the road. The sections of the road are raised by cranes and attached to the hanging cables. The road sections have steel supports under them to add some stiffness to an otherwise flexible roadway, and help support the weight of certain vehicles.


The arrows indicate the main cable (down below.)



Interesting Facts

Golden Gate Bridge

Only 11 worker died during the construction of the Golden Gate Bridge, which is a safety record for the time. Around 35 people were expected to die during the construction. A net was suspended under the floor which saved lives of 19 men!

The steel cables are long enough to circle the world 3 times.

The Golden Gate bridge is the most photographed bridge in the world.


Akashi-Kaikyo Bridge

Akashi-Kaikyo Bridge is 12,828 feet which is 4 times longer than Brooklyn Bridge. It took 2 million workers and 10 years to construct it.

The steel cables used are long enough to circle the world 7 times.

It was built because in 1955 a ferry carrying over 100 children collided with another ferry which sank with 168 adults and children in the busy shipping lane.


Brooklyn Bridge

Brooklyn Bridge is one of the oldest suspension bridges in the US.

It is the first steel wire bridge.

Brooklyn Bridge was opened for use on May 24, 1883. On that day 1800 vehicles and 150,300 people crossed the bridge.


Storebaelt Bridge

Storebaet Bridge is 6,790 metres long in length from the islands of Zealand and Sprongo.

It is the world's second largest supension bridge with a main span of 1,624 metres.

The pylons are 254 metres above sea-level which is 26 metres higher than those of the Golden-Gate Bridge.


Tsing Ma Bridge

The Tsing Ma Bridge is the world's largest suspension bridge.

It has 2 decks that carry both road and rail traffic.

The total length of wire used in the construction is 160,000.

The height of the pylons is 206 metres.