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Suspension Bridges - Design Technology

Suspension bridges in their simplest form were originally made from rope and wood.
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.

Light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet far longer than any other kind of bridge. They are ideal for covering busy waterways.

With any bridge project the choice of materials and form usually comes down to cost.
Suspension bridges tend to be the most expensive to build. A suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. These cables rest on top of high towers and have to be securely anchored into the bank at either end of the bridge.
The towers enable the main cables to be draped over long distances. Most of the weight or load of the bridge is transferred by the cables to the anchorage systems. These are imbedded in either solid rock or huge concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

An image of the Golden Gate Bridge in San Fancisco.

Currently, the Humber bridge in England has world's longest center span measuring 4,624 feet

The diagram below shows the tension in the cables of a suspension bridge. These cables are capable of withstanding tension but offer no resistance to compression. These types of bridges work in a completely different way to the arch bridge.

New Tacoma Narrows Bridge


Structural Failure

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.

Eventually on November 7, 1940, only four months after it was built the bridge collapsed in a wind of 42 mph. The bridge was designed to withstand winds of up to 120 mph.

Some experts have blamed the collapse of the bridge upon a phenomenon called resonance. When a body vibrates at its natural frequency it can shatter. Resonance is the same force that can shatter a glass when exposed to sound vibrations from an opera singers voice.

Today all new bridges prototypes have to be tested in a wind tunnel before being constructed. The Tacoma Narrows bridge was rebuilt in 1949.


The four shapes below are pin-jointed simple frameworks which when loaded will collapse. Redraw the shapes but include a few additional cross members to stop the frames from distorting under load.


Solutions to Pin Frameworks

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