Suspension types

The longest bridges in the world are suspension bridges or their cousins, the cable-stayed bridge. The deck is hung from suspenders of wire rope, eye bars or other materials. Materials for the other parts also vary: piers may be steel or masonry; the deck may be made of girders or trussed.

How a suspension bridge works

http://www.aiacincinnati.org/community/abc/curriculum/fivebridgetypes.pdf

The cables that go from the top of the towers down to the ground are the backstays. The backstays are connected to big rock or concrete piers buried in the ground. The backstays keep the towers from bending in.

Suspension bridges are very light. This allows them to span very long distances. The longest suspension bridge in the world is the Askashi Kaikyo Bridge in Japan. In addition to the long span, this bridge was designed to resist huge earthquakes and hurricane force winds.

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 became 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, 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.

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

Source: http://www.aiacincinnati.org/community/abc/curriculum/fivebridgetypes.pdf