Suspension

Suspension bridges have been in existence since ancient times and evolved in different civilisations with no relationship between them. They may have developed initially from having a single cable with a basket suspended from it crossing a valley. It may then have progressed to three cables, one for walking on and two for handrails. Many primitive forms of suspension bridge are still in use today e.g. isolated areas in Nepal.

The suspension bridge may be considered as an inverted arch bridge where the cables take the place of the arch. It was an American, James Finely, though, who first identified the key components of the suspension bridge, namely, a level truss deck hung from vertical cables or chains, which in turn were supported by the main catenary cables. These catenary cables have to be at a particular curvature, pass over the top of the supporting towers and are fixed, normally, at the end of the side spans in very large anchorages cast into the ground. Finely’s first bridge was built in Pennsylvania, USA, around 1800, with a span of 21 metres. This simple categorisation of the principle of the suspension bridge rapidly led to spans being constructed which were longer than the truss and box girder bridges.

The suspension bridge has become famous because since the beginning of its use it has been used to bridge areas of particular difficulty, such as the Menai Strait Bridge (1826) in Wales, Brooklyn Bridge (1885) in New York, Golden Gate in San Francisco (1937) and the Bosporus Bridge in Turkey (1973). The Roebling family of father and son were responsible for the Brooklyn Bridge but it was D. Steinnman and O. Amman who made the greatest advances in America and R. Freeman who optimised its use in Europe.

The greatest advantage, and disadvantage, of the suspension bridge is its lightness. The lightness allowed for quick construction but it also made them highly flexible. As a result many bridges collapsed and had to be rebuilt. The suspension bridge is also more susceptible to changes in traffic loads compared to other bridge types. In order to increase rigidity to the deck various systems have been tried, such as cables connected directly from the towers to the deck (e.g. Brooklyn Bridge) or, more recently an open truss type deck was introduced, which spreads the loads over a greater number of cables and reduces wind resistance. The truss also allowed for the construction of two levels of transport for either road traffic or trains at the top of the truss and at the bottom. This can be seen on the Verrazano Bridge (1964) in New York and many others. In Europe the deck rigidity was provided by an aerofoil deck which was similar to a box girder with wings. While this did not allow for the provision of a lower deck for transport, until the construction of the Tsing Ma Bridge (1997) in Hong Kong, it introduced the concept of aerodynamics to bridge design. This type of design reduces the wind impact on the deck.

The longest suspension bridge for many years was the Humber Bridge (1981 – 1998) in England with a centre span of 1410 metres. It was superseded by the Akashi - Kaikyo Bridge in Japan which has a centre span of 1991 metres and two side spans of 950 metres each. It was completed in 1998 and forms just one of a chain of bridges linking Honshu to Shikoku.