Suspension bridges

The suspension concept dates back to antiquity, and it is because of their fundamental simplicity and economy of structural action that suspension bridges are used for the longest of modern bridge spans. The deck of a suspension bridge is supported by vertical tension hangers, which are supported in turn by large tension cables extending over two towers from anchorage to anchorage. (A diversion into inclined hangers, to give an additional structural damping effect, was used at the original Severn and Bosphorus bridges, but it has not found widespread support elsewhere.)

A stiffening girder running the full length of each span is an essential part of a suspension bridge. It distributes the concentrated traffic loads and provides stiffness against bending, twisting and oscillation. For single decks, the trend is to use box girders to minimise weight and give maximum torsional stiffness. However, where twin level decks are required (eg to carry road traffic and railway traffic), then the general trend is to use truss girders.

Aesthetically, the graceful curve of the suspension bridge combined with the strong visual line of the deck give a pleasing effect. The combination of grace and grandeur in such situations leads to the acknowledged view that many of the world’s most exciting bridges are suspension bridges.

Currently, the longest span suspension bridge in the world is the Akashi Kaikyo bridge in Japan (1998), with a main span of 1990m (shown right). The bridge is part of the major Honshu-Shikoku link scheme. The two pylons are almost 300m tall, giving a sag to span ratio of approximately 10, and the stiffening girder is a 14m deep truss.

This table below shows details of famous suspension bridges. The Golden Gate bridge was well ahead of its time, with a span of 1280m in 1937, and Britain led the world with major bridges in the 1960s and 1970s. More recently the focus of major suspension bridge construction has been very much SE Asia.

However, there are plans for a bridge across the Straits of Messina (Italy-Sicily) with a span of around 3300m.