Future Bridge: s the future of bridges all suspended?

The proposed Gibraltar Straits bridge

Recent talk of a road bridge between Europe and Africa, spanning the Straits of Gibraltar, has centred on a slightly different type of suspension bridge from the purely ‘hanging’ form we have discussed so far.

Iron cable-stay Albert Bridge. London.

This is the so-called 'cable stay' suspension bridge. The first modern example dates only from the 1950s, although there are earlier bridges that use iron links in a similar way, among them the 1873 Albert Bridge, which spans the River Thames in London.

The cable stay design

In the cable stay version of the suspension bridge, the deck is hung from diagonal cables that exert a force towards the towers as well as vertically. This makes the tension in the steel cables extremely high, and hence they are very stiff. In addition, the cables effectively stabilise the towers from both sides.

Cable-stayed bridges can be constructed in a huge variety of designs. They are often used to span river mouths, sometimes themselves resembling giant sailing boats

New Sunshine Skyway Cable Stayed Bridge, Tampa Bay, Florida, USA . Showing cable stays

In some ways, it is the cable-stayed bridge that represents the state-of-the-art in bridge-building at the beginning of the twenty-first century, but, with technology giving us lighter and stiffer materials to build with, there are already all-plastic bridges. Such 'composites' may well make massive concrete towers and reinforced steel decks a thing of the past, just as solid-stone arch bridges were superseded by lighter steel and concrete ones.

The variety of 'one-off' cable-stayed designs reminds us of the importance of individuality to humans: one size doesn't fit all. In fact, as we shall see, the single thing that characterises bridge-building today is that no single thing characterises bridge-building today!

Lets glance towards the future of bridges in the next section.