In every sphere of life, technology offers the modern world a choice between 'bigger and better' and 'small is beautiful', between quantity and quality. We can achieve either, but which of them we choose defines our vision for the future.
Bridges are thus in some sense symbols, or indices, of the choices made by society as a whole.
Bigger and better
In mediaeval times, multi-span arch bridges competed with soaring cathedrals as the most impressive feats of building technology.
With the advent of iron and steel, mighty arch and cantilever bridges became the dominant man-made structures, together with huge steam-driven ships.
In the last century, some of the most potent symbols of the technological dominance of the United States were undoubtedly her skyscrapers and her many famous suspension bridges – many of them concentrated around New York and other key US cities.
Recently, the arena of massive-scale engineering has largely moved to the Far East, where modern city centres boast high-rise skylines and spectacular cable-stay bridges. Outside the cities, waterways are crossed by up-to-the-minute cantilever and suspension bridges, often designed by British engineers and architects.
As mentioned above, the nearly two-kilometre main span of Japan's Akashi-Kaikyo Bridge is currently the world’s longest – and it's just one part of the largest construction project ever seen, linking Japan's main island with its smallest neighbour.
Even longer bridges loom on the horizon … Sicily to Italy? Spain to Africa? The United States to Russia? All of these ambitious projects are government-funded and driven by potential economic gain – the latter is dependent, for example, on major world powers competing to control the oil supplies of the future.
Can you think of some of the arguments for and against the proposed bridges between Sicily and Italy, and between Gibraltar and Morocco? Think back to some of the opening comments about the role of bridges in bringing peoples and ideas together. Is this always to everyone's advantage? Who might be for and who might be against the idea of closer ties?
Small is beautiful?
There's another way technology can help us. Rather than try to control nature, we can use innovations to help us live more efficiently and less destructively within it. A striking example of this is the recently built Leonardo Bridge in Norway, which uses traditional wood and a Leonardo da Vinci design from 1502.
Notice how the weight of the bridge is spread over a wide, forking base. The bridge was never actually built in Leonardo's time, as there was no way of knowing beforehand whether the forces within the continuous flowing design would really be transferred safely throughout the structure.
So it remained unbuilt for over 400 years, until software engineers developed computer programs that can run so-called 'Finite Particle Analysis' (FPA). This is a simulation of the way complex structures react to external loads, and can be used to test the wings of aeroplanes, the strength of ship bulkheads and the stresses within a bridge.
Once the computers had done their work and modifications been made to the original da Vinci design, it was time for the craftsmen to take over. Following a Norwegian tradition, local timber was used for the project. The bowed arches so critical to the bridge's elegant form were created by a process of lamination still common in Norway.
The Norwegian Leonardo Bridge was completed and opened to foot and bicycle traffic on 31 October 2001.
It is important to remember that this kind of design innovation is easiest to achieve with bridges at the small end of the scale. As has been stressed throughout this module, many of the problems of load and stress dissipation become far worse as the scale of a bridge increases.
Leonardo da Vinci originally designed this bridge to be on a vast scale, and his preferred material was solid granite. There is little doubt that his bridge could not have been realised when he was alive, and recent calculations have shown it to be unlikely that any such solid bridge will ever be possible unless we discover radically lighter new materials with which to build it.
Recent research has shown how 'fibres' often lend strength to slender structures in Nature (among them stalks, insect wings, spiders' webs and so on).
Spinning threads out of molten glass, carbon or aramid has produced stiff man-made fibres that have revolutionised everyday objects like tennis racquets and tent poles. Embedded within a plastic or resin base, these retain their stiffness but lose their brittleness.
Much stronger than steel for the same weight, these 'composites' offer far less solid-looking bridge-building prospects for the future. They are already being used in the form of 'patches' to repair and strengthen bridges where weaknesses have been identified. Repairing a bridge is a better option all-round than knocking it down and starting again.
There aren't many examples of whole bridges made of materials such as plastic - yet. The lightness brings its own problems and there are issues arising from long-term safety trials.
While the possibilities opened up by these newer materials and by computer-aided design are being explored, the older staples of bridge construction are still being used – often in fresh combinations and reincarnations. These are called 'hybrid' bridge designs.