Spans

The biggest step, in both length and in ratio of lengths, was made in 1931, by the large and magnificent George Washington bridge. There was another big step quite recently. The lines on the graph have, of course, no physical meaning, and a curve could just as well have been drawn.

But the bend corresponds roughly with the date, around 1880, when the price of steel became low enough for it to be considered as an important structural material. From then on it began to displace iron.

It is dangerous to draw a conclusion from a graph with so few points. showing only exceptional spans. A graph showing large numbers of more typical spans might look rather different.

How will the graph continue? Surely the spans of bridges cannot increase indefinitely - firstly, because there must come a point where cables can only just hold themselves up without a bridge, secondly because costs at some stage must become excessive, and thirdly, because the earth will have no more places where long crossings are needed badly enough.

On the other hand, it is quite difficult to forecast the traffic. The George Washington bridge was given a second deck, while the Severn suspension bridge was refurbished and given a sister bridge nearby. But traffic on the Humber bridge, which was once a record holder, remains light - so far.

This graph suggests that the record spans of bridges have been growing almost exponentially. This cannot continue indefinitely. The graph does not show the kink that was drawn on the linear graph.

Will there be a 10000 foot or 3000 metre span by the year 2040? Click here to calculate the longest possible span.

There is, of course, much more to bridges than length of span. There are beautiful bridges, elegant bridges, graceful bridges, interesting bridges, and amusing bridges. And, very sadly, there are some dull ones, unsuitable ones, and even ugly ones. When you see a dramatic or beautiful location, with exactly the right bridges, you feel that it has been enhanced. There is a magnificent site at Cruseille, north of Annecy in Haute-Savioe, with two fine bridges crossing a very deep gorge in limestone country. Click here to see more about the appearance of bridges.

What a waste of a fine site, when someone has erected a bridge that does not do justice to the location. Until it needs replacing, such an object prevents anything better being built. Luckily, more new bridges are attractive than unattractive. And the imagination of designers and builders shows no sign of running out of ideas.