Steel in the spotlight ... cranes in London. Photograph: Peter Macdiarmid/Getty
You never see them in architectural models. You never think of them when you call to mind your favourite cityscape. Yet they're a consistent feature of our urban skyline, particularly during periods of economic growth. They're not thought of as architectural but they're a particularly pure example of form following function, an article of faith for architects working in the modernist tradition.
Ladies and gentlemen, I give you the crane.
As we speak, the UK is experiencing a construction boom. A strong economy, the low cost of borrowing and changes in local and national government policy mean that things are getting built - houses and offices, groundscrapers and skyscrapers.
It's the big projects that send cranes soaring into the sky of course, and the real forests only spring up in places with a bona fide downtown. Manchester and Birmingham are on the way, but London's definitely there already. If you visit the Square Mile at the moment you'll find it stalked by a host of gangly invaders. Eventually they weave a steel and concrete skeleton that gets clad in glass. Then, just as mysteriously as they arrived, they disappear, ready for the suits to move in.
The kind of crane we're now so used to seeing is a post-war invention. German engineer Hans Liebherr created the Turmkran or tower crane in 1949 (his children are now billionaires). This is the kind of crane I used to dream of sitting in - the one with the cab 100 feet up. Alas, Jim never fixed it for me. But I can still admire their lightness and grace. They are, after all, designed to lift the heaviest of building materials, but they manage to make it look effortless, gently swinging round, the triangular braces that make up their bodies meant to give maximum strength and stability.
Though cranes have only recently become part of the urban scene, they do have their historical forerunners. If you walk over the Thames in the direction of St Paul's, which seems to be the epicentre of the current London cluster, it's possible to see the cranes taking their place in a line of ephemeral objects that have floated in and out of this view over the centuries. They echo the tall ships and port jibs that once jostled with the city churches for space.
The point is that it's easy to slip into thinking of the city as a fixed, set form, when it has always been much more dynamic. Ignore the maps, models and computer renderings because real life isn't like that. Cranes are one unmissable expression of the fact that cities are, ultimately, temporary structures. They're impressive enough, for as long as they last. Enjoy them as they stalk your town.
