The Way of the Wobble

Peter Campbell

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The meal is over. On the tablecloth there are corks, an orange, a few walnut shells, an empty glass and a coffee spoon. Those of us whose instinct is to see if we can somehow balance these objects, one on the other, are generally found to be annoying. Conversation falters. People wait for over ambitious configurations to collapse. Structural doodling is our way of playing at being engineers. We search for the point of balance as weight is transferred from one item to another. We load a folded card or a drinking straw until it buckles, replicating in miniature tests made with hefty presses and rigs in the basements of engineering schools.

The desire to play at building comes not so much from memories of the look of things as from memories of movement: memories as clear as the memories of smells or sounds, and sometimes more dramatic. One of my earliest is the shudder, pause, then the next shudder of an earthquake. Crockery rattled on the shelves. The whole house was in motion. We moved with it. There was not much damage – a cracked chimney stack had to be demolished (the house itself was timber frame), maybe there were broken plates – but the sense that a building is not a solid mass but something made only more or less stable by the stiffness of beams and walls, that the whole construction is flexible and unpredictable in its deflections, stayed with me. That was in New Zealand, in Wellington, where a fault cuts off one side of the harbour in a ruler-straight line. There, when you look down into excavations for new foundations you see concrete lap round a reassuringly dense weave of reinforcing rods. But you know that when the big one comes these buildings will shake and bend a little, even if they do not fall. And there are always forces which cannot be rationally planned for. Timber-frame houses exploded like paper bags in the great storm that hit Wellington in 1968.

I have another early kinetic memory, of swing bridges on tramping tracks. I remember their sway and the way the narrow-planked walkway bounced up to meet your advancing steps. It was worse if you didn’t cross one person at a time. Short in span, and made of wire for crossing streams rather than of lianas for crossing gorges, these bridges were otherwise pretty much the same as those liable to be cut away under Indiana Jones as he is chased through the South American jungle. New Zealand’s swing bridges came to mind when it turned out that the Millennium Bridge had a wobble. Reading about it I felt it in my feet.

Suspension bridge technology has been pushed forward by a need, on the one hand, and an ambition, on the other. The need is to prevent the rhythmic, self-destructive fluttering that can be generated when the cadence of a live load (marching men, wind-driven eddies) matches one of the bridge’s modes of vibration. The ambition is to cross wider spans with lighter, more economical structures. The two aims are to a degree in conflict, because light decks are likely to deflect in unpredictable and unpleasant ways. The result has been monuments that mark advances in civil engineering: Telford’s bridge over the Menai Straits, the Roeblings’ Brooklyn Bridge, and bridges by Freeman Fox over the Severn and the Bosphorus. A suspension bridge also provided what is probably – because recorded on film – the most widely publicised of all civil engineering failures, the collapse in 1940, owing to wind-induced oscillations, of the Tacoma Narrows Bridge in Washington State.

There is no reason to think that engineers make more mistakes than professionals in other fields, but they cannot bury them and are not allowed to forget them. A structure on which the safety of a trainload of passengers depends is a public performance. It’s easy to see that civil engineering could be, or should be, an anxious business and why engineers have, from time to time, been thought too conservative. Post-mortems on glitches and disasters make good reading for amateur engineers: they go back through the design process and tell you something about how engineers think.

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[*] Published in Ingenia (Issue 9, August 2001).