The question of whether a supposedly pure engineering structure is better or worse than one in which an architect has taken a major role will never be resolved. Particularly in the case of those who, like Santiago Calatrava, are themselves both architect and engineer and so afford the spectacle of two halves of the same brain in dialogue. I was doubly lucky in being made aware, quite young, of two of the finest "engineer's bridges" in the United Kingdom. At university in Durham, most days I had to cross the Kingsgate footbridge over the gorge of the River Wear. Though I did not know it at the time, it was the last project to be personally designed and supervised by the great engineer Ove Arup, in 1962-3. The trough of the concrete footway felt as if it should have been an aqueduct, in the tradition of the Roman Pont du Gard in France, or Thomas Telford's great Georgian canal aqueduct at Pontcysyllte in North Wales, its ironwork superstructure perched lightly high above the valley of the River Dee. Arup's bridge shows you how it is made: two precisely balanced sections built parallel to the river banks that were then rotated into position and linked with a pair of simple, fully exposed, bronze roller expansion joints. It is clearly alive, this bridge, quivering slightly as the gaggles of students career across. But just as interesting is its setting. It joins ancient Durham, with its cathedral and castle on a wooded peninsula, with the modern Durham of new university buildings. It has to leap not just a river, but a thousand years. It achieves this task effortlessly.

Shortly after this, I saw the Humber Bridge being built in north-east England. By William Brown and others of Freeman Fox and Partners, at the time it was the longest single-span suspension bridge in the world, comfortably outstripping the 1964 Verrazano Narrows bridge across New York harbour that had previously held the title. It is so long, indeed, that its towers had to be angled slightly to allow for the curvature of the earth. Visiting the East Riding of Yorkshire at intervals, I received a stop-motion picture of its progress. First the towers, then the fragile pilot threads, then the cables spun across, and then, what seemed to be the truly miraculous part: sections of deck were floated out into the tideway and hoisted into thin air. The narrow band of the deck advanced from either shore - ribbons in the wind, given the extraordinary height and length of the bridge. The distances were such that the eye was deceived. Cables visually dissolved in the mist of the estuary. The impression was of impossibly slender cantilevers poised high above the waves, as if in a weightless environment. When they finally joined, and the bridge became a bridge, it retained that delicate, evanescent quality. It is an extraordinarily beautiful response to the land, water and sky: very different to the urban setting of the Verrazano Narrows. In more recent times perhaps only the marvellous 1994 cable-stay bridge of the Pont du Normandie in northern France (another great estuary crossing) approaches the atmosphere of the Humber Bridge.