Leonardo's bridge to the future

It may have taken 500 years, but a bridge designed by Leonardo da Vinci to cross the Golden Horn has at last become a reality in Norway. Jay Merrick reports

The snow blows in tiny, whirling chips across the E18 dual carriageway half an hour north of Oslo. The land is gently undulating, its features unremarkable. The road, heavily gritted but still with a skim of dirty ice, runs through granite and conifer country. Drivers have to concentrate here; the surface grips, and then it doesn't. And yet here, approaching the otherwise unremarkable town of As, it's necessary to remember that you are several thousand miles away from Istanbul and a kilometre or two away from the work of the world's first polymathic genius.

My driver slows as the first buildings of As materialise through the gauzy light. Then, at first ghostly and then pin-sharp on winter's pale fresnel screen, there it is; the snow-obscured object of desire, an absurd, beautiful time-warp thing, the fruit of an absurd, beautiful, time-warp idea that more than a few of Oslo's great and good regard as something uppity, a scandalette.

Leonardo da Vinci would have thought otherwise. He designed the scandalette, the elegant bridge that crosses the dual carriageway at As so lightly and gracefully in sections of glue-laminated pine.

It was a great design then, and it's a great design now. The rhythm of the structure, the bifurcation of its arches, might easily have been conjured up by today's most self-consciously graceful architect, Santiago Calatrava, he of the spinal-cum-Gothic arches. One wonders what Leonardo might have knocked up with Apple Mac software.

The bridge at As actually belongs in Istanbul, or Constantinople as was. Leonardo's design was for a bridge made of stone, 350 metres long with a 234m arch reaching a height of 40m, to cross the Golden Horn, from Galata to Constantinople; an amazing proposal, considering that the engineering knowledge to achieve it did not exist.

Only one drawing remains, the one he sent to Sultan Bazajet II in 1502. It shows, in a wonderfully economical sketch, the form of the bridge, both in plan and elevation. In elevation, Leonardo envisaged an arch supported on abutments split in two at each end. The top edge of the arch clearly follows part of the radius of a circle; and, in plan, the curving line of the arch appears to match that same radius. The moment you look at these two drawings, they feel agreeable; more than that, you imagine the bridge would have worked, structurally.

But the most delightful realisation is that Leonardo's bridge would have been beautiful, an eighth wonder of the world, fabulous in its grandeur. Leonardo's key intention, according to a letter he wrote to the Duke of Milan, was to create a new kind of masonry bridge with a "light, strong arch".

The thinking behind his solution is remarkable. The bridge would have been very high, and the unusual bifurcation of the span was anything but an aesthetic nicety. By splitting the bridge at the abutments and creating two radii, Leonardo reasoned that wind forces could compress the arch at its narrowest point, adding to its structural security. The other departure from known practice was that the thickness of the arch was not constant, but tapered to a depth of just nine metres at the mid-point.

Both ideas were hugely avant-garde, according to the structural engineer Dr Olav Olsen, who worked on the design of the bridge at As. "Leonardo's ideas about the bridge design indicate a vision that does not just extend far beyond his own time, but right into our own times," he says. "A stone bridge with corresponding dimensions has never been built in one span, and almost 450 years would pass before a span of this length was mastered in concrete."

The trick, at As, was to master it in glue-laminated wood. And, before that, to convince the highways authority that As not only needed a bridge across the E18, but that it should be something special. The key ingredient was a "rogue" artist, Vebjorn Sand, noted for his skill in producing classically inspired oils andin generating publicity. The latter has a habit of bringing results – and isolating him from the sniffier elements of Oslo's art establishment.

He wanted to bring Leonardo's bridge to life, perhaps not as big but following the same structural rationale. He wanted it built in stone, but the "glulam" option took over: it offered the chance to design and build something that tested timber technology and manufacturing right to the edge. And it made more sense, aesthetically. However beautiful a stone bridge might have looked, slinging it across a dual carriageway on the edge of a small town would have been overkill. The key point, though, is that Sand lobbied the highways department so doggedly that they finally agreed to the NKr10m (£890,000) project.

One person who relished the switch to wood was Tarja Koskinen, of Selberg Arkitektkontor in Oslo. As project architect, it was she who faced the challenge of not turning Leonardo's architectural gold into lumpen base metal. She didn't face it alone, of course. While Koskinen was largely responsible for the general form of the bridge, a great deal of design was infused with the structural and materials knowledge of Age Holmestad, an engineer at Sweden's leading glulam manufacturer, Moelven.

"If you follow the rules of mechanics, you usually get a beautiful object," says Koskinen. "And if you have only a few shapes, the shapes have to be perfect." Those shapes are based on a wonderfully graceful 110m span, giving a 42m arch over the road before splitting into two pairs of splayed 50m supporting arches. Like Leonardo, Koskinen and the engineers were determined to minimise the thickness of the glulam arches, and at no point are they more than a metre thick.

The structure at As is the kind of achievement that Britain's Wood for Good campaign would love to see transplanted here. Imagine catching sight of Leonardo's bridge on the Uckfield bypass: what a relief to humdrum reality, and what a reminder that beauty can crop up in a thousand different ways.

An art critic on Oslo's daily, Aftenposten, said the bridge was too good to be sited over a dual carriageway in Hicksville. But, as it receded into a soft-focus whiteout on the return journey, one couldn't help thinking back to Leonardo's original sketch: a dazzling dream that stood the test not only of time, but of aspiration. The bridge at As may be a second-hand design, but it reminds us that great architecture is indestructible.