One of Leonardo's more grandiose schemes was for a grand single-span bridge to leap 1,155 feet (347 meters) across the Golden Horn inlet near Istanbul. At the time, it would have been the longest bridge of any kind in the world, and it would have dwarfed any single span bridge built in several succeeding centuries. Leonardo proposed his bridge to the visionary Ottoman Sultan, Bayazit II, in 1502. Bayazit (there are lots of alternate spellings) reigned from 1481 until 1512, and he had many notable accomplishments, including opening mental hospitals (1484 and later), protecting Christian monasteries from further gubernatorial depredations (decree of 1491) and rescuing the Jews expelled from Spain by Ferdinand and (1492.) The latter two acts certainly contributed to the intellectual excellence that characterized his reign. But even Bayazit and his brilliant advisors did not understand Leonardo's concept.
Leonardo's bridge had to wait 500 years to be built, albeit on a smaller scale (slightly less than one-fifth), of different materials (wood rather than stone) and far removed from the Golden Horn (in the small town of Aas in southern Norway). A really ugly modern bridge was finally built at Galata, the site of Leonardo's proposed bridge across the Golden Horn, in the 20th century. But the Aas bridge, which opened in November of 2001, proved Leonardo's concept, and there are plans for other bridges on his design. What took so long?
Leonardo's design was well known, at least among bridge builders and engineers. What was missing -- as it had been in Leonardo's own time -- was faith in his calculations, which may have accompanied his actual proposal letter to Bayazit, but which were not with drawings in his notebooks or with surviving documentation of the proposal. Working only from drawings, no one could really know if a real bridge would stand. Models had been made, displayed, and tested in engineering schools around the world, and they certainly stood up to static stress and pressure tests. But bridges aren't static. Even in modern times, there have been dramatic failures including the much-filmed collapse of the first Tacoma Narrows bridge in 1940 due to resonance in high winds and the fall of the Tasman Bridge after being struck by a ship in 1975. There have also been dramatic near-failures: only good luck saved the Golden Gate Bridge, whose central suspension span dramatically flattened beneath throngs of admirers during its 50th anniversary celebration in 1987: folks were quietly ushered off, and there was no horrendous news story that night. And London's Southwark Millennium Bridge, a 350-meter suspended pedestrian span, the city's first new river bridge in more than a century, had to be closed immediately after its June, 2000, opening weekend, because there was unexpected and scarily dramatic swaying.
Modern computers might be able handle active bridge-stress models, but the software designers and the engineers providing the parameters have to do perfect work, which, of course, they can't. So there's always a leap of faith. And with bridge spans you can't simply "overbuild" -- make them more robust than even worst nightmares might demand -- your like you do in some other structures. "Robustness" in bridge spans simply adds more weight.
So why was Leonardo's bridge so scary for so long? First, it looks too thin even to support its own weight. That's an illusion, of course, but city fathers (and Sultans) who have to put up the cash are deterred by such considerations. Arched bridges were not unknown, but the integrity of an arch is mostly based on its ability to translate forces that push outward into forces that push downward. If you want it to push downward, you have to build a high semi-circular arch. You can buttress arches to help to control residual outward forces (even after the fact, as with the buttresses added to the church of St. Francis in Assisi), but any attempt to flatten the arch increases the out-thrusting component. Flatten the arch too much and even the strongest bases and buttresses won't hold. Leonardo's arch is "obviously" too flat -- obvious, at least, to non-engineer bill-payers.
Leonardo knew that no soil or even bedrock could withstand the accumulated outward thrust of a long flat arch unless that thrust could be spread out over a much wider area. His design accomplishes this by splitting and splaying out the ends of the lower supporting arch. The wide ends press down and outward over a very much wider area, distributing and dispersing the forces. The roadway is carried on another superimposed arch, which is recurved like an archery bow and springs from between the splits in the broadened ends. It is supported at its center by the top of the lower arch.
It's not so hard to understand once you've seen Leonardo's actual drawing. (The ship drawn below the bridge in the lower part of the drawing is there to indicate the drawing's scale. Its high stern of shows that it is one of the larger vessels of its day.)
Whereabouts: Topkapi Palace
