Popular Mechanics (South Africa)
Constructing Da Vinci’s forgotten bridge (of course it works)
WHEN SULTAN Bayezid II of the Ottoman Empire requested proposals for a bridge connecting capital city Constantinople (now Istanbul, Turkey) with the neighbouring city, Galata (Karaköy, Turkey) at the turn of the 16th century, famed inventor Leonardo da Vinci was eager to win the contract.
He didn’t get the job, but more than 500 years later, a team of researchers at the Massachusetts Institute of Technology suggests that maybe he should have. Karly Bast, who graduated from MIT in 2019 and is now at Buro Happold Engineering, worked alongside structural engineer and MIT professor John Ochsendorf and undergraduate Michelle Xie to analyse Leonardo’s sketches and letters (pulled from Manuscript L, a small Codex stored in the Institut de France in Paris). They also researched historically accurate materials and construction methods to prove that Leonardo’s unique bridge design would have held up.
The team created a 1:500 scale model of the bridge (making it about one metre long), which the inventor described in a letter to the Sultan as ‘a masonry bridge as high as a building, [so] even tall ships will be able to sail under it’. Leonardo’s concept was for a flattened bridge with a long arch – a radically different style from anything that had previously been constructed. At the time, many bridges featured a semicircular arch, often half as tall as the span of the bridge, instead of a sweeping parabolic arch.
Leonardo’s bridge would have spanned roughly 280 metres (or 918 feet) – neither measurement had been invented yet, so he used a unit of measurement called braccia. It would have been about 10 times longer than typical bridges of that period in order to span the river, Bast says.
Like the classic masonry bridges of ancient Rome, with which Leonardo would have been familiar, his design
relied solely on the forces of physics and gravity with no need for fasteners or mortar. Critically, the design also included wing walls, or abutments that splayed out on either side, which would have steadied the bridge against high winds.
Strong winds have forced even relatively modern bridges from the 20th century – such as Galloping Gertie, the famed swaying bridge in Washington State – into lateral oscillations, which cause the bridge to sway back and forth and can lead to collapse.
The inventor didn’t specify what materials he would use. So, based on the materials and technology that would have been available at the time, Bast and her colleagues figured stone was the likeliest choice. They calculated that wood, for example, wouldn’t have been strong enough to hold up a bridge of that length. Unreinforced masonry structures are ‘driven by geometry because the stone has an incredibly high compressive strength’, meaning all the pieces press tightly against one another to keep the bridge standing, Bast says. The team used a gypsum powder 3D printer to create each of the 126 blocks, which needed to be a uniform weight and create enough friction to keep each stone locked in place. It took the 3D printer one hour to produce each piece.
The crucial moment came when Bast dropped the final piece, called the keystone, into place. ‘I had to really lock it in,’ Bast says. ‘That was the best sign that it was being supported by the stones around it instead of relying on the scaffolding.’ Newer, stronger materials and updated designs have made unreinforced masonry bridges all but obsolete. But one thing is certain: Leonardo’s bridge likely would have stood the test of time.