MIT

It's 1502 A.D. and Sultan Bayezid II sends out a request for bids: He wants someone to build an enormous bridge, spanning the Golden Horn and connecting Istanbul to neighboring Galata. If you're Leonardo da Vinci, you don't have modern rebar or asphalt to rely on. Forgoing wood planks and even mortar joints, your design uses only three geometrically daring principles: the pressed-bow, the parabolic curve and the keystone arch. With these, you design what at the time would have been the world's longest bridge, with an unprecedented single span of 790 feet.

And after the sultan's rejection, you would have to wait more than 500 years for your bridge design to be tested by a team of ambitious MIT engineers and their handy 3D printer.

"It was time-consuming, but 3D printing allowed us to accurately recreate this very complex geometry," MIT graduate student Karly Bast said in a release on Thursday.

Bast worked with a team of engineering academics to finally bring to life a faithful 1-to-500 scale model of da Vinci's famously rejected bridge design, putting the Renaissance man's long-questioned geometry to the test by slicing the complex shapes into 126 individual blocks, then assembling them with only the force of gravity. The group, which presented its work this week in Barcelona, relied on the sketches and descriptions found in da Vinci's letter bidding for the job, along with their own analysis of the era's construction methods.

The structure is held together only by compression -- the MIT team wanted to show that the forces were all being transferred within the structure, said Bast. "When we put it in, we had to squeeze it in."

Bast said she had her doubts, but when she put the keystone in, she realized it was going to work. When the group took the scaffolding out, the bridge stayed up.

"It's the power of geometry," she said.