Seems that Leonardo designed a 280 meter single-span bridge in response to a sultan's request. The configuration is a very flat arch, with wide "feet" at each end to spread the loads. Nothing in antiquity was remotely close to this in a single span, at that time bridges were built with circular arches with columns at each end (many columns sitting in the riverbed). While the shape of this bridge has been approximated with modern materials, this time the researchers wanted to determine if it could have been built with what was on site -- big, cut stone. Story is here, http://news.mit.edu/2019/leonardo-da-vinci-bridge-test-1010 [mit.edu]
Spoiler alert: Leonardo knew what he was doing.
The bridge would have been about 280 meters long (though Leonardo himself was using a different measurement system, since the metric system was still a few centuries off), making it the longest span in the world at that time, had it been built. “It’s incredibly ambitious,” Bast says. “It was about 10 times longer than typical bridges of that time.”
The design also featured an unusual way of stabilizing the span against lateral motions — something that has resulted in the collapse of many bridges over the centuries. To combat that, Leonardo proposed abutments that splayed outward on either side, like a standing subway rider widening her stance to balance in a swaying car.
In his notebooks and letter to the Sultan, Leonardo provided no details about the materials that would be used or the method of construction. Bast and the team analyzed the materials available at the time and concluded that the bridge could only have been made of stone, because wood or brick could not have carried the loads of such a long span. And they concluded that, as in classical masonry bridges such as those built by the Romans, the bridge would stand on its own under the force of gravity, without any fasteners or mortar to hold the stone together.
To prove that, they had to build a model and demonstrate its stability. That required figuring out how to slice up the complex shape into individual blocks that could be assembled into the final structure. While the full-scale bridge would have been made up of thousands of stone blocks, they decided on a design with 126 blocks for their model, which was built at a scale of 1 to 500 (making it about 32 inches long). Then the individual blocks were made on a 3D printer, taking about six hours per block to produce.
There is also a 2 minute video (but I'm sure no one here will bother to watch it!)