Some wonks at MIT just did a recreation of a bridge proposal from Leonardo Da Vince, and, if their reconstruction from his notes is correct,^* his bridge was centuries ahead of the state of the art:

Some 500 years after his death, researchers are still discovering just how talented and brilliant Leonardo da Vinci was. Architects and civil engineers at the Massachusetts Institute of Technology used a 3D printer to create a replica of a bridge da Vinci designed, but never built. To their surprise, not only did it work, but it would have also revolutionized bridge design five centuries ago.

As the story goes, in 1502 A.D. the Sultan Bayezid II wanted to build a bridge to connect the city of Istanbul to its neighbor, Galata. One of the proposed designs came from Leonardo da Vinci, who had already made a name for himself in the arts and sciences at the time. In a letter he sent to the sultan, accompanied by a notebook full of sketches, da Vinci described a bridge that would span the proposed distance using a single, flattened arch design, supported by bases on either shore. Bridges at the time were typically made using a series of semicircular arches, and to span the distance between the two cities would have required at least 10 evenly spaced piers in between to support the entire structure. Da Vinci’s design, which would have easily allowed sailboats to pass beneath it, was radically different (and centuries ahead of its time), which is probably why the sultan decided not to take the risk. Half a millennium later, researchers were curious if it would have succeeded.

………

Not only did the bridge work, remaining strong and stable without the use of any mortars or fasteners, but the team at MIT also realized that da Vinci had even engineered a way to minimize unwanted lateral movements in the structure, which would have quickly led to its collapse. The footings on either side of the arched bridge featured designs that splayed outwards to add a considerable amount of stability. The bridge would have even survived most earthquakes, which were common at the time in that area, as the MIT researchers discovered by putting their replica on two movable platforms. It wasn’t indestructible, but it would have been an ancient architectural marvel.

There are a number of “Ifs” here:

• Did the technology of the day allow for the construction of abutments to handle the not-inconsiderable thrust loads.
• Does the material handling technology of the day allow for the handling of the stone blocks.
• could the barge and scaffolding technology of the day effectively provide for the support of the structure when under construction?

My guess is that Da Vinci never looked at the nitty-gritty details involved in actually putting up such a bridge, because he was never really a details kind of guy.

^*That is a VERY big if.