Da Vinci designed the bridge as part of a civil engineering project for Sultan Bajazet II of Constantinople (Istanbul.) The bridge was to span the Golden Horn, an inlet at the mouth of the Bosphorus River in what is now Turkey.
The Bridge was never built.
http://www.vebjorn-sand.com/thebridge.htm

U.N. ice bridge is reminder of melting Antarctica
U.N.’s Ban says global warming is an emergency

An ice bridge made of frozen Antarctic water erected at U.N. headquarters on Monday is expected to melt within a week to dramatize the dangers of melting glaciers.

Norwegian artist Vebjorn Sand brought the fresh water to New York to fashion a temporary bridge based on a Leonardo da Vinci design. It replicates another span he built on a glacier in Antarctica a year ago which he hopes will never melt.

"Our future lays underneath that ice glacier. So to erect it on that glacier, and that part of Antarctica, (it) must never melt," Sand said outside U.N. headquarters.

"The one outside the United Nations is intended to melt to show that Antarctica is melting," he said, highlighting that 70 percent of the earth's fresh water is held in Antarctic ice.

The bridge's elegant arc, based on da Vinci's 1502 "Golden Horn" design, is roughly 30 feet long and anchors an exhibition highlighting the "fragile beauty" of polar regions.

The bridge was unveiled two days after nearly 200 nations agreed at U.N.-led talks in Bali to launch negotiations on a new pact to fight global warming.

In May, satellite data analyzed by NASA showed vast areas of Antarctic snow and ice, roughly the size of California, melted in 2005 and then re-froze, the most significant thawing in 30 years.

Some experts believe the thaw of Antarctic ice is outpacing predictions by the U.N. climate panel and could in the worst case drive up world sea levels by 6 feet by 2100.