What Is Slenderness?
"Slenderness" is an engineering definition. Structural engineers generally consider skyscrapers with a minimum 1:10 or 1:12 ratio (of the width of the building's base to its height) to be "slender." Slenderness is a proportion based on the width of the base to the height of the building.
The World Trade Center North Tower was the tallest building in the world on its completion in 1971. But at a height of 1,368 feet and with a big square floor plate, 209 feet on each side, the ratio of its base to height was less than 1:7. This image compares at the same scale the former 1 WTC and the residential tower 432 Park Avenue, now under construction. The base of the apartment building is 93 feet square, and the shaft will rise to 1,396 feet, making its slenderness ratio 1:15. To visualize a 1:12 ratio, we show a ruler 1-inch wide and set on end. The eighteen towers on our chart range from a ratio of 1:10 to an extraordinary 1:23 at 111 W. 57 Street.
Tall is not Big
Tall and BIG are not the same thing. This chart places behind the line up of New York's slender towers the silhouettes, to scale, of several of the world's tallest and largest skyscrapers, all of which include residences or hotel spaces, except for New York's One WTC. From left
to right they are: FinanceCentre, One WTC, Lotte World Tower, Mecca Royal Clock Tower, Shanghai Tower, Burj Khalifa. The smallest total floor area (GFA) in any of these skyscrapers is Lotte World Tower with 304,081 m² / 3,273,101 ft² of GFA.
How Tall? How Big?
Lining up the super-slenders by ascending height does not correlate to arranging them by their size in gross floor area (GFA). An extreme example is the difference in floor area of two tallest towers on the chart: Central Park Tower will comprise slightly more than 1 million sq. ft., while 111 West 57th Street will contain less than a third of that total. There is a more than five-fold range of size among the super-slenders: the two towers with the smallest floor areas– less than 200,000 sq. ft. – are 520 Park Avenue and One Madison.
Below, the Museum's line-up of buildings in the chart above are detailed, starting from Sky House (completed in 2008), which is the both earliest and the shortest of the slender tower type and ending with the tallest, Central Park Tower (in May 2016 in the early stages of construction), which is reported will rise to 1,569 feet. For a history of slenderness, the influence of zoning, and role of air rights click here.