Simanaitis Says
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HIGH ON MANHATTAN PART 1
IN “NEW YORK CITY’S Evolving Skyline,” Stefano Chen writes that this year “could be the city’s busiest year ever for new skyscrapers.” Here, in Parts 1 and 2 today and tomorrow, are tidbits from Chen’s article, which was published in The New York Times, June 5, 2019.
The “Old Days”—Up to the 1970s. Three buildings dominated the NYC skyline from the early 1930s into the 1970s: 40 Wall Street (now the Trump Building, not to be confused with Trump Tower ), 927 ft., opened May 26, 1930; the Chrysler Building, 1047 ft, May 27, 1930; and the Empire State Building, 1250 ft., May 1, 1931.
The New York City skyline, 1931. This image, from Underwood Archives/IUG/Rex via Shutterstock, and others following appeared in The New York Times, June 9, 2019.
In a prescient slight to our national narcissist, the Chrysler Building’s architect secretly added a 125-ft. spire to its design, thus besting 40 Wall Street’s height. The Empire State Building’s design was modified 15 times to ensure its claim as the world’s tallest building. Including its antenna, it rises 1454 ft.
The Empire State Building lost its U.S. claim in 1971 with completion of the North Tower of NYC’s World Trade Center. As of 2019, the Empire State Building is the fifth tallest skyscraper in the U.S. and 28th-tallest in the world.
Manhattan versus St. Thomas, U.S.V.I. I used to live on St. Thomas, a Caribbean island topographically not unlike Manhattan, particularly if a giant green sheet were draped over the latter. St. Thomas is about 12 miles by an irregular 3 miles, with its highest elevation at 1555 ft.
The population of St. Thomas is around 55,000, a density of 1653 people/sq. mi. The population of Manhattan is around 1,665,000, with about 73,000 people per sq. mi.
Ever wonder why Manhattan’s buildings grow tall?
Manhattan’s West Side, 2019. This and the following image by Tony Cenicola/The New York Times.
Stronger Concrete, Quicker Elevators, Better Aerodynamics, and Anti-Sway. Manhattan’s skyscraper have increasingly scraped the sky, especially in the last decade. Reasons for this include stronger construction materials, quicker and more efficiently controlled elevators, improved knowledge of structural aerodynamics, enhanced computer modeling, and a proliferation of sway dampers.
Chen reports in The New York Times that “A better understanding of aerodynamics has produced skinny towers that can sway from four to six feet in any direction at the top.” Also, rarely used before 2005, mechanical dampers atop towers reduce sway in about a dozen of NYC buildings.
New York City’s East Side is home to the U.N. complex. Note as well the Chrysler Building (in the photo to the U.N.’s right) and the Empire State Building both peeking over its shoulder.
All sites worth celebrating. Tomorrow in Part 2, we’ll hear about some of the gaming played to exploit this proclivity for being high on Manhattan.
© Dennis Simanaitis, SimanaitisSays.com, 2019
Simanaitis Says 
According to ANSI/AISC 360 wind sway limits for steel building structures should fall in the region of H/400 to H/500, which would translate into 2 to 2.5 ft. for a 1000 ft. building. Seismic limits are higher. Drift/sway limits are governed by features such as interior and exterior finishes and the ability of these materials to experience drift deflections with no damage (cracked plaster, broken glass, etc).
In New York, the One World Trade Center at 1,776 ft sways 3 ft. (1776/400 = 4.4ft.). In Dubai, the Burj Khalifa at 2,717 ft. sways 5 ft. (2717/400 = 6.8 ft.). The Willis Tower (formerly Sears Tower) in Chicago at 1,354 ft. sways about 12” (1354/400 = 3.4 ft.). Conversely 432 Park Avenue, NY, NY, at 1,379 ft. sways 4 to 5 feet (1379/400 = 3.4 ft.), which according to reports (https://gizmodo.com/video-the-tallest-apartment-building-in-america-sways-1651431417) means that the $95 million apartment near the top can be distinctly uncomfortable, inducing nausea. Heck the idea of paying $95 million for an apartment gives me nausea.
Human perception of building sway is less related to the distance moved (assuming no cracking or glass breakage), but more to the acceleration of the movement. Accelerations of between 0.5%g and 1.5%g become perceptible to humans. ISO guidelines suggest a 1.3%g peak yearly acceleration for office buildings and 0.9%g peak yearly for residential buildings.
In the National Building Code of Canada buildings over 60m or height greater than 4 times the least base dimension can be designed with the dynamic wind procedure (computations using some equations and charts) or via wind tunnel testing. Buildings with height greater than 6 times the least base dimension must be designed via wind tunnel testing only. As a result most skyscrapers spend some time in the wind tunnel via scale models of course.
I had the fortune of attending the University of Western Ontario, in London, Ontario where the Boundary Layer Wind Tunnel (https://www.uwo.ca/projects/heritage/heritage4/) was a leading authority in the design of buildings and structures all around the world, including to name a few: Original WORLD TRADE CENTER TOWERS, New York; FIRST NATIONAL BANK BUILDING, Seattle; US STEEL BUILDING, Pittsburgh; SEARS BUILDING, Chicago; CN TOWER, Toronto; HONG KONG AND SHANGHAI BANK BUILDING, Hong Kong; THE SUNSHINE SKYWAY BRIDGE, Tampa, Florida; BANK OF CHINA BUILDING, Hong Kong; BURJ KHALIFA, Dubai, UAE; and STRAIT OF MESSINA BRIDGE, ITALY, planned as world’s longest suspension bridge with main span of 3.3km.
The envelope wind loads procedure in ASCE 7 was based on research at the wind tunnel, and on the lighter side, they studied WIND EFFECTS FOR AMEN CORNER, AUGUSTA NATIONAL GOLF CLUB.