"Slender" towers are beginning to pop up all over the world, notably, in cities like New York, where real estate is scarce, but the desire to maximize ROI is strong. "The limited space for new buildings in places like New York City generally involves small parcels of land, which means that these new structures tend to be both tall and slender," explains WSP director of building structures, Silvian Marcus, in a recent story by Civil Engineering, the official magazine of the American Society of Civil Engineers.

The structural acrobatics required for such structures, when coupled with the soaring real estate costs for a place like Manhattan, means that these slim buildings cater almost exclusively to the ultra-luxury market. These buildings often have "just a single apartment or condominium per floor," for example, with some penthouses selling "for as much as $90 million," according to the article. 

What constitutes a bonafide "slender" building designation, exactly? According to New York City's Skyscraper Museum's 2012 exhibition, Sky High & the Logic of Luxury, a slender building is one with "a [height to width] ratio of 10:1 or even 12:1." Take SHoP Architects' new 111 West 57th tower in New York City due to open in 2020: When completed, the tower will be the world's most slender skyscraper, boasting a 1,435 feet of height and just 59 feet in terms of width. 

These structures, according to Civil Engineering, are possible today due to the "advances in engineering and improvements in the strengths of building materials." For example, decades ago, the strength of concrete would max-out at 10,000 or 12,000 pounds-per-square-inch, but today, concrete can easily can take up to 18,000 or even 20,000 pounds-per-square-inch, SOM's director of engineering Charles Besjak explained to Civil Engineering. 

When it comes to tall, skinny buildings, the sky's the limit.