Sky tubes

A Bangladeshi engineer named Fazlur Rahman Khan revolutionised the design of skyscrapers by modelling them on bamboo tubes.

Tomislav Medak, CC BY-SA 2.0, via Wikimedia Commons

As a building gets taller, the engineering challenges increase. This is down to two factors: the weight of the building itself, and the forces like wind that act on the building. For a long time, these factors were a natural constraint on the height of a skyscraper. Each floor of the building had to be able to carry the full weight of the floors above it. So, with each new floor, the lower floors needed thicker columns and less usable floor space. (The Empire State Building, for forty years the tallest skyscraper in the world, is packed full of steel columns.) The steel frames that held up skyscrapers had an upper limit. Beyond a certain point, extra height became impractical. This is a nice summary:

The person who freed the skyscraper from this constraint was a Bangladeshi-American architect / structural engineer named Fazlur Rahman Khan. His innovation was to transfer the load from the frame to the exterior of the building. Rather than putting many columns on the inside of the building, Khan’s skyscraper design pushed most of the columns to the outside. If you wanted more space between the columns, you could bind them together with diagonal beams. This design gave the whole structure unparalleled strength and wind resistance. In essence, his skyscrapers functioned like long tubes of bamboo.

The first building to use this tubular design was the DeWitt-Chestnut Apartment Building in Chicago. With that proof of concept done, Khan and his colleagues designed such iconic structures as the John Hancock Center (pictured above) and the Willis Tower:

Willis Tower tubes
I, Daniel Schwen, CC BY-SA 3.0, via Wikimedia Commons

The Willis Tower (formerly the Sears Tower) is actually a series of tubes all bundled together, as is the current tallest skyscraper in the world (the Burj Khalifa). In fact, most skyscrapers since this innovation use a sky tube design.

Khan died in 1982; his influence on skyscraper construction was so central that the lifetime achievement award from the Tall Building Awards is named in his honour.

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