Prefabrication technology allows giant skyscrapers to grow even faster which makes them even more profitable and desirable. But as skyscrapers soar higher into the clouds they become exposed to a new enemy, one that exploits every weakness – the wind.

To build the 442 metre Sears / Willis Tower in Chicago, the proverbial windy city, engineers must turn the skyscraper inside out. In 1970 the architects building the new headquarters for Sears and Roebuck in Chicago face a problem, their skyscraper the Sears Tower, will be over 100 floors tall, a height that exposes it to huge wind forces. Building this skyscraper using a traditional steel skeleton would cause massive problems as the taller the skeleton gets the more susceptible it is to bending in high winds. Gusts off Lake Michigan can buffet a skyscraper at up to 80kph, causing the upper floors to sway and affecting the workers inside.

The architects of the Sears Tower invent a technology that can beat the wind, they shift the steel framework from the inside of the building, to the outside. This so-called exoskeleton makes it very hard for the wind to bend the building. In the Seers Tower nine such tubes lock together to make the building rock solid. The exoskeleton is the best way of resisting wind ever invented, even at wind speeds of over 90kph the top floor of the Seers Tower only moves 15cm.

The world’s tallest skyscraper, the Burj Khalifa, is almost twice as high as the Sears Tower. At this extreme height fighting the wind with a rigid exoskeleton is not good enough. To stop the residents from getting motion sickness the architects turn to highly advanced aerodynamics.

At high speeds wind can be extremely dangerous for a skyscraper. Air rushes around the building and forms mini tornadoes, called vortices. These areas of low pressure suck the building sideways, and the taller the building, the more dangerous the vortices become. So for the Burj Khalifa, rather than fight the wind, the design team decide to deceive it. They don’t make the tower flat and rectangular, but instead give it a more unpredictable shape. Each section of the tower is designed to deflect the wind in a different way, this disrupts the power of the vortices and breaks the hold of the wind on the building.

Clip from the “Big Bigger Biggest” documentary series exploring the engineering breakthroughs that have enabled us to develop some of the largest structures in existence.

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