How did turning a skyscraper into a hollow tube let towers climb with less steel?

Once buildings were tall enough, the main enemy stopped being their own weight and became the wind. As a building gets taller, lateral loads such as wind and earthquake forces begin to dominate its structural system. Tubular structures are stiff and significantly reduce the structural material required. In the 1960s an engineer named Fazlur Rahman Khan, working in Chicago for Skidmore, Owings & Merrill, proposed something different: make the whole building act like a hollow tube, a thin-walled cylinder cantilevered up from the ground.

The idea in its simplest form is to put the strength on the outside. Closely spaced exterior columns are tied together by deep beams, forming a rigid frame that amounts to a dense and strong wall around the building. Because the outside resists all the lateral loads, the interior needs only to hold up the floors, with few columns, mostly at the core. About half the exterior surface can still be windows, and the inside is more usable space.

Why is a tube such a good shape? Engineers note that the most efficient shape for bending in any direction is a cylindrical shell or tube, and a skyscraper in a gale is really a giant cantilever beam bending in whatever direction the wind blows. Khan said his idea was inspired by the bamboo around Dhaka, his hometown in Bangladesh: he found that a hollow tube lent a high-rise vertical durability. He had not seen a skyscraper in person until he was 21.

The result was dramatic. The 100-storey John Hancock Center, with its famous exterior X-bracing, needed only 145 kilograms of steel per square metre, compared with 206 for the Empire State Building and 275 for One Chase Manhattan Plaza. Today most buildings of over 40 storeys built since the 1960s are of this structural type.