The X That Holds Up the Hancock

Look at the dark tower ahead and you will notice something unusual: giant X shaped braces running up its sides, visible from the outside instead of hidden inside the walls. That is not decoration. It is the building's skeleton, turned inside out.

The engineer behind it, Fazlur Khan, realized that a skyscraper's outer skin could do structural work that had always been left to a hidden steel cage buried in the center. By bracing the exterior into a kind of rigid tube, he let the tower stand taller while using far less steel than earlier giants like the Empire State Building, and he freed the interior floors from the forest of support columns older skyscrapers needed. When it topped out in May of nineteen sixty eight, with the finished tower opening the following year, it was the second tallest building on Earth, behind only the Empire State Building, and the first skyscraper anywhere designed from the start to mix offices and homes in a single tower.

That mixed use design is still visible if you know where to look. Around the forty fourth floor, the building changes character entirely: offices fill the floors below it, apartments fill the floors above, and the forty fourth floor itself holds a sky lobby where residents transfer elevators, along with a swimming pool. Measured by height above the ground, it is the highest swimming pool anywhere in the country, a strange amenity to build so far into the sky, but a popular one with the people who actually live up there.

The tower has gone by more than one name over the years, but the engineering underneath never changed. Khan's X braced tube became one of the most copied ideas in skyscraper design anywhere in the world.