How Do Skyscrapers Stand Up? Steel, Wind and the Lift That Made Them Possible
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The tallest building in the world before 1885 was limited by its walls, which had to be thicker at the bottom the higher they went, until a sixteen-storey building in Chicago had walls nearly two metres thick at ground level and almost no room inside them. The Home Insurance Building of that year put the weight on a skeleton of iron and steel instead and hung the walls from it, and the skyscraper was born as a frame. Everything since, from the Empire State to the 828-metre Burj Khalifa, is a variation on that idea, plus the solution to two problems the frame created: the wind, and getting people to the top.
The frame
A skyscraper's weight and the loads on its floors are carried by columns, which run vertically through the building to foundations that spread the load over rock or, in soft ground, onto piles driven or bored down to it; the Burj Khalifa stands on 192 piles 50 metres deep. Beams span between the columns to carry the floors, and the floors themselves are concrete slabs on steel decking. The walls, the curtain wall of glass and metal that gives a tower its face, hang from the floor edges and carry nothing but themselves and the wind that hits them, which is why a skyscraper can be glass from top to bottom. Steel took over from iron because it is stronger and more consistent, and reinforced concrete competes with it, especially outside the United States, since it is cheaper, stiffer and better at damping the sway.
The wind
Gravity is the easy load; the wind is the hard one. A tall building is a lever with the wind pushing at its top and the ground holding its foot, and the pressure rises with height and with the square of the wind speed, so that a 400-metre tower in a storm is taking a sideways force of thousands of tonnes and trying to bend. The engineering answer is to make the building stiff against bending in the way that best uses its shape:
- •The core: a concrete tube around the lifts and stairs, acting as a spine; most towers up to 300 metres rely on it
- •The braced or framed tube: the outer columns and beams made stiff enough that the whole building acts as a hollow tube, as in the twin towers of New York and the diagonal bracing of the John Hancock tower in Chicago
- •Outriggers: stiff arms tying the core to the outer columns so that they resist bending together, used in the Burj Khalifa and most supertall towers since
- •Shape: setbacks, tapering and softened corners that break up the wind and stop it forming the rhythmic eddies that can rock a tower; the Burj's spiralling profile confuses the wind at every height
- •Dampers: a tuned mass, a 660-tonne steel ball hung near the top of Taipei 101, that swings against the sway and cancels it, or tanks of water that slosh out of phase
Sway
Every tall building moves. The top of a 400-metre tower may sway a metre or more in a strong wind, and the structure is designed to do so without harm; what limits the movement is not the steel but the stomachs of the people inside, who feel the acceleration as seasickness at levels far below anything structural. Occupant comfort is the governing criterion for the tallest towers, and it is why the dampers exist. The first tenants of the Hancock tower in Boston, in the 1970s, watched their windows fall out as the building twisted; the tuned damper added afterwards fixed it, and every supertall since has had the wind-tunnel test that the Hancock did not.
The lift
A tall building is useless if nobody can reach its floors, and the invention that made the skyscraper possible was less the steel frame than Elisha Otis's safety lift of 1853, which locked to its guide rails if the cable broke and made people willing to ride one; the first passenger lift went into a New York shop in 1857. Lifts set the limits still. A cable is limited to about 500 metres because above that its own weight becomes the load, so the tallest towers use sky lobbies where passengers change from one bank of lifts to another, and the core of a supertall building is largely lift shaft; the Burj Khalifa has 57. Speeds have reached 20 metres a second, ears pop, and carbon-fibre ropes and cable-free lifts that climb on linear motors, moving sideways as well as up, are the technologies meant to let buildings pass a kilometre.
Why build them
The skyscraper was an answer to expensive land, first in Chicago after the fire of 1871 and then in Manhattan, where a small island and a growing city made it worth building up rather than out; the Empire State's 102 storeys went up in 410 days in 1930 and 1931 and stood as the tallest for forty years. The towers of the Gulf and Asia that overtook it are as much statements as offices, since a building above about 70 storeys costs more per floor than it earns, and the height race is a race for the record. The engineering that makes them possible, the frame, the tube, the outrigger, the damper and the lift, is now used at every scale, and most of the world's tall buildings are not records but the ordinary 30- and 40-storey blocks that house the people of cities where the ground ran out.
The takeaway
A skyscraper stands on a frame of steel or concrete columns and beams that carries every load to deep foundations, with a curtain wall that holds nothing up, and it resists the wind, which is the governing force at height, with a stiff core, an outer tube, outriggers, a wind-confusing shape and tuned dampers that cancel the sway people would otherwise feel. Otis's safety lift made height usable, cable limits set the sky lobbies, and the whole form was an answer to land too expensive to build across.