How do skyscrapers not fall over?

1pinned deep

It is natural to imagine a very tall building as something balanced, ready to topple. It is not. A skyscraper is pinned into the ground: the Burj Khalifa stands on a thick foundation mat held by close to two hundred concrete piles driven deep into the earth. The real question is not tipping but what keeps a tall slim structure from bending, cracking or swaying too much.

A skyscraper is pinned into the ground on deep piles, not balanced like blocks.

2two jobs

A tall building has two jobs. The first is to carry its own weight, which presses down on the columns and is something steel and concrete are good at. The second is to resist wind, which pushes sideways, and whose pressure increases with height. For supertall towers, wind is often the load that governs the design.

Weight pushes down; wind pushes sideways, and wind often decides the design of the tallest towers.

3a stiff beam

Engineers treat a tall tower like a giant beam fixed at the bottom, a cantilever. Wind pushes on the whole side of the building and is strongest near the top, so it tries to bend the beam over, much as you would bend a ruler pinned to a table. The structure has to be stiff enough that the bend stays small.

A tower acts like a stiff beam fixed at the base; wind tries to bend it.

4core and outriggers

At the heart of the tower is a stiff core. To make the whole building share the load, tall towers add outriggers: stiff arms, often with a belt truss around the edge, that tie the core to the outer columns. Taipei 101 has outrigger trusses every eight floors. The outer columns then help the core resist the bend, so the building acts as one wide beam.

A stiff core plus outrigger arms make the full width of the tower resist the wind.

5shaped for wind

Wind flowing past a tall building creates swirling vortices that can make it sway. The Burj Khalifa's designers used wind tunnel tests and reshaped the tower, with a Y-shaped plan and setbacks, to limit those wind loads. The varied shape means the wind meets a different form at each stage, so the swirls do not organise into a steady rhythm.

The Burj Khalifa's stepped Y shape was tuned in wind tunnels to break up wind swirls.

6the pendulum

Some towers add a tuned mass damper: a huge pendulum tuned to the building's own rhythm. In Taipei 101 it is a ball of stacked steel plates weighing about 660 tonnes, hung near the top. When the tower sways in the wind, the ball moves out of step with it and soaks up the energy, which is then dissipated by dampers. Not every skyscraper needs one.

A tuned pendulum swings against the sway and soaks up the energy; Taipei 101's weighs about 660 tonnes.

7the whole idea

The whole answer on one page: a skyscraper is a stiff beam pinned deep into the ground on piles; its core and outrigger arms make the full width resist wind, its shape is tuned in wind tunnels so swirls break up, and a few towers add a pendulum that swings against the sway.

Deep piles, a stiff core with outriggers, a wind-tuned shape and sometimes a pendulum.