Explained
How a skyscraper works.
Weight and wind
A skyscraper has to solve two problems at once. The first is gravity: every floor, every person and every desk has to be carried down to the ground. The second is wind, which pushes on the side of a tall building with surprising force and tries to bend it like a tree. In most modern towers these jobs are shared between a stiff central core and a ring of columns around the edge.
The core is a reinforced-concrete tube that runs the full height of the building. It holds the lift shafts, stairs and the risers that carry water, power and air up the tower, and because it's a closed tube it's very stiff against bending. The columns around the perimeter carry the floors' weight straight down. Lower columns support every floor above them, so they're often bigger than the ones near the top. The floor plates, concrete slabs often poured onto ribbed steel decking, span between the core and the columns and tie them together, so the whole frame acts as one.
A skin that hangs
The glass you see on the outside isn't holding anything up. It's a curtain wall: a lightweight grid of aluminium mullions and glass panels hung from the edge of each floor slab. It carries only its own weight and the wind load on it, and passes both back to the structure. Separating the skin from the structure is what allows towers to be wrapped in glass, and it lets the panels be made in a factory and lifted into place.
Lifts and foundations
Tall buildings only became practical with safe passenger lifts. In 1854 Elisha Otis demonstrated a safety brake that stopped a falling platform if the rope broke, and traction lifts with counterweights followed. In the model two lift cars run up and down inside the core.
All of that weight has to go somewhere. Near the surface the soil is often too soft to carry a tower, so long concrete piles are driven or drilled down to firmer ground or rock. A thick concrete raft, or pile cap, on top spreads the load between them. Chicago's 1885 Home Insurance Building is often called the first skyscraper because of its load-bearing metal frame. Today Dubai's Burj Khalifa, 828 metres tall, has held the record since 2010, using the same basic ideas at enormous scale.
Parts glossary
- Concrete core
- A reinforced-concrete tube in the middle of the tower houses the lifts, stairs and service risers. It acts as the building's spine and resists much of the sideways force from wind.
- Columns
- Steel or concrete columns around the edge carry the weight of every floor straight down to the foundations. Lower columns carry more floors, so they're often larger.
- Floor plates
- Each floor is a concrete slab, often poured onto profiled steel decking, spanning between the core and the columns. The slabs also tie the columns to the core so they work together.
- Curtain wall
- The glass skin hangs off the edge of each floor and carries no building weight, only its own weight and the wind pushing on it. Aluminium mullions frame each glass panel. Fun fact: It's called a curtain wall because it hangs from the structure like a curtain instead of holding the building up.
- Elevators
- Lift cars run in shafts inside the core. In a traction lift, steel ropes run over a motor-driven sheave to a counterweight that balances the car. Fun fact: In 1854 Elisha Otis publicly demonstrated his safety brake by having the hoist rope cut while he stood on the raised platform.
- Foundation piles
- Long concrete piles are drilled or driven down until they reach strong soil or rock. A thick raft on top spreads the tower's weight across all of them.
