Why did older stone buildings have such small windows? They had thick walls with small windows, because the walls held up the heavy roof. Before steel frames, tall buildings had thick heavy masonry walls, and that limited how high they could go.

A tall building now stands on a strong frame of steel beams and columns, like a skeleton. Engineers use steel beams and columns to hold up most skyscrapers because steel is very strong. The weight pushes down on the columns and squeezes them. Because the frame carries the weight, the outer wall can have many windows, which stone walls could not.

Builders began reinforcing concrete with iron in the 1850s and with steel by the 1880s. Why add metal at all? Concrete is strong when it is squeezed, but weak when it is pulled apart. To make it strong when pulled, builders put steel bars inside. Compression squeezes a material and tension stretches it. When a beam bends, one side is squeezed and the other is stretched.

The first passenger elevator was installed in a New York store in 1857. It made buildings taller than four or five stories practical. Elisha Otis showed his safety brake by having the rope cut while he stood on the platform, and the platform held. The brake is a steel spring that grabs a ratchet if the rope breaks, which stops the platform from falling.

Steel frames and elevators together made the first skyscraper possible, in Chicago in 1885. The Home Insurance Building there, 10 stories and 138 feet tall, is called the first skyscraper. The Home Insurance Company wanted a fireproof building, and masonry covered the metal frame to protect it from fire. The word skyscraper was used for very tall buildings starting in the 1880s.

Here is a bold idea. Fazlur Khan's idea was to put the strong parts of a skyscraper in its outside walls so that the building acts like a hollow tube.

Wind is a bigger task for taller buildings. They face stronger and more complicated wind forces than short ones. Every tall structure sways a little, and engineers watch sway because people on high floors can feel it. Taipei 101 has a huge pendulum at the top that weighs 660 tonnes. It swings the opposite way from the tower, which helps cancel out its swaying. This was true in November 2026.

The Burj Khalifa is built of reinforced concrete, which is concrete with steel bars inside. It has a Y-shaped plan with three wings that hold each other up around a six-sided center. It is 828 meters (2,717 feet) tall with 163 floors.

The ground can be a problem too. To keep the Leaning Tower of Pisa from leaning more, engineers laid lead weights on a concrete ring on its north side. To straighten it, they slowly removed soil from beneath its high north side through boreholes.

Earthquakes are a different test. Earthquake shaking is the main cause of damage to buildings, and a building built straight on the ground moves with the shaking. A base isolator is a flexible bearing of rubber and lead under a building, so the building moves less when the ground shakes.

Words to know

• masonry: stone or brick
• tension: stretching force
• pendulum: swinging weight
• isolator: shaking blocker