How Skyscrapers Are Built to Stand Tall

A tall building is, from an engineer's point of view, a remarkable balancing act between the forces that push a structure up and the forces that try to bring it down. The most obvious of these forces is gravity, which pulls the enormous weight of concrete, steel and glass towards the ground. This downward load, known as the dead load, is the constant weight of the building itself. Added to it is the live load, the changing weight of the people, furniture and equipment that a building must carry as it is used. Together these vertical loads must travel safely down through the structure and into the earth, and the taller the tower, the greater the burden that its lowest levels are asked to bear.

For centuries the height of buildings was limited by the materials used to make them. Stone and brick are strong when squeezed but weak when stretched or bent, so walls had to be made thicker and thicker to support the floors above. In a very tall masonry building the walls at the base would need to be so massive that they would swallow up much of the usable space inside. The breakthrough that made the modern skyscraper possible was the development of a strong internal skeleton, usually made of steel or reinforced concrete. Instead of resting the weight on solid outer walls, engineers hang the floors and the light outer covering from a frame of columns and beams. The outer wall no longer holds the building up; it merely keeps out the weather.

Reinforced concrete is central to this achievement. Concrete on its own is excellent at resisting compression but cracks easily under tension. By embedding steel bars inside it, engineers combine the compressive strength of the concrete with the tensile strength of the steel, producing a material that can cope with both kinds of stress. The columns carry the crushing weight downwards, while the horizontal beams and floor slabs spread the loads across the frame and pass them to the columns. In this way the whole skeleton works as a single connected system rather than as a stack of separate pieces.

Yet weight is not the only enemy of a tall tower. Wind is often the more difficult problem. A skyscraper presents a huge flat surface to the sky, and a strong gale pressing against that surface can push the top of the building noticeably to one side. Engineers therefore design towers to sway, because a structure that is completely rigid would be far more likely to crack than one that can bend a little and recover. The aim is not to prevent all movement but to keep it small enough that the building remains safe and that the people inside do not feel uncomfortable. Very tall towers are tested in wind tunnels, where scale models are exposed to carefully controlled currents of air so that engineers can study how the design behaves before it is built.

Several clever techniques are used to keep wind movement under control. The shape of the tower itself can be adjusted, with corners rounded or the outline gradually twisted, so that the wind cannot form the regular swirling patterns that would otherwise set the building rocking. Inside, engineers may place a heavy weight near the top of the tower, held by springs or fluid, which swings in the opposite direction to the building and cancels out much of the sway. Such a device, known as a tuned mass damper, acts rather like a counterbalance, absorbing energy that would otherwise be felt as motion.

Everything a skyscraper does above ground depends on what lies beneath it. The foundations must carry the entire weight of the tower and prevent it from sinking or tilting. Where the ground is soft, engineers sink deep columns called piles far down until they reach firm rock or dense soil that can bear the load. In many towers a thick slab of reinforced concrete, sometimes combined with these piles, spreads the weight over a wide area of ground. The design of the foundation depends heavily on the type of ground at the site, which is why engineers study the soil and rock carefully before construction begins.

A finished skyscraper, then, is not simply a tall pile of floors but a carefully tuned instrument for managing forces. Its skeleton channels gravity safely to the earth, its shape and internal devices tame the wind, and its foundations anchor it against sinking and toppling. As materials improve and computers allow ever more precise calculation of how a structure will behave, buildings continue to climb higher. Each new record, however, rests on the same basic principles that first allowed engineers to lift habitable floors hundreds of metres into the air and keep them there safely.