Engineers cannot be absolutely sure a structure will resist every possible load; they can only give it large enough margins of safety that failure is acceptably unlikely. Most of a skyscraper's load is its own weight: the dead load (the structure) is larger than the live load (people, furniture, vehicles). So the lower levels need much more structural material than the upper ones; wind pressure increases with height, and for very tall buildings the wind loads are larger than the dead or live loads.
A building's shape does not always show how it carries load. The Empire State Building's setbacks came from the 1916 Zoning Resolution, not from structural need, while the John Hancock Center's shape is the direct result of how it supports loads.
Historians do not agree on the first skyscraper. The Home Insurance Building had an iron skeleton with steel floor beams and masonry still carrying part of the load, and Liverpool's Oriel Chambers (1864) already had an iron frame and glass curtain wall. Architectural historians define a skyscraper by its steel skeleton, as opposed to load-bearing masonry, which reached its practical limit in 1891 with Chicago's Monadnock Building. The Flaxmill in Shrewsbury, England (1797), only partly iron framed, is the oldest iron-framed building in the world and is seen as the grandfather of skyscrapers.
A plain steel frame gets inefficient in very tall buildings; as height grows, supports must be closer together, which adds more material to support. Above about 40 stories it becomes uneconomic because columns eat usable floor space and more steel is needed. Steel use per square metre of floor fell as designs improved. The Empire State Building (1931) needed about 206 kilograms, 28 Liberty Street (1961) needed 275, and the John Hancock Center with its trussed tube needed only 145.
The tube system treats a tall building as a hollow cylinder standing on the ground like a cantilever, so that the whole building resists lateral loads such as wind, earthquake and impact. Fazlur Rahman Khan introduced the system while working at Skidmore, Owings & Merrill in Chicago. One account says Khan's tube idea was inspired by the bamboo around his hometown of Dhaka: he found that a hollow tube, like bamboo, gives a tall structure vertical durability.
The bundled tube was said to be efficient structurally and economically, and also flexible for architecture: towers no longer had to be box-like, because tube units could take different shapes and be grouped in different ways; hybrid structures use the basic tube idea plus other supports; engineers choose them when a building is so thin that one system alone cannot give enough strength or stiffness.
The outrigger and belt truss system joins the tube to the central core wall with very stiff outriggers and belt trusses at one or more levels. BHP House was the first building to use it. For mid-rise towers Khan developed the shear wall frame interaction system, combining shear walls and frames against sideways forces. The 35-story Brunswick Building, finished in 1965, was the tallest reinforced concrete structure of its time.
Elevators shape a skyscraper's design: a taller building needs more elevators, but the shafts use up valuable floor space, and a service core that grows too big cuts the building's profit; some towers use sky lobbies: express elevators carry people to upper floors that act as bases for local elevators, so shafts can be stacked on top of each other to save space.
Most skyscrapers have a steel frame and shear walls that hold up curtain walls, which either rest on the frame below or hang from the frame above, instead of being load-bearing walls.
Engineers choose deep foundations for skyscrapers because of very large design loads, poor soil near the surface, or site limits such as property lines; driving a pile compresses the surrounding soil, which raises friction on the pile's sides and its load capacity. When one pile cannot carry the load, groups of piles are connected by a pile cap, a large concrete block holding the pile heads, which spreads the load among them. An end-bearing pile is drilled until the hole reaches a sufficient depth into a strong layer, such as rock or hardpan.
Wind tunnel tests are used to find the wind speeds around buildings and bridges and the wind forces acting on them. Ground-level wind is highly turbulent, so environmental wind tunnels use spires and small cubes on the floor to make the air approaching the model behave like the atmosphere's boundary layer, unlike the steady air of vehicle tunnels.
A tuned mass damper is a mass on damped springs mounted in a structure; its oscillation frequency is tuned to match the structure's resonant frequency, and it reduces the structure's maximum motion while weighing much less than the structure. Wind can move the top of a skyscraper more than a meter, swaying or twisting it, and some wind angles can cause motion sickness. A damper works best when tuned to the building's resonant frequency, which can drift with wind, temperature and humidity; practical dampers are tuned either to move the main vibration mode away from a troublesome excitation frequency, or to add damping to a resonance that is hard or expensive to damp directly. During Typhoon Soudelor in August 2015, the damper of Taipei 101 swung 1 meter (as of November 2026).
Base isolation is a set of structural elements that largely decouples a building from the shaking ground beneath it, so earthquake motion is not passed fully to the structure above; isolation can be achieved with rubber bearings, friction bearings, ball bearings, spring systems and other means. The layered rubber-and-steel bearing with a lead core was invented by Dr Bill Robinson in New Zealand in 1974, and base isolator bearings were pioneered there in the 1970s.
The Citicorp Center in New York was completed in 1977; a light steel frame made the tower so efficient that it was light and more likely to sway in wind, and a tuned mass damper sat at its top. In 1978 a student working on a project about the tower phoned the engineer LeMessurier, a year after it was completed. The student said the wind checks covered winds blowing straight at the faces but not quartering winds that hit the corners.
LeMessurier found that the braces' connections had been bolted instead of welded as designed, a change made during construction; with the corrected figures, he estimated about a 1 in 16 chance of failure under the right quartering wind conditions. Fixing the problem meant welding steel plates over the bolted joints, with welders working at night while the building was in use. The repair was a race against the hurricane season, and the engineer, the architects and Citicorp fixed the issue as a hurricane threatened.
The public did not learn of the repair until The New Yorker told the story in 1995. Accounts disagree on whether New York's building code required checking quartering winds: one says it did not call for it, another says both wind types were mandatory. The Citicorp story was later written up as a case study in engineering ethics, and it became a staple of engineering ethics teaching.
Words to know
- cantilever: fixed-end beam
- outriggers: core-to-wall arms
- turbulent: swirling, uneven
- ethics: right conduct
