Skyscrapers are built upward for economic reasons: when land is expensive enough, building up lowers the land cost per unit of floor area, which is why they cluster in the centers of large cities. 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-frame construction overtook brick and mortar from the end of the 19th century, and in the 20th century it and reinforced concrete took over as steel got cheaper and labor got more expensive. 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. In a tube building the exterior frame is made strong enough to resist all the sideways loads, so the inside only has to be framed to carry gravity loads. In the simplest tube, closely spaced columns around the outside are tied together by deep beams with rigid connections. Together they form a dense, strong structural wall around the building's perimeter.

The tube system can be built in steel, concrete or a composite of both, and suits offices, apartments and mixed-use buildings. Most buildings over 40 stories built since the 1960s use this type of structure. Since about 1960 tubular designs have let towers use less material and still go higher. Willis Tower uses a third less steel than the Empire State Building, and the design leaves fewer interior columns and more usable floor space.

A framed tube lets about half of the exterior surface be windows and needs fewer interior columns, so more floor space is usable. Where a large opening such as a garage door is needed, the tube must be interrupted, and transfer girders keep the structure sound. A trussed, or braced, tube has fewer, more widely spaced exterior columns. Steel bracing or concrete shear walls along the outer walls make up for the missing columns by tying them together. Khan's X-braced tube, first used in the John Hancock Center, reduced the lateral load on the building by passing it into the exterior columns. A bundled tube is not one tube but several tied together to resist lateral forces. Willis Tower and One Magnificent Mile are examples.

A hull-and-core tower has an inner core tube for elevators and services plus an outer tube, and the outer tube usually takes most of the gravity and sideways loads because it is stronger. A tube-in-tube system adds an inner tube of core shear walls to the outer tube. The two work together against gravity and sideways loads and add stiffness, so the top does not deflect much. A building's core is a tube in the middle that holds the elevators, stairs and pipes. A skyscraper core acts like a vertical cantilever fixed at its base, so the core must resist large bending forces from wind. A core of concrete or steel linked to the outer columns by outriggers makes tall, thin buildings possible without thick walls, by adding stiffness and cutting the bending stress in the core.

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.

Every tall structure sways a little, and engineers watch sway because people on high floors can feel it. Taller buildings face stronger and more complicated wind forces than short ones. Wind swirls around a tall building in an unstable way, creating alternating low-pressure areas called vortices. Wind has steady pressure and gusty, changing pressure; the changing part can make a tall building sway. Tall, thin buildings sway more, because the larger the height-to-width ratio, the greater the swaying. Wind can move the top of a skyscraper more than a meter, swaying or twisting it, and some wind angles can cause motion sickness.

Most of a skyscraper's load is its own weight: the dead load (the structure) is larger than the live load (people, furniture, vehicles). Wind pressure increases with height. For very tall buildings the wind loads are larger than the dead or live loads, and the sideways wind load on supertall structures is generally the governing factor in structural design. Some structural engineers define a high-rise as any vertical construction where wind is a more significant load than earthquake or weight.

Engineers study wind forces on tall buildings so they can build with a minimum of material and still be safe in very high winds. Wind tunnel tests are used to find the wind speeds around buildings and bridges and the wind forces acting on them. Buildings with sharp corners or lopsided faces get uneven wind pressure that can twist them, so designers round corners and taper tops.

In skyscrapers, dampers are typically huge concrete blocks or steel bodies that move against the building's resonant oscillations, supported by springs, fluid or pendulums. 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. Many include a frictional or hydraulic part that turns the motion energy into heat, like a car's shock absorber. Taipei 101 has a huge pendulum at the top that weighs 660 tonnes (as of November 2026). The giant pendulum swings the opposite way from the tower, which helps cancel out its swaying. During Typhoon Soudelor in August 2015, its damper swung 1 meter, and a bumper ring limits its swing to 1.5 meters. New techniques let skyscrapers get narrower while getting taller: mass dampers reduce vibration and sway, and gaps let air pass through the building, which lessens wind forces.

Piles are long, slender columns of steel, concrete or sometimes timber that are put deep into the ground. Where the ground near the surface is too weak, builders use deep foundations that carry the load down to stronger layers. Engineers choose them 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. An end-bearing pile is drilled until the hole reaches a sufficient depth into a strong layer, such as rock or hardpan. 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. The Burj Khalifa's foundation has 192 piles, each 1.5 meters wide and 43 meters long, buried more than 50 meters deep. It was designed to carry a building weight of about 450,000 tonnes. The whole Burj Khalifa tower took 330,000 cubic meters of concrete and 55,000 tonnes of steel rebar.

A drilled shaft is made by boring a hole, then placing concrete, often with reinforcing, in it. Rotary boring can make larger piles than any other method and can cut through dense or hard layers. A pneumatic caisson is a bottomless box, sealed at the top and filled with compressed air to keep water and mud out at depth. An airlock gives access to the chamber. A caisson is lowered through soft mud until it reaches suitable foundation material. Bedrock is preferred, but a stable, hard mud is sometimes used when bedrock is too deep. A pneumatic caisson gives dry working conditions, which is better for placing concrete, and it suits foundations where other methods might make nearby structures settle.

The Citicorp Center in New York was a 59-story tower designed by architect Hugh Stubbins. It was completed in 1977. A church stayed on one corner of its site, so the tower floated above that corner, and LeMessurier put its support columns at the middle of each side instead of at the corners. 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 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. 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
• lateral: sideways
• pile: long deep column
• ethics: right conduct