Reading passage
A skyscraper is generally defined as a tall, continuously habitable building whose weight is carried by a structural frame rather than by masonry walls. The type was born in Chicago in 1885, when the Home Insurance Building rose ten storeys on a skeleton of steel and cast iron. Although later research showed that its frame was only partly self-supporting, the building proved a principle that would transform cities: once walls stop carrying loads, height is limited by engineering and economics rather than by the compressive strength of brick.
Three technologies made the skyscraper practical. Cheap structural steel became available after the Bessemer process spread in the 1870s, giving frames far greater strength than iron. The safety elevator, patented by Elisha Otis in 1852 and demonstrated dramatically at the 1854 New York Crystal Palace exhibition, removed the fear that a snapped cable would send passengers plunging down the shaft. Finally, pneumatic caissons allowed foundations to be sunk through soft ground until they reached bedrock, so that enormous loads could be carried safely even on the marshy soils where many commercial cities happened to stand.
Yet the decisive force behind the skyscraper was not technology but the price of land. In districts such as downtown Chicago and Manhattan, plots became so expensive that owners could profit only by multiplying the floor area standing on each square metre of ground. A twelve-storey building could, in effect, stack twelve sites on one footprint and collect rent from all of them. When New York introduced its zoning law in 1916, requiring towers to step back as they rose so that sunlight could still reach the streets, architects turned the setback itself into a celebrated style.
As buildings grew taller, wind replaced gravity as the controlling design problem. A tall tower behaves like a giant sail, and its top can drift sideways by more than a metre in a storm, which alarms occupants even when the structure is perfectly safe. To calm this motion, engineers install a tuned mass damper, a huge weight that swings out of phase with the building and absorbs its energy. The most famous example is the 660-tonne steel pendulum suspended between the 87th and 92nd floors of Taipei 101, which tourists now photograph as an attraction in its own right.
Materials have kept pace with ambition. High-strength concrete can now be pumped to heights once thought impossible, and the Burj Khalifa in Dubai, completed in 2010, rises 828 metres over 163 storeys on a Y-shaped, buttressed core that resists both wind and its own colossal weight. Engineers involved in the project report that concrete for the upper floors had to be placed at night, when cooler desert temperatures prevented the mixture from setting inside the pumps before it arrived at the top of the building.
The environmental record of tall buildings, however, is under growing scrutiny. Early glass towers leaked heat in winter and overheated in summer, so their air-conditioning systems consumed vast quantities of energy. Newer designs answer with double-skin facades that trap a buffer of air between two layers of glass, with shades that track the sun, and with ventilation strategies borrowed from termite mounds. Certification schemes now reward towers that cut energy use, harvest rainwater, and reuse the heat expelled by their own lifts and computers.
Tall buildings also stir argument about the kind of city people want. Critics complain that clusters of towers cast long shadows, create windy canyons at street level, and concentrate crowds that strain transport systems. The recent shift towards home working has added a practical question, as vacant office floors lose their tenants. In response, several cities now encourage the conversion of older office towers into apartments, arguing that reusing an existing frame saves the carbon that demolition and rebuilding would release.
Meanwhile, a quieter revolution is taking place in timber. Engineered wood products such as cross-laminated panels are strong, light, and store carbon instead of emitting it. The Mjostarnet tower in Norway, at 85.4 metres, showed in 2019 that wood can reach genuine skyscraper heights, at least by the standards of earlier centuries. Whether the future skylines of great cities are drawn in steel, concrete, or timber, the contest between height, cost, safety, and sustainability that began in Chicago is far from over.