Even on a calm day, the tower sways around 6 inches (15 cm) in both directions. In comparison, the Empire State Building is designed to move only an inch and change even in gusts of up to 100 miles per hour (160 kilometers per hour). The movement is caused by wind pressures acting on the building's slender shape.
In fact, the Empire State Building is one of the most rigid structures ever built. It was constructed using what are now known as "Chicago school" techniques, which means that its frame is made of steel girders without any wood framing inside the exterior walls or roof. This allows the architects to use very thin walls (30 feet [9 m] thick at its base) for its weight capacity of 10,000 pounds per square foot (141 kg/m2).
The building is actually free-standing except for the base which rests on large blocks of concrete held in place by metal ties. This design decision was made so that people can walk under it if they want to go from one side of the street to another, rather than having to cross its entire width.
It was also thought that this would make the building more resistant to damage from heavy winds - although this purpose has been questioned by some engineers who think that it could cause problems of its own.
"The Empire State Building does not waver in any manner. It provides The structure gives 1.48 inches in a 110-mile-per-hour wind. Movement off center is never larger than one quarter inch, therefore quantifiable movement is just one half inch on any side." -- Structural engineer Richard Roberts.
According to Discovery, the Empire State Building in New York City, with a roof height of 1,250 feet, is predicted to shift one inch in high winds. The Willis Tower in Chicago, with a roof height of 1,450 feet, has an average wobble of six inches off its "true center," but is planned to shift no more than three feet. The Petronas Twin Towers in Kuala Lumpur, Malaysia, with a roof height of 535 feet, are expected to lean up to four degrees east or west in high winds.
The question of whether tall buildings can be swayed by wind is important because strong winds can cause large structures to bend or even collapse. As buildings get taller, they require stronger connections between their different parts: beams to hold up the floors, walls, and ceilings; and ties to connect them together into a solid foundation. As these connections become longer, they can develop weaknesses, which can lead to collapse.
When a building does not have a clear owner or manager, it can be difficult or impossible to get repairs done efficiently. This often leads to buildings being designed for specific loads rather than natural conditions, which increases their cost and may make them obsolete when heavy winds do strike.
It is possible for buildings to collapse due to other causes too, such as when an earthquake causes part of a structure to crumble down on itself. However, if a building's frame is still intact but the building is mostly made up of empty rooms, then it could suffer major damage without collapsing.