Tall buildings are designed to sway, and this movement is not a structural defect but a necessary function of modern engineering. The slight, controlled motion allows a skyscraper to safely absorb and dissipate external forces, much like a tree bending in a strong wind. If a tall structure were perfectly rigid, it would be unable to flex and would instead accumulate stress, leading to material fatigue and potential failure. Engineers intentionally build this flexibility into the design, ensuring the building remains intact and operational.
The Primary Causes of Building Movement
The vast majority of perceptible skyscraper movement is caused by wind, which exerts forces that push and pull the structure. A significant factor is vortex shedding, where wind flowing around the building separates and forms alternating swirling air masses called vortices on opposite sides. This pattern, often called a Kármán vortex street, creates alternating zones of low pressure that pull the building sideways. If the frequency of this vortex shedding aligns with the building’s natural frequency, resonance occurs, which can amplify the motion to uncomfortable levels. Significant seismic activity is the other major force that induces building movement.
Designing for Flexibility and Frequency
The core philosophy in tall building design is to create a structure flexible enough to absorb energy without snapping. Engineers must determine the structure’s natural frequency, which is the specific rate at which it will naturally vibrate when disturbed, similar to a tuning fork. This frequency is determined by the building’s mass and stiffness, and for super-tall buildings, it is inherently low. To avoid the destructive effects of resonance, engineers manipulate the building’s design so its natural frequency does not match the expected frequency of wind forces. This structural tuning often involves modifying the exterior shape through features like tapering or rounded corners, which disrupt the formation of vortices and spread wind energy over a wide frequency spectrum.
Active and Passive Systems for Sway Control
Engineers often employ specialized hardware to actively manage excessive movement once the foundational structural design is complete. The most prominent solution is the Tuned Mass Damper (TMD), which functions as a massive, dynamic shock absorber for the building. A TMD consists of a large mass, often steel or concrete, that is mounted on springs or pendulums near the top of the skyscraper, where movement is most pronounced. The mass is “tuned” to synchronize with the building’s sway frequency. When the skyscraper begins to move, the TMD’s inertia causes the heavy mass to move in the opposite direction, transferring the kinetic energy from the building’s frame into the damper system.
Human Perception and Safety Standards
While structural sway is necessary for safety, it must be controlled to ensure the comfort of occupants, who can experience motion sickness or anxiety from the movement. Human perception of sway is highly subjective, but engineers rely on established serviceability criteria to set limits on acceleration, especially at the highest floors. These comfort thresholds are expressed in milli-g, with a peak acceleration of around five milli-g being the point where movement becomes noticeable. The movement is often only perceptible on the upper floors. Building codes require that the maximum sway magnitude remains within limits that ensure structural integrity, even during severe weather. These codes limit a building’s top-level deflection to a small fraction of its total height.